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What does the three dot '...' syntax do in svelte? [duplicate]

What does the ... do in this React (using JSX) code and what is it called?
<Modal {...this.props} title='Modal heading' animation={false}>

That's property spread notation. It was added in ES2018 (spread for arrays/iterables was earlier, ES2015), but it's been supported in React projects for a long time via transpilation (as "JSX spread attributes" even though you could do it elsewhere, too, not just attributes).
{...this.props} spreads out the "own" enumerable properties in props as discrete properties on the Modal element you're creating. For instance, if this.props contained a: 1 and b: 2, then
<Modal {...this.props} title='Modal heading' animation={false}>
would be the same as
<Modal a={this.props.a} b={this.props.b} title='Modal heading' animation={false}>
But it's dynamic, so whatever "own" properties are in props are included.
Since children is an "own" property in props, spread will include it. So if the component where this appears had child elements, they'll be passed on to Modal. Putting child elements between the opening tag and closing tags is just syntactic sugar — the good kind — for putting a children property in the opening tag. Example:
class Example extends React.Component {
render() {
const { className, children } = this.props;
return (
<div className={className}>
{children}
</div>
);
}
}
ReactDOM.render(
[
<Example className="first">
<span>Child in first</span>
</Example>,
<Example className="second" children={<span>Child in second</span>} />
],
document.getElementById("root")
);
.first {
color: green;
}
.second {
color: blue;
}
<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>
Spread notation is handy not only for that use case, but for creating a new object with most (or all) of the properties of an existing object — which comes up a lot when you're updating state, since you can't modify state directly:
this.setState(prevState => {
return {foo: {...prevState.foo, a: "updated"}};
});
That replaces this.state.foo with a new object with all the same properties as foo except the a property, which becomes "updated":
const obj = {
foo: {
a: 1,
b: 2,
c: 3
}
};
console.log("original", obj.foo);
// Creates a NEW object and assigns it to `obj.foo`
obj.foo = {...obj.foo, a: "updated"};
console.log("updated", obj.foo);
.as-console-wrapper {
max-height: 100% !important;
}

... are called spread attributes which, as the name represents, it allows an expression to be expanded.
var parts = ['two', 'three'];
var numbers = ['one', ...parts, 'four', 'five']; // ["one", "two", "three", "four", "five"]
And in this case (I'm going to simplify it).
// Just assume we have an object like this:
var person= {
name: 'Alex',
age: 35
}
This:
<Modal {...person} title='Modal heading' animation={false} />
is equal to
<Modal name={person.name} age={person.age} title='Modal heading' animation={false} />
So in short, it's a neat short-cut, we can say.

The three dots represent the spread operator in ES6. It allows us to do quite a few things in JavaScript:
Concatenate arrays
var shooterGames = ['Call of Duty', 'Far Cry', 'Resident Evil'];
var racingGames = ['Need For Speed', 'Gran Turismo', 'Burnout'];
var games = [...shooterGames, ...racingGames];
console.log(games) // ['Call of Duty', 'Far Cry', 'Resident Evil', 'Need For Speed', 'Gran Turismo', 'Burnout']
Destructuring an array
var shooterGames = ['Call of Duty', 'Far Cry', 'Resident Evil'];
var [first, ...remaining] = shooterGames;
console.log(first); //Call of Duty
console.log(remaining); //['Far Cry', 'Resident Evil']
Combining two objects
var myCrush = {
firstname: 'Selena',
middlename: 'Marie'
};
var lastname = 'my last name';
var myWife = {
...myCrush,
lastname
}
console.log(myWife); // {firstname: 'Selena',
// middlename: 'Marie',
// lastname: 'my last name'}
There's another use for the three dots which is known as Rest Parameters and it makes it possible to take all of the arguments to a function in as one array.
Function arguments as array
function fun1(...params) {
}

... (three dots in JavaScript) is called the Spread Syntax or Spread Operator. This allows an iterable such as an array expression or string to be expanded or an object expression to be expanded wherever placed. This is not specific to React. It is a JavaScript operator.
All these answers here are helpful, but I want to list down the mostly used practical Use Cases of the Spread Syntax (Spread Operator).
1. Combine Arrays (Concatenate Arrays)
There are a variety of ways to combine arrays, but the spread operator allows you to place this at any place in an array. If you'd like to combine two arrays and place elements at any point within the array, you can do as follows:
var arr1 = ['two', 'three'];
var arr2 = ['one', ...arr1, 'four', 'five'];
// arr2 = ["one", "two", "three", "four", "five"]
2. Copying Arrays
When we wanted a copy of an array, we used to have the Array.prototype.slice() method. But, you can do the same with the spread operator.
var arr = [1,2,3];
var arr2 = [...arr];
// arr2 = [1,2,3]
3. Calling Functions without Apply
In ES5, to pass an array of two numbers to the doStuff() function, you often use the Function.prototype.apply() method as follows:
function doStuff (x, y, z) {}
var args = [0, 1, 2];
// Call the function, passing args
doStuff.apply(null, args);
However, by using the spread operator, you can pass an array into the function.
doStuff(...args);
4. Destructuring Arrays
You can use destructuring and the rest operator together to extract the information into variables as you'd like them:
let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
console.log(x); // 1
console.log(y); // 2
console.log(z); // { a: 3, b: 4 }
5. Function Arguments as Rest Parameters
ES6 also has three dots (...) which indicates a rest parameter that collects all remaining arguments of a function into an array.
function f(a, b, ...args) {
console.log(args);
}
f(1, 2, 3, 4, 5); // [3, 4, 5]
6. Using Math Functions
Any function where spread is used as the argument can be used by functions that can accept any number of arguments.
let numbers = [9, 4, 7, 1];
Math.min(...numbers); // 1
7. Combining Two Objects
You can use the spread operator to combine two objects. This is an easy and cleaner way to do it.
var carType = {
model: 'Toyota',
yom: '1995'
};
var carFuel = 'Petrol';
var carData = {
...carType,
carFuel
}
console.log(carData);
// {
// model: 'Toyota',
// yom: '1995',
// carFuel = 'Petrol'
// }
8. Separate a String into Separate Characters
You can use the spread operator to spread a string into separate characters.
let chars = ['A', ...'BC', 'D'];
console.log(chars); // ["A", "B", "C", "D"]
You can think of more ways to use the Spread Operator. What I have listed here are the popular use cases of it.

The three dots in JavaScript are the spread / rest operator.
Spread operator
The spread syntax allows an expression to be expanded in places where multiple arguments are expected.
myFunction(...iterableObj);
[...iterableObj, 4, 5, 6]
[...Array(10)]
Rest parameters
The rest parameter syntax is used for functions with a variable number of arguments.
function(a, b, ...theArgs) {
// ...
}
The spread / rest operator for arrays was introduced in ES6. There's a State 2 proposal for object spread / rest properties.
TypeScript also supports the spread syntax and can transpile that into older versions of ECMAScript with minor issues.

This is a feature of ES6, which is used in React as well. Look at the below example:
function Sum(x, y, z) {
return x + y + z;
}
console.log(Sum(1, 2, 3)); // 6
This way is fine if we have a maximum of three parameters. But, what if we need to add, for example, 110 parameters. Should we define them all and add them one by one?
Of course there is an easier way to do, which is called spread.
Instead of passing all those parameters you write:
function (...numbers){}
We have no idea how many parameters we have, but we know there are heaps of those.
Based on ES6, we can rewrite the above function as below and use the spread and mapping between them to make it as easy as a piece of cake:
let Sum = (...numbers) => {
return numbers.reduce((prev, current) => prev + current);
}
console.log(Sum(1, 2, 3, 4, 5, 6, 7, 8, 9)); // 45

Kudos to Brandon Morelli. He explained perfectly here, but links may die so I am just pasting the content below:
The spread syntax is simply three dots: ...
It allows an iterable to expand in places where 0+ arguments are expected.
Definitions are tough without context. Let's explore some different use cases to help understand what this means.
Example 1 — Inserting Arrays
Take a look at the code below. In this code, we don’t use the spread syntax:
var mid = [3, 4];
var arr = [1, 2, mid, 5, 6];
console.log(arr);
Above, we’ve created an array named mid. We then create a second array which contains our mid array. Finally, we log out the result. What do you expect arr to print? Click run above to see what happens. Here is the output:
[1, 2, [3, 4], 5, 6]
Is that the result you expected?
By inserting the mid array into the arr array, we’ve ended up with an array within an array. That’s fine if that was the goal. But what if you want only a single array with the values of 1 through 6? To accomplish this, we can use the spread syntax! Remember, the spread syntax allows the elements of our array to expand.
Let’s look at the code below. Everything is the same — except we’re now using the spread syntax to insert the mid array into the arr array:
var mid = [3, 4];
var arr = [1, 2, ...mid, 5, 6];
console.log(arr);
And when you hit the run button, here’s the result:
[1, 2, 3, 4, 5, 6]
Awesome!
Remember the spread syntax definition you just read above? Here’s where it comes into play. As you can see, when we create the arr array and use the spread operator on the mid array, instead of just being inserted, the mid array expands. This expansion means that each and every element in the mid array is inserted into the arr array. Instead of nested arrays, the result is a single array of numbers ranging from 1 to 6.
Example 2 — Math
JavaScript has a built-in math object that allows us to do some fun math calculations. In this example we’ll be looking at Math.max(). If you’re unfamiliar, Math.max() returns the largest of zero or more numbers. Here are a few examples:
Math.max();
// -Infinity
Math.max(1, 2, 3);
// 3
Math.max(100, 3, 4);
// 100
As you can see, if you want to find the maximum value of multiple numbers, Math.max() requires multiple parameters. You unfortunately can’t simply use a single array as input. Before the spread syntax, the easiest way to use Math.max() on an array is to use .apply().
var arr = [2, 4, 8, 6, 0];
function max(arr) {
return Math.max.apply(null, arr);
}
console.log(max(arr));
It works, it’s just really annoying.
Now take a look at how we do the same exact thing with the spread syntax:
var arr = [2, 4, 8, 6, 0];
var max = Math.max(...arr);
console.log(max);
Instead of having to create a function and utilize the apply method to return the result of Math.max() , we only need two lines of code! The spread syntax expands our array elements and inputs each element in our array individually into the Math.max() method!
Example 3 — Copy an Array
In JavaScript, you can’t just copy an array by setting a new variable equal to already existing array. Consider the following code example:
var arr = ['a', 'b', 'c'];
var arr2 = arr;
console.log(arr2);
When you press run, you’ll get the following output:
['a', 'b', 'c']
Now, at first glance, it looks like it worked — it looks like we’ve copied the values of arr into arr2. But that’s not what has happened. You see, when working with objects in JavaScript (arrays are a type of object) we assign by reference, not by value. This means that arr2 has been assigned to the same reference as arr. In other words, anything we do to arr2 will also affect the original arr array (and vice versa). Take a look below:
var arr = ['a', 'b', 'c'];
var arr2 = arr;
arr2.push('d');
console.log(arr);
Above, we’ve pushed a new element d into arr2. Yet, when we log out the value of arr, you’ll see that the d value was also added to that array:
['a', 'b', 'c', 'd']
No need to fear though! We can use the spread operator!
Consider the code below. It’s almost the same as above. Instead though, we’ve used the spread operator within a pair of square brackets:
var arr = ['a', 'b', 'c'];
var arr2 = [...arr];
console.log(arr2);
Hit run, and you’ll see the expected output:
['a', 'b', 'c']
Above, the array values in arr expanded to become individual elements which were then assigned to arr2. We can now change the arr2 array as much as we’d like with no consequences on the original arr array:
var arr = ['a', 'b', 'c'];
var arr2 = [...arr];
arr2.push('d');
console.log(arr);
Again, the reason this works is because the value of arr is expanded to fill the brackets of our arr2 array definition. Thus, we are setting arr2 to equal the individual values of arr instead of the reference to arr like we did in the first example.
Bonus Example — String to Array
As a fun final example, you can use the spread syntax to convert a string into an array. Simply use the spread syntax within a pair of square brackets:
var str = "hello";
var chars = [...str];
console.log(chars);

For someone who wants to understand this simple and fast:
First of all, this is not a syntax only to React. This is syntax from ES6 called spread syntax which iterate (merge, add, etc.) the array and object. Read more about it here.
So to answer the question:
Let's imagine you have this tag:
<UserTag name="Supun" age="66" gender="male" />
And you do this:
const user = {
"name": "Joe",
"age": "50"
"test": "test-val"
};
<UserTag name="Supun" gender="male" {...user} age="66" />
Then the tag will be equal to this:
<UserTag name="Joe" gender="male" test="test-val" age="66" />
So when you used the spread syntax in a React tag, it took the tag's attribute as object attributes which merge (replace if it exists) with the given object user. Also, you might have noticed one thing that it only replaces before attribute, not after attributes. So in this example, age remains as it is.

It's just defining props in a different way in JSX for you!
It's using ... array and object operator in ES6 (object one not fully supported yet), so basically if you already define your props, you can pass it to your element this way.
So in your case, the code should be something like this:
function yourA() {
const props = {name='Alireza', age='35'};
<Modal {...props} title='Modal heading' animation={false} />
}
so the props you defined, now separated and can be reused if necessary.
It's equal to:
function yourA() {
<Modal name='Alireza' age='35' title='Modal heading' animation={false} />
}
These are the quotes from React team about spread operator in JSX:
JSX Spread Attributes
If you know all the properties that you want to place on a component
ahead of time, it is easy to use JSX:
var component = <Component foo={x} bar={y} />;
Mutating Props is Bad If you don't know which properties you want to set, you might be tempted to add them onto the object later:
var component = <Component />;
component.props.foo = x; // bad
component.props.bar = y; // also bad
This is an anti-pattern because it means that we can't help you check
the right propTypes until way later. This means that your propTypes
errors end up with a cryptic stack trace.
The props should be considered immutable. Mutating the props object
somewhere else could cause unexpected consequences so ideally it would
be a frozen object at this point.
Spread Attributes Now you can use a new feature of JSX called spread attributes:
var props = {};
props.foo = x;
props.bar = y;
var component = <Component {...props} />;
The properties of the object that you pass in are copied onto the
component's props.
You can use this multiple times or combine it with other attributes.
The specification order is important. Later attributes override
previous ones.
var props = { foo: 'default' };
var component = <Component {...props} foo={'override'} />;
console.log(component.props.foo); // 'override'
What's with the weird ... notation? The ... operator (or spread operator) is already supported for arrays in ES6. There is also
an ECMAScript proposal for Object Rest and Spread Properties. We're
taking advantage of these supported and developing standards in order
to provide a cleaner syntax in JSX.

For those who come from the Python world, JSX Spread Attributes are equivalent to
Unpacking Argument Lists (the Python **-operator).
I'm aware this is a JSX question, but working with analogies sometimes helps to get it faster.

Three dots ... represent spread operators or rest parameters.
It allows an array expression or string or anything which can be iterating to be expanded in places where zero or more arguments for function calls or elements for array are expected.
Merge two arrays
var arr1 = [1,2,3];
var arr2 = [4,5,6];
arr1 = [...arr1, ...arr2];
console.log(arr1); //[1, 2, 3, 4, 5, 6]
Copying array:
var arr = [1, 2, 3];
var arr2 = [...arr];
console.log(arr); //[1, 2, 3]
Note: Spread syntax effectively goes one level deep while copying an
array. Therefore, it may be unsuitable for copying multidimensional
arrays as the following example shows (it's the same with
Object.assign() and spread syntax).
Add values of one array to other at specific index e.g 3:
var arr1 = [4, 5]
var arr2 = [1, 2, 3, ...arr1, 6]
console.log(arr2); // [1, 2, 3, 4, 5, 6]
When calling a constructor with new:
var dateFields = [1970, 0, 1]; // 1 Jan 1970
var d = new Date(...dateFields);
console.log(d);
Spread in object literals:
var obj1 = { foo: 'bar', x: 42 };
var obj2 = { foo: 'baz', y: 13 };
var clonedObj = { ...obj1 };
console.log(clonedObj); // {foo: "bar", x: 42}
var mergedObj = { ...obj1, ...obj2 };
console.log(mergedObj); // {foo: "baz", x: 42, y: 13}
Note that the foo property of obj1 has been overwritten by the obj2 foo property.
As a rest parameter syntax which allows us to represent an indefinite number of arguments as an array:
function sum(...theArgs) {
return theArgs.reduce((previous, current) => {
return previous + current;
});
}
console.log(sum(1, 2, 3)); //6
console.log(sum(1, 2, 3, 4)); //10
Note: The spread syntax (other than in the case of spread properties) can be applied only to iterable objects:
So the following will throw an error:
var obj = {'key1': 'value1'};
var array = [...obj]; // TypeError: obj is not iterable
Reference1
Reference2

The ...(spread operator) is used in React to:
provide a neat way to pass props from parent to child components. E.g., given these props in a parent component,
this.props = {
username: "danM",
email: "dan#mail.com"
}
they could be passed in the following manner to the child,
<ChildComponent {...this.props} />
which is similar to this
<ChildComponent username={this.props.username} email={this.props.email} />
but way cleaner.

The three dots (...) are called the spread operator, and this is conceptually similar to the ES6 array spread operator, JSX
taking advantage of these supported and developing standards in order to provide a cleaner syntax in JSX
Spread properties in object initializers copies own enumerable
properties from a provided object onto the newly created object.
let n = { x, y, ...z };
n; // { x: 1, y: 2, a: 3, b: 4 }
References:
Spread Properties
JSX In Depth

It is common practice to pass props around in a React application. In doing this we able to apply state changes to the child component regardless of whether it is Pure or Impure (stateless or stateful). There are times when the best approach, when passing in props, is to pass in singular properties or an entire object of properties. With the support for arrays in ES6 we were given the "..." notation and with this we are now able to achieve passing an entire object to a child.
The typical process of passing props to a child is noted with this syntax:
var component = <Component foo={x} bar={y} />;
This is fine to use when the number of props is minimal but becomes unmanageable when the prop numbers get too much higher. A problem with this method occurs when you do not know the properties needed within a child component and the typical JavaScript method is to simple set those properties and bind to the object later. This causes issues with propType checking and cryptic stack trace errors that are not helpful and cause delays in debugging. The following is an example of this practice, and what not to do:
var component = <Component />;
component.props.foo = x; // bad
component.props.bar = y;
This same result can be achieved but with more appropriate success by doing this:
var props = {};
props.foo = x;
props.bar = y;
var component = Component(props); // Where did my JSX go?
But does not use JSX spread or JSX so to loop this back into the equation we can now do something like this:
var props = {};
props.foo = x;
props.bar = y;
var component = <Component {...props} />;
The properties included in "...props" are foo: x, bar: y. This can be combined with other attributes to override the properties of "...props" using this syntax:
var props = { foo: 'default' };
var component = <Component {...props} foo={'override'} />;
console.log(component.props.foo); // 'override'
In addition we can copy other property objects onto each other or combine them in this manner:
var oldObj = { foo: 'hello', bar: 'world' };
var newObj = { ...oldObj, foo: 'hi' };
console.log(newObj.foo); // 'hi';
console.log(newObj.bar); // 'world';
Or merge two different objects like this (this is not yet available in all react versions):
var ab = { ...a, ...b }; // merge(a, b)
Another way of explaining this, according to Facebook's react/docs site is:
If you already have "props" as an object, and you want to pass it in JSX, you can use "..." as a SPREAD operator to pass the whole props object. The following two examples are equivalent:
function App1() {
return <Greeting firstName="Ben" lastName="Hector" />;
}
function App2() {
const props = {firstName: 'Ben', lastName: 'Hector'};
return <Greeting {...props} />;
}
Spread attributes can be useful when you are building generic containers. However, they can also make your code messy by making it easy to pass a lot of irrelevant props to components that don't care about them. This syntax should be used sparingly.

It is called spreads syntax in JavaScript.
It use for destructuring an array or object in JavaScript.
Example:
const objA = { a: 1, b: 2, c: 3 }
const objB = { ...objA, d: 1 }
/* Result of objB will be { a: 1, b: 2, c: 3, d: 1 } */
console.log(objB)
const objC = { ....objA, a: 3 }
/* result of objC will be { a: 3, b: 2, c: 3, d: 1 } */
console.log(objC)
You can do it same result with Object.assign() function in JavaScript.
Reference: Spread syntax

The spread operator (triple operator) introduced in ECMAScript 6 (ES6). ECMAScript (ES6) is a wrapper of JavaScript.
The spread operator enumerable properties in props.
this.props =
{
firstName: 'Dan',
lastName: 'Abramov',
city: 'New York',
country: 'USA'
}
<Modal {...this.props} title='Modal heading' animation={false}>
{...this.props} = { firstName: 'Dan',
lastName: 'Abramov',
city: 'New York',
country: 'USA' }
But the main feature spread operator is used for a reference type.
For example,
let person= {
name: 'Alex',
age: 35
}
person1 = person;
person1.name = "Raheel";
console.log( person.name); // Output: Raheel
This is called a reference type. One object affects other objects, because they are shareable in memory. If you are getting a value independently means spread memory and both use the spread operator.
let person= {
name: 'Alex',
age: 35
}
person2 = {...person};
person2.name = "Shahzad";
console.log(person.name); // Output: Alex

... 3 dots represent the spread operator in JS.
Without a spread operator.
let a = ['one','one','two','two'];
let unq = [new Set(a)];
console.log(a);
console.log(unq);
Output:
(4) ['one', 'one', 'two', 'two']
[Set(2)]
With spread operator.
let a = ['one','one','two','two'];
let unq = [...new Set(a)];
console.log(a);
console.log(unq);
Output:
(4) ['one', 'one', 'two', 'two']
(2) ['one', 'two']

Spread operator! As most ppl have already answered the question elegantly, I wanted to suggest a quick list of ways to use the spread operator:
The ... spread operator is useful for many different routine tasks in JavaScript, including the following:
Copying an array
Concatenating or combining arrays
Using Math functions
Using an array as arguments
Adding an item to a list
Adding to state in React
Combining objects
Converting NodeList to an array
Check out the article for more details. How to use the Spread Operator. I recommend getting used to it. There are so many cool ways you can use spread operators.

Those 3 dots ... is not React terms. Those are JavaScript ES6 spread operators. Which helps to create a new array without disturbing the original one to perform a deep copy. This can be used for objects also.
const arr1 = [1, 2, 3, 4, 5]
const arr2 = arr1 // [1, 2, 3, 4, 5]
/*
This is an example of a shallow copy.
Where the value of arr1 is copied to arr2
But now if you apply any method to
arr2 will affect the arr1 also.
*/
/*
This is an example of a deep copy.
Here the values of arr1 get copied but they
both are disconnected. Meaning if you
apply any method to arr3 it will not
affect the arr1.
*/
const arr3 = [...arr1] // [1, 2, 3, 4, 5]

<Modal {...{ title: "modal heading", animation: false, ...props} />
Much cleaner.

Those are called spreads. Just as the name implies, it means it's putting whatever the value of it in those array or objects.
Such as:
let a = [1, 2, 3];
let b = [...a, 4, 5, 6];
console.log(b);
> [1, 2, 3, 4, 5, 6]

The spread syntax allows an data structures like array and object to destructure
them for either to extract a value from them or to add a value to them.
e.g
const obj={name:"ram",age:10} const {name}=obj
from above example we can say that we destructured the obj and extracted name from that object.
similarly,
const newObj={...obj,address:"Nepal"}
from this example we added a new property to that object.
This is similar in case of array too.

The Spread operator lets you expand an iterable like an object, string, or array into its elements while the Rest operator does the inverse by reducing a set of elements into one array.

Sum of two multi dimensional array

I have a array in JavaScript like this.
var arr=
[
['A'],[1,2,3,4],
['A'],[4,3,2,1],
['B'],[10,12,3,1],
['B'],[1,2,3,4],
.
.
.
.
['AZ'],[1,2,3,4]
]
and I want the output to summarize the array like -
var output=
[
['A'],[5,5,5,5],
['B'],[11,14,6,5],
['AZ'],[1,2,3,4]
]
Thanks.

Script
You can use the following script to achieve what you want to do.
const arr = [
["A"],
[1, 2, 3, 4],
["A"],
[4, 3, 2, 1],
["B"],
[10, 12, 3, 1],
["B"],
[1, 2, 3, 4],
["AZ"],
[1, 2, 3, 4],
];
/**
* Generator to return key-value pairs with array[i] being the key and array[i+1] being the value
* #param {Array<any>} array
*/
function* keyValue(array) {
// make sure we can build pairs (other ways of handling this are also possible)
if (array.length % 2 !== 0)
throw new RangeError(
"Array length must be dividable by 2 without remainder!"
);
for (let i = 0; i < array.length; i += 2) {
yield [array[i], array[i + 1]];
}
}
// here the created key-value pairs
console.log("Key-value pairs created by keyValue() generator function:");
console.log([...keyValue(arr)]);
// loop over key value pairs and sum up all the individul arrays based on the letter assigned to them
const result = [...keyValue(arr)].reduce((all, [[key], array]) => {
// if we don't have values for this letter, assing copy of the array to that letter
if (!all[key]) all[key] = [...array];
// we have some values for that letter already, sum up each value
else all[key] = all[key].map((prev, idx) => prev + array[idx]);
return all;
}, {});
// this would be a "better" result to my mind as there is no point wrapping single string values in arrays
// When using objects the values can easily be accessed in O(1)
console.log(result);
// now transform JS object to array of arrays
console.log("Result:");
const transformed = Object.entries(result).flatMap(([key, value]) => [[key], value]);
console.log(transformed);
.as-console-wrapper { max-height: 100% !important; top: 0; }
Please note: This implementation assumes that the arrays for a given letter have the same length (as is the case in your example).
Explanation
First of all, I use a generator function keyValue() to always group two consecutive values in the array (a key and a value) together. One could also do this differently but once you understand how generators work that's an easy and elegant approach, I think. For this demo I just throw an error if the array is not dividable by 2 without remainder, but one could also handle this more gracefully.
Then, using reduce(), I iterate over the array created by using keyValue() and for each element in the array I check if I've encountered that value before. If I have not, I create a copy of the array (for immutablility) and assign it to the key i.e. a letter. If I have encountered a certain letter before I add up the values that I have previously saved assigned to that letter with the ones I am currently processing. After iteration all sums are calculated and I have a JavaScript object containing the results.
To my mind, this would be a good output because your output is a bit odd, since there is no point storing single letters in an array or even arrays of arrays. Using a JavaScript object is much more convenient and faster for lookups.
Nevertheless, you can easily deduct your result from the created object using flatMap().

what is ...{} (three dots followed by curly brackets) in React [duplicate]

What does the ... do in this React (using JSX) code and what is it called?
<Modal {...this.props} title='Modal heading' animation={false}>

That's property spread notation. It was added in ES2018 (spread for arrays/iterables was earlier, ES2015), but it's been supported in React projects for a long time via transpilation (as "JSX spread attributes" even though you could do it elsewhere, too, not just attributes).
{...this.props} spreads out the "own" enumerable properties in props as discrete properties on the Modal element you're creating. For instance, if this.props contained a: 1 and b: 2, then
<Modal {...this.props} title='Modal heading' animation={false}>
would be the same as
<Modal a={this.props.a} b={this.props.b} title='Modal heading' animation={false}>
But it's dynamic, so whatever "own" properties are in props are included.
Since children is an "own" property in props, spread will include it. So if the component where this appears had child elements, they'll be passed on to Modal. Putting child elements between the opening tag and closing tags is just syntactic sugar — the good kind — for putting a children property in the opening tag. Example:
class Example extends React.Component {
render() {
const { className, children } = this.props;
return (
<div className={className}>
{children}
</div>
);
}
}
ReactDOM.render(
[
<Example className="first">
<span>Child in first</span>
</Example>,
<Example className="second" children={<span>Child in second</span>} />
],
document.getElementById("root")
);
.first {
color: green;
}
.second {
color: blue;
}
<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>
Spread notation is handy not only for that use case, but for creating a new object with most (or all) of the properties of an existing object — which comes up a lot when you're updating state, since you can't modify state directly:
this.setState(prevState => {
return {foo: {...prevState.foo, a: "updated"}};
});
That replaces this.state.foo with a new object with all the same properties as foo except the a property, which becomes "updated":
const obj = {
foo: {
a: 1,
b: 2,
c: 3
}
};
console.log("original", obj.foo);
// Creates a NEW object and assigns it to `obj.foo`
obj.foo = {...obj.foo, a: "updated"};
console.log("updated", obj.foo);
.as-console-wrapper {
max-height: 100% !important;
}

... are called spread attributes which, as the name represents, it allows an expression to be expanded.
var parts = ['two', 'three'];
var numbers = ['one', ...parts, 'four', 'five']; // ["one", "two", "three", "four", "five"]
And in this case (I'm going to simplify it).
// Just assume we have an object like this:
var person= {
name: 'Alex',
age: 35
}
This:
<Modal {...person} title='Modal heading' animation={false} />
is equal to
<Modal name={person.name} age={person.age} title='Modal heading' animation={false} />
So in short, it's a neat short-cut, we can say.

The three dots represent the spread operator in ES6. It allows us to do quite a few things in JavaScript:
Concatenate arrays
var shooterGames = ['Call of Duty', 'Far Cry', 'Resident Evil'];
var racingGames = ['Need For Speed', 'Gran Turismo', 'Burnout'];
var games = [...shooterGames, ...racingGames];
console.log(games) // ['Call of Duty', 'Far Cry', 'Resident Evil', 'Need For Speed', 'Gran Turismo', 'Burnout']
Destructuring an array
var shooterGames = ['Call of Duty', 'Far Cry', 'Resident Evil'];
var [first, ...remaining] = shooterGames;
console.log(first); //Call of Duty
console.log(remaining); //['Far Cry', 'Resident Evil']
Combining two objects
var myCrush = {
firstname: 'Selena',
middlename: 'Marie'
};
var lastname = 'my last name';
var myWife = {
...myCrush,
lastname
}
console.log(myWife); // {firstname: 'Selena',
// middlename: 'Marie',
// lastname: 'my last name'}
There's another use for the three dots which is known as Rest Parameters and it makes it possible to take all of the arguments to a function in as one array.
Function arguments as array
function fun1(...params) {
}

... (three dots in JavaScript) is called the Spread Syntax or Spread Operator. This allows an iterable such as an array expression or string to be expanded or an object expression to be expanded wherever placed. This is not specific to React. It is a JavaScript operator.
All these answers here are helpful, but I want to list down the mostly used practical Use Cases of the Spread Syntax (Spread Operator).
1. Combine Arrays (Concatenate Arrays)
There are a variety of ways to combine arrays, but the spread operator allows you to place this at any place in an array. If you'd like to combine two arrays and place elements at any point within the array, you can do as follows:
var arr1 = ['two', 'three'];
var arr2 = ['one', ...arr1, 'four', 'five'];
// arr2 = ["one", "two", "three", "four", "five"]
2. Copying Arrays
When we wanted a copy of an array, we used to have the Array.prototype.slice() method. But, you can do the same with the spread operator.
var arr = [1,2,3];
var arr2 = [...arr];
// arr2 = [1,2,3]
3. Calling Functions without Apply
In ES5, to pass an array of two numbers to the doStuff() function, you often use the Function.prototype.apply() method as follows:
function doStuff (x, y, z) {}
var args = [0, 1, 2];
// Call the function, passing args
doStuff.apply(null, args);
However, by using the spread operator, you can pass an array into the function.
doStuff(...args);
4. Destructuring Arrays
You can use destructuring and the rest operator together to extract the information into variables as you'd like them:
let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
console.log(x); // 1
console.log(y); // 2
console.log(z); // { a: 3, b: 4 }
5. Function Arguments as Rest Parameters
ES6 also has three dots (...) which indicates a rest parameter that collects all remaining arguments of a function into an array.
function f(a, b, ...args) {
console.log(args);
}
f(1, 2, 3, 4, 5); // [3, 4, 5]
6. Using Math Functions
Any function where spread is used as the argument can be used by functions that can accept any number of arguments.
let numbers = [9, 4, 7, 1];
Math.min(...numbers); // 1
7. Combining Two Objects
You can use the spread operator to combine two objects. This is an easy and cleaner way to do it.
var carType = {
model: 'Toyota',
yom: '1995'
};
var carFuel = 'Petrol';
var carData = {
...carType,
carFuel
}
console.log(carData);
// {
// model: 'Toyota',
// yom: '1995',
// carFuel = 'Petrol'
// }
8. Separate a String into Separate Characters
You can use the spread operator to spread a string into separate characters.
let chars = ['A', ...'BC', 'D'];
console.log(chars); // ["A", "B", "C", "D"]
You can think of more ways to use the Spread Operator. What I have listed here are the popular use cases of it.

The three dots in JavaScript are the spread / rest operator.
Spread operator
The spread syntax allows an expression to be expanded in places where multiple arguments are expected.
myFunction(...iterableObj);
[...iterableObj, 4, 5, 6]
[...Array(10)]
Rest parameters
The rest parameter syntax is used for functions with a variable number of arguments.
function(a, b, ...theArgs) {
// ...
}
The spread / rest operator for arrays was introduced in ES6. There's a State 2 proposal for object spread / rest properties.
TypeScript also supports the spread syntax and can transpile that into older versions of ECMAScript with minor issues.

This is a feature of ES6, which is used in React as well. Look at the below example:
function Sum(x, y, z) {
return x + y + z;
}
console.log(Sum(1, 2, 3)); // 6
This way is fine if we have a maximum of three parameters. But, what if we need to add, for example, 110 parameters. Should we define them all and add them one by one?
Of course there is an easier way to do, which is called spread.
Instead of passing all those parameters you write:
function (...numbers){}
We have no idea how many parameters we have, but we know there are heaps of those.
Based on ES6, we can rewrite the above function as below and use the spread and mapping between them to make it as easy as a piece of cake:
let Sum = (...numbers) => {
return numbers.reduce((prev, current) => prev + current);
}
console.log(Sum(1, 2, 3, 4, 5, 6, 7, 8, 9)); // 45

Kudos to Brandon Morelli. He explained perfectly here, but links may die so I am just pasting the content below:
The spread syntax is simply three dots: ...
It allows an iterable to expand in places where 0+ arguments are expected.
Definitions are tough without context. Let's explore some different use cases to help understand what this means.
Example 1 — Inserting Arrays
Take a look at the code below. In this code, we don’t use the spread syntax:
var mid = [3, 4];
var arr = [1, 2, mid, 5, 6];
console.log(arr);
Above, we’ve created an array named mid. We then create a second array which contains our mid array. Finally, we log out the result. What do you expect arr to print? Click run above to see what happens. Here is the output:
[1, 2, [3, 4], 5, 6]
Is that the result you expected?
By inserting the mid array into the arr array, we’ve ended up with an array within an array. That’s fine if that was the goal. But what if you want only a single array with the values of 1 through 6? To accomplish this, we can use the spread syntax! Remember, the spread syntax allows the elements of our array to expand.
Let’s look at the code below. Everything is the same — except we’re now using the spread syntax to insert the mid array into the arr array:
var mid = [3, 4];
var arr = [1, 2, ...mid, 5, 6];
console.log(arr);
And when you hit the run button, here’s the result:
[1, 2, 3, 4, 5, 6]
Awesome!
Remember the spread syntax definition you just read above? Here’s where it comes into play. As you can see, when we create the arr array and use the spread operator on the mid array, instead of just being inserted, the mid array expands. This expansion means that each and every element in the mid array is inserted into the arr array. Instead of nested arrays, the result is a single array of numbers ranging from 1 to 6.
Example 2 — Math
JavaScript has a built-in math object that allows us to do some fun math calculations. In this example we’ll be looking at Math.max(). If you’re unfamiliar, Math.max() returns the largest of zero or more numbers. Here are a few examples:
Math.max();
// -Infinity
Math.max(1, 2, 3);
// 3
Math.max(100, 3, 4);
// 100
As you can see, if you want to find the maximum value of multiple numbers, Math.max() requires multiple parameters. You unfortunately can’t simply use a single array as input. Before the spread syntax, the easiest way to use Math.max() on an array is to use .apply().
var arr = [2, 4, 8, 6, 0];
function max(arr) {
return Math.max.apply(null, arr);
}
console.log(max(arr));
It works, it’s just really annoying.
Now take a look at how we do the same exact thing with the spread syntax:
var arr = [2, 4, 8, 6, 0];
var max = Math.max(...arr);
console.log(max);
Instead of having to create a function and utilize the apply method to return the result of Math.max() , we only need two lines of code! The spread syntax expands our array elements and inputs each element in our array individually into the Math.max() method!
Example 3 — Copy an Array
In JavaScript, you can’t just copy an array by setting a new variable equal to already existing array. Consider the following code example:
var arr = ['a', 'b', 'c'];
var arr2 = arr;
console.log(arr2);
When you press run, you’ll get the following output:
['a', 'b', 'c']
Now, at first glance, it looks like it worked — it looks like we’ve copied the values of arr into arr2. But that’s not what has happened. You see, when working with objects in JavaScript (arrays are a type of object) we assign by reference, not by value. This means that arr2 has been assigned to the same reference as arr. In other words, anything we do to arr2 will also affect the original arr array (and vice versa). Take a look below:
var arr = ['a', 'b', 'c'];
var arr2 = arr;
arr2.push('d');
console.log(arr);
Above, we’ve pushed a new element d into arr2. Yet, when we log out the value of arr, you’ll see that the d value was also added to that array:
['a', 'b', 'c', 'd']
No need to fear though! We can use the spread operator!
Consider the code below. It’s almost the same as above. Instead though, we’ve used the spread operator within a pair of square brackets:
var arr = ['a', 'b', 'c'];
var arr2 = [...arr];
console.log(arr2);
Hit run, and you’ll see the expected output:
['a', 'b', 'c']
Above, the array values in arr expanded to become individual elements which were then assigned to arr2. We can now change the arr2 array as much as we’d like with no consequences on the original arr array:
var arr = ['a', 'b', 'c'];
var arr2 = [...arr];
arr2.push('d');
console.log(arr);
Again, the reason this works is because the value of arr is expanded to fill the brackets of our arr2 array definition. Thus, we are setting arr2 to equal the individual values of arr instead of the reference to arr like we did in the first example.
Bonus Example — String to Array
As a fun final example, you can use the spread syntax to convert a string into an array. Simply use the spread syntax within a pair of square brackets:
var str = "hello";
var chars = [...str];
console.log(chars);

For someone who wants to understand this simple and fast:
First of all, this is not a syntax only to React. This is syntax from ES6 called spread syntax which iterate (merge, add, etc.) the array and object. Read more about it here.
So to answer the question:
Let's imagine you have this tag:
<UserTag name="Supun" age="66" gender="male" />
And you do this:
const user = {
"name": "Joe",
"age": "50"
"test": "test-val"
};
<UserTag name="Supun" gender="male" {...user} age="66" />
Then the tag will be equal to this:
<UserTag name="Joe" gender="male" test="test-val" age="66" />
So when you used the spread syntax in a React tag, it took the tag's attribute as object attributes which merge (replace if it exists) with the given object user. Also, you might have noticed one thing that it only replaces before attribute, not after attributes. So in this example, age remains as it is.

It's just defining props in a different way in JSX for you!
It's using ... array and object operator in ES6 (object one not fully supported yet), so basically if you already define your props, you can pass it to your element this way.
So in your case, the code should be something like this:
function yourA() {
const props = {name='Alireza', age='35'};
<Modal {...props} title='Modal heading' animation={false} />
}
so the props you defined, now separated and can be reused if necessary.
It's equal to:
function yourA() {
<Modal name='Alireza' age='35' title='Modal heading' animation={false} />
}
These are the quotes from React team about spread operator in JSX:
JSX Spread Attributes
If you know all the properties that you want to place on a component
ahead of time, it is easy to use JSX:
var component = <Component foo={x} bar={y} />;
Mutating Props is Bad If you don't know which properties you want to set, you might be tempted to add them onto the object later:
var component = <Component />;
component.props.foo = x; // bad
component.props.bar = y; // also bad
This is an anti-pattern because it means that we can't help you check
the right propTypes until way later. This means that your propTypes
errors end up with a cryptic stack trace.
The props should be considered immutable. Mutating the props object
somewhere else could cause unexpected consequences so ideally it would
be a frozen object at this point.
Spread Attributes Now you can use a new feature of JSX called spread attributes:
var props = {};
props.foo = x;
props.bar = y;
var component = <Component {...props} />;
The properties of the object that you pass in are copied onto the
component's props.
You can use this multiple times or combine it with other attributes.
The specification order is important. Later attributes override
previous ones.
var props = { foo: 'default' };
var component = <Component {...props} foo={'override'} />;
console.log(component.props.foo); // 'override'
What's with the weird ... notation? The ... operator (or spread operator) is already supported for arrays in ES6. There is also
an ECMAScript proposal for Object Rest and Spread Properties. We're
taking advantage of these supported and developing standards in order
to provide a cleaner syntax in JSX.

For those who come from the Python world, JSX Spread Attributes are equivalent to
Unpacking Argument Lists (the Python **-operator).
I'm aware this is a JSX question, but working with analogies sometimes helps to get it faster.

Three dots ... represent spread operators or rest parameters.
It allows an array expression or string or anything which can be iterating to be expanded in places where zero or more arguments for function calls or elements for array are expected.
Merge two arrays
var arr1 = [1,2,3];
var arr2 = [4,5,6];
arr1 = [...arr1, ...arr2];
console.log(arr1); //[1, 2, 3, 4, 5, 6]
Copying array:
var arr = [1, 2, 3];
var arr2 = [...arr];
console.log(arr); //[1, 2, 3]
Note: Spread syntax effectively goes one level deep while copying an
array. Therefore, it may be unsuitable for copying multidimensional
arrays as the following example shows (it's the same with
Object.assign() and spread syntax).
Add values of one array to other at specific index e.g 3:
var arr1 = [4, 5]
var arr2 = [1, 2, 3, ...arr1, 6]
console.log(arr2); // [1, 2, 3, 4, 5, 6]
When calling a constructor with new:
var dateFields = [1970, 0, 1]; // 1 Jan 1970
var d = new Date(...dateFields);
console.log(d);
Spread in object literals:
var obj1 = { foo: 'bar', x: 42 };
var obj2 = { foo: 'baz', y: 13 };
var clonedObj = { ...obj1 };
console.log(clonedObj); // {foo: "bar", x: 42}
var mergedObj = { ...obj1, ...obj2 };
console.log(mergedObj); // {foo: "baz", x: 42, y: 13}
Note that the foo property of obj1 has been overwritten by the obj2 foo property.
As a rest parameter syntax which allows us to represent an indefinite number of arguments as an array:
function sum(...theArgs) {
return theArgs.reduce((previous, current) => {
return previous + current;
});
}
console.log(sum(1, 2, 3)); //6
console.log(sum(1, 2, 3, 4)); //10
Note: The spread syntax (other than in the case of spread properties) can be applied only to iterable objects:
So the following will throw an error:
var obj = {'key1': 'value1'};
var array = [...obj]; // TypeError: obj is not iterable
Reference1
Reference2

The ...(spread operator) is used in React to:
provide a neat way to pass props from parent to child components. E.g., given these props in a parent component,
this.props = {
username: "danM",
email: "dan#mail.com"
}
they could be passed in the following manner to the child,
<ChildComponent {...this.props} />
which is similar to this
<ChildComponent username={this.props.username} email={this.props.email} />
but way cleaner.

The three dots (...) are called the spread operator, and this is conceptually similar to the ES6 array spread operator, JSX
taking advantage of these supported and developing standards in order to provide a cleaner syntax in JSX
Spread properties in object initializers copies own enumerable
properties from a provided object onto the newly created object.
let n = { x, y, ...z };
n; // { x: 1, y: 2, a: 3, b: 4 }
References:
Spread Properties
JSX In Depth

It is common practice to pass props around in a React application. In doing this we able to apply state changes to the child component regardless of whether it is Pure or Impure (stateless or stateful). There are times when the best approach, when passing in props, is to pass in singular properties or an entire object of properties. With the support for arrays in ES6 we were given the "..." notation and with this we are now able to achieve passing an entire object to a child.
The typical process of passing props to a child is noted with this syntax:
var component = <Component foo={x} bar={y} />;
This is fine to use when the number of props is minimal but becomes unmanageable when the prop numbers get too much higher. A problem with this method occurs when you do not know the properties needed within a child component and the typical JavaScript method is to simple set those properties and bind to the object later. This causes issues with propType checking and cryptic stack trace errors that are not helpful and cause delays in debugging. The following is an example of this practice, and what not to do:
var component = <Component />;
component.props.foo = x; // bad
component.props.bar = y;
This same result can be achieved but with more appropriate success by doing this:
var props = {};
props.foo = x;
props.bar = y;
var component = Component(props); // Where did my JSX go?
But does not use JSX spread or JSX so to loop this back into the equation we can now do something like this:
var props = {};
props.foo = x;
props.bar = y;
var component = <Component {...props} />;
The properties included in "...props" are foo: x, bar: y. This can be combined with other attributes to override the properties of "...props" using this syntax:
var props = { foo: 'default' };
var component = <Component {...props} foo={'override'} />;
console.log(component.props.foo); // 'override'
In addition we can copy other property objects onto each other or combine them in this manner:
var oldObj = { foo: 'hello', bar: 'world' };
var newObj = { ...oldObj, foo: 'hi' };
console.log(newObj.foo); // 'hi';
console.log(newObj.bar); // 'world';
Or merge two different objects like this (this is not yet available in all react versions):
var ab = { ...a, ...b }; // merge(a, b)
Another way of explaining this, according to Facebook's react/docs site is:
If you already have "props" as an object, and you want to pass it in JSX, you can use "..." as a SPREAD operator to pass the whole props object. The following two examples are equivalent:
function App1() {
return <Greeting firstName="Ben" lastName="Hector" />;
}
function App2() {
const props = {firstName: 'Ben', lastName: 'Hector'};
return <Greeting {...props} />;
}
Spread attributes can be useful when you are building generic containers. However, they can also make your code messy by making it easy to pass a lot of irrelevant props to components that don't care about them. This syntax should be used sparingly.

It is called spreads syntax in JavaScript.
It use for destructuring an array or object in JavaScript.
Example:
const objA = { a: 1, b: 2, c: 3 }
const objB = { ...objA, d: 1 }
/* Result of objB will be { a: 1, b: 2, c: 3, d: 1 } */
console.log(objB)
const objC = { ....objA, a: 3 }
/* result of objC will be { a: 3, b: 2, c: 3, d: 1 } */
console.log(objC)
You can do it same result with Object.assign() function in JavaScript.
Reference: Spread syntax

The spread operator (triple operator) introduced in ECMAScript 6 (ES6). ECMAScript (ES6) is a wrapper of JavaScript.
The spread operator enumerable properties in props.
this.props =
{
firstName: 'Dan',
lastName: 'Abramov',
city: 'New York',
country: 'USA'
}
<Modal {...this.props} title='Modal heading' animation={false}>
{...this.props} = { firstName: 'Dan',
lastName: 'Abramov',
city: 'New York',
country: 'USA' }
But the main feature spread operator is used for a reference type.
For example,
let person= {
name: 'Alex',
age: 35
}
person1 = person;
person1.name = "Raheel";
console.log( person.name); // Output: Raheel
This is called a reference type. One object affects other objects, because they are shareable in memory. If you are getting a value independently means spread memory and both use the spread operator.
let person= {
name: 'Alex',
age: 35
}
person2 = {...person};
person2.name = "Shahzad";
console.log(person.name); // Output: Alex

... 3 dots represent the spread operator in JS.
Without a spread operator.
let a = ['one','one','two','two'];
let unq = [new Set(a)];
console.log(a);
console.log(unq);
Output:
(4) ['one', 'one', 'two', 'two']
[Set(2)]
With spread operator.
let a = ['one','one','two','two'];
let unq = [...new Set(a)];
console.log(a);
console.log(unq);
Output:
(4) ['one', 'one', 'two', 'two']
(2) ['one', 'two']

Spread operator! As most ppl have already answered the question elegantly, I wanted to suggest a quick list of ways to use the spread operator:
The ... spread operator is useful for many different routine tasks in JavaScript, including the following:
Copying an array
Concatenating or combining arrays
Using Math functions
Using an array as arguments
Adding an item to a list
Adding to state in React
Combining objects
Converting NodeList to an array
Check out the article for more details. How to use the Spread Operator. I recommend getting used to it. There are so many cool ways you can use spread operators.

Those 3 dots ... is not React terms. Those are JavaScript ES6 spread operators. Which helps to create a new array without disturbing the original one to perform a deep copy. This can be used for objects also.
const arr1 = [1, 2, 3, 4, 5]
const arr2 = arr1 // [1, 2, 3, 4, 5]
/*
This is an example of a shallow copy.
Where the value of arr1 is copied to arr2
But now if you apply any method to
arr2 will affect the arr1 also.
*/
/*
This is an example of a deep copy.
Here the values of arr1 get copied but they
both are disconnected. Meaning if you
apply any method to arr3 it will not
affect the arr1.
*/
const arr3 = [...arr1] // [1, 2, 3, 4, 5]

<Modal {...{ title: "modal heading", animation: false, ...props} />
Much cleaner.

Those are called spreads. Just as the name implies, it means it's putting whatever the value of it in those array or objects.
Such as:
let a = [1, 2, 3];
let b = [...a, 4, 5, 6];
console.log(b);
> [1, 2, 3, 4, 5, 6]

The spread syntax allows an data structures like array and object to destructure
them for either to extract a value from them or to add a value to them.
e.g
const obj={name:"ram",age:10} const {name}=obj
from above example we can say that we destructured the obj and extracted name from that object.
similarly,
const newObj={...obj,address:"Nepal"}
from this example we added a new property to that object.
This is similar in case of array too.

The Spread operator lets you expand an iterable like an object, string, or array into its elements while the Rest operator does the inverse by reducing a set of elements into one array.

make comma separated strings from multidimensional array

I have JavaScript multidimensional array like this:
a 1
b 2
c 3
I want it converted into comma separated value like this:
function reg() {
var result = [];
var comma_value;
for (var i = 0; i < arrc.length; i++) {
if (parseInt(arrc[i].value)) {
result.push([arrp[i].value, arrc[i].value]);
}
// result is an array
// My desired result:
// comma_value = "a,1;b,2;c,3"
}
$('#str').val(JSON.stringify(result));
console.table(result);
console.log(result.join(', '));
}

Given your comments I assume you are simply looking for Array.prototype.join(';'):
let array = [
['a', 1],
['b', 2],
['c', 3]
];
let comma_value = array.join(';');
console.log(comma_value); // 'a,1;b,2;c,3'
This works due to the implicit array element ['a', 1] to string 'a,1' conversion performed within the array.join(';') method call.
If you are curious how this works: When you call array.join(';'), the individual array elements are first converted to strings via the Array.prototype.toString() method:
['a', 1].toString() // returns 'a,1'
Subsequently, those strings are joined with the ';' separator in between.
However, I don't see how this and your comments relate to the given reg() function. That code features two one-dimensional arrays with {value: ...} objects as elements.

alert( [["a",1],["b",2]].map(e=>e.join()).join(";"));
Join the inner Arrays, then the outer...

Creating range in JavaScript - strange syntax

I've run into the following code in the es-discuss mailing list:
Array.apply(null, { length: 5 }).map(Number.call, Number);
This produces
[0, 1, 2, 3, 4]
Why is this the result of the code? What's happening here?

Understanding this "hack" requires understanding several things:
Why we don't just do Array(5).map(...)
How Function.prototype.apply handles arguments
How Array handles multiple arguments
How the Number function handles arguments
What Function.prototype.call does
They're rather advanced topics in javascript, so this will be more-than-rather long. We'll start from the top. Buckle up!
1. Why not just Array(5).map?
What's an array, really? A regular object, containing integer keys, which map to values. It has other special features, for instance the magical length variable, but at it's core, it's a regular key => value map, just like any other object. Let's play with arrays a little, shall we?
var arr = ['a', 'b', 'c'];
arr.hasOwnProperty(0); //true
arr[0]; //'a'
Object.keys(arr); //['0', '1', '2']
arr.length; //3, implies arr[3] === undefined
//we expand the array by 1 item
arr.length = 4;
arr[3]; //undefined
arr.hasOwnProperty(3); //false
Object.keys(arr); //['0', '1', '2']
We get to the inherent difference between the number of items in the array, arr.length, and the number of key=>value mappings the array has, which can be different than arr.length.
Expanding the array via arr.length does not create any new key=>value mappings, so it's not that the array has undefined values, it does not have these keys. And what happens when you try to access a non-existent property? You get undefined.
Now we can lift our heads a little, and see why functions like arr.map don't walk over these properties. If arr[3] was merely undefined, and the key existed, all these array functions would just go over it like any other value:
//just to remind you
arr; //['a', 'b', 'c', undefined];
arr.length; //4
arr[4] = 'e';
arr; //['a', 'b', 'c', undefined, 'e'];
arr.length; //5
Object.keys(arr); //['0', '1', '2', '4']
arr.map(function (item) { return item.toUpperCase() });
//["A", "B", "C", undefined, "E"]
I intentionally used a method call to further prove the point that the key itself was never there: Calling undefined.toUpperCase would have raised an error, but it didn't. To prove that:
arr[5] = undefined;
arr; //["a", "b", "c", undefined, "e", undefined]
arr.hasOwnProperty(5); //true
arr.map(function (item) { return item.toUpperCase() });
//TypeError: Cannot call method 'toUpperCase' of undefined
And now we get to my point: How Array(N) does things. Section 15.4.2.2 describes the process. There's a bunch of mumbo jumbo we don't care about, but if you manage to read between the lines (or you can just trust me on this one, but don't), it basically boils down to this:
function Array(len) {
var ret = [];
ret.length = len;
return ret;
}
(operates under the assumption (which is checked in the actual spec) that len is a valid uint32, and not just any number of value)
So now you can see why doing Array(5).map(...) wouldn't work - we don't define len items on the array, we don't create the key => value mappings, we simply alter the length property.
Now that we have that out of the way, let's look at the second magical thing:
2. How Function.prototype.apply works
What apply does is basically take an array, and unroll it as a function call's arguments. That means that the following are pretty much the same:
function foo (a, b, c) {
return a + b + c;
}
foo(0, 1, 2); //3
foo.apply(null, [0, 1, 2]); //3
Now, we can ease the process of seeing how apply works by simply logging the arguments special variable:
function log () {
console.log(arguments);
}
log.apply(null, ['mary', 'had', 'a', 'little', 'lamb']);
//["mary", "had", "a", "little", "lamb"]
//arguments is a pseudo-array itself, so we can use it as well
(function () {
log.apply(null, arguments);
})('mary', 'had', 'a', 'little', 'lamb');
//["mary", "had", "a", "little", "lamb"]
//a NodeList, like the one returned from DOM methods, is also a pseudo-array
log.apply(null, document.getElementsByTagName('script'));
//[script, script, script, script, script, script, script, script, script, script, script, script, script, script, script, script, script, script, script, script]
//carefully look at the following two
log.apply(null, Array(5));
//[undefined, undefined, undefined, undefined, undefined]
//note that the above are not undefined keys - but the value undefined itself!
log.apply(null, {length : 5});
//[undefined, undefined, undefined, undefined, undefined]
It's easy to prove my claim in the second-to-last example:
function ahaExclamationMark () {
console.log(arguments.length);
console.log(arguments.hasOwnProperty(0));
}
ahaExclamationMark.apply(null, Array(2)); //2, true
(yes, pun intended). The key => value mapping may not have existed in the array we passed over to apply, but it certainly exists in the arguments variable. It's the same reason the last example works: The keys do not exist on the object we pass, but they do exist in arguments.
Why is that? Let's look at Section 15.3.4.3, where Function.prototype.apply is defined. Mostly things we don't care about, but here's the interesting portion:
Let len be the result of calling the [[Get]] internal method of argArray with argument "length".
Which basically means: argArray.length. The spec then proceeds to do a simple for loop over length items, making a list of corresponding values (list is some internal voodoo, but it's basically an array). In terms of very, very loose code:
Function.prototype.apply = function (thisArg, argArray) {
var len = argArray.length,
argList = [];
for (var i = 0; i < len; i += 1) {
argList[i] = argArray[i];
}
//yeah...
superMagicalFunctionInvocation(this, thisArg, argList);
};
So all we need to mimic an argArray in this case is an object with a length property. And now we can see why the values are undefined, but the keys aren't, on arguments: We create the key=>value mappings.
Phew, so this might not have been shorter than the previous part. But there'll be cake when we finish, so be patient! However, after the following section (which'll be short, I promise) we can begin dissecting the expression. In case you forgot, the question was how does the following work:
Array.apply(null, { length: 5 }).map(Number.call, Number);
3. How Array handles multiple arguments
So! We saw what happens when you pass a length argument to Array, but in the expression, we pass several things as arguments (an array of 5 undefined, to be exact). Section 15.4.2.1 tells us what to do. The last paragraph is all that matters to us, and it's worded really oddly, but it kind of boils down to:
function Array () {
var ret = [];
ret.length = arguments.length;
for (var i = 0; i < arguments.length; i += 1) {
ret[i] = arguments[i];
}
return ret;
}
Array(0, 1, 2); //[0, 1, 2]
Array.apply(null, [0, 1, 2]); //[0, 1, 2]
Array.apply(null, Array(2)); //[undefined, undefined]
Array.apply(null, {length:2}); //[undefined, undefined]
Tada! We get an array of several undefined values, and we return an array of these undefined values.
The first part of the expression
Finally, we can decipher the following:
Array.apply(null, { length: 5 })
We saw that it returns an array containing 5 undefined values, with keys all in existence.
Now, to the second part of the expression:
[undefined, undefined, undefined, undefined, undefined].map(Number.call, Number)
This will be the easier, non-convoluted part, as it doesn't so much rely on obscure hacks.
4. How Number treats input
Doing Number(something) (section 15.7.1) converts something to a number, and that is all. How it does that is a bit convoluted, especially in the cases of strings, but the operation is defined in section 9.3 in case you're interested.
5. Games of Function.prototype.call
call is apply's brother, defined in section 15.3.4.4. Instead of taking an array of arguments, it just takes the arguments it received, and passes them forward.
Things get interesting when you chain more than one call together, crank the weird up to 11:
function log () {
console.log(this, arguments);
}
log.call.call(log, {a:4}, {a:5});
//{a:4}, [{a:5}]
//^---^ ^-----^
// this arguments
This is quite wtf worthy until you grasp what's going on. log.call is just a function, equivalent to any other function's call method, and as such, has a call method on itself as well:
log.call === log.call.call; //true
log.call === Function.call; //true
And what does call do? It accepts a thisArg and a bunch of arguments, and calls its parent function. We can define it via apply (again, very loose code, won't work):
Function.prototype.call = function (thisArg) {
var args = arguments.slice(1); //I wish that'd work
return this.apply(thisArg, args);
};
Let's track how this goes down:
log.call.call(log, {a:4}, {a:5});
this = log.call
thisArg = log
args = [{a:4}, {a:5}]
log.call.apply(log, [{a:4}, {a:5}])
log.call({a:4}, {a:5})
this = log
thisArg = {a:4}
args = [{a:5}]
log.apply({a:4}, [{a:5}])
The later part, or the .map of it all
It's not over yet. Let's see what happens when you supply a function to most array methods:
function log () {
console.log(this, arguments);
}
var arr = ['a', 'b', 'c'];
arr.forEach(log);
//window, ['a', 0, ['a', 'b', 'c']]
//window, ['b', 1, ['a', 'b', 'c']]
//window, ['c', 2, ['a', 'b', 'c']]
//^----^ ^-----------------------^
// this arguments
If we don't provide a this argument ourselves, it defaults to window. Take note of the order in which the arguments are provided to our callback, and let's weird it up all the way to 11 again:
arr.forEach(log.call, log);
//'a', [0, ['a', 'b', 'c']]
//'b', [1, ['a', 'b', 'c']]
//'b', [2, ['a', 'b', 'c']]
// ^ ^
Whoa whoa whoa...let's back up a bit. What's going on here? We can see in section 15.4.4.18, where forEach is defined, the following pretty much happens:
var callback = log.call,
thisArg = log;
for (var i = 0; i < arr.length; i += 1) {
callback.call(thisArg, arr[i], i, arr);
}
So, we get this:
log.call.call(log, arr[i], i, arr);
//After one `.call`, it cascades to:
log.call(arr[i], i, arr);
//Further cascading to:
log(i, arr);
Now we can see how .map(Number.call, Number) works:
Number.call.call(Number, arr[i], i, arr);
Number.call(arr[i], i, arr);
Number(i, arr);
Which returns the transformation of i, the current index, to a number.
In conclusion,
The expression
Array.apply(null, { length: 5 }).map(Number.call, Number);
Works in two parts:
var arr = Array.apply(null, { length: 5 }); //1
arr.map(Number.call, Number); //2
The first part creates an array of 5 undefined items. The second goes over that array and takes its indices, resulting in an array of element indices:
[0, 1, 2, 3, 4]

Disclaimer: This is a very formal description of the above code - this is how I know how to explain it. For a simpler answer - check Zirak's great answer above. This is a more in depth specification in your face and less "aha".
Several things are happening here. Let's break it up a bit.
var arr = Array.apply(null, { length: 5 }); // Create an array of 5 `undefined` values
arr.map(Number.call, Number); // Calculate and return a number based on the index passed
In the first line, the array constructor is called as a function with Function.prototype.apply.
The this value is null which does not matter for the Array constructor (this is the same this as in the context according to 15.3.4.3.2.a.
Then new Array is called being passed an object with a length property - that causes that object to be an array like for all it matters to .apply because of the following clause in .apply:
Let len be the result of calling the [[Get]] internal method of argArray with argument "length".
As such, .apply is passing arguments from 0 to .length , since calling [[Get]] on { length: 5 } with the values 0 to 4 yields undefined the array constructor is called with five arguments whose value is undefined (getting an undeclared property of an object).
The array constructor is called with 0, 2 or more arguments.
The length property of the newly constructed array is set to the number of arguments according to the specification and the values to the same values.
Thus var arr = Array.apply(null, { length: 5 }); creates a list of five undefined values.
Note: Notice the difference here between Array.apply(0,{length: 5}) and Array(5), the first creating five times the primitive value type undefined and the latter creating an empty array of length 5. Specifically, because of .map's behavior (8.b) and specifically [[HasProperty] .
So the code above in a compliant specification is the same as:
var arr = [undefined, undefined, undefined, undefined, undefined];
arr.map(Number.call, Number); // Calculate and return a number based on the index passed
Now off to the second part.
Array.prototype.map calls the callback function (in this case Number.call) on each element of the array and uses the specified this value (in this case setting the this value to `Number).
The second parameter of the callback in map (in this case Number.call) is the index, and the first is the this value.
This means that Number is called with this as undefined (the array value) and the index as the parameter. So it's basically the same as mapping each undefined to its array index (since calling Number performs type conversion, in this case from number to number not changing the index).
Thus, the code above takes the five undefined values and maps each to its index in the array.
Which is why we get the result to our code.

As you said, the first part:
var arr = Array.apply(null, { length: 5 });
creates an array of 5 undefined values.
The second part is calling the map function of the array which takes 2 arguments and returns a new array of the same size.
The first argument which map takes is actually a function to apply on each element in the array, it is expected to be a function which takes 3 arguments and returns a value.
For example:
function foo(a,b,c){
...
return ...
}
if we pass the function foo as the first argument it will be called for each element with
a as the value of the current iterated element
b as the index of the current iterated element
c as the whole original array
The second argument which map takes is being passed to the function which you pass as the first argument. But it would not be a, b, nor c in case of foo, it would be this.
Two examples:
function bar(a,b,c){
return this
}
var arr2 = [3,4,5]
var newArr2 = arr2.map(bar, 9);
//newArr2 is equal to [9,9,9]
function baz(a,b,c){
return b
}
var newArr3 = arr2.map(baz,9);
//newArr3 is equal to [0,1,2]
and another one just to make it clearer:
function qux(a,b,c){
return a
}
var newArr4 = arr2.map(qux,9);
//newArr4 is equal to [3,4,5]
So what about Number.call ?
Number.call is a function that takes 2 arguments, and tries to parse the second argument to a number (I'm not sure what it does with the first argument).
Since the second argument that map is passing is the index, the value that will be placed in the new array at that index is equal to the index. Just like the function baz in the example above. Number.call will try to parse the index - it will naturally return the same value.
The second argument you passed to the map function in your code doesn't actually have an effect on the result. Correct me if I'm wrong, please.

An array is simply an object comprising the 'length' field and some methods (e.g. push). So arr in var arr = { length: 5} is basically the same as an array where the fields 0..4 have the default value which is undefined (i.e. arr[0] === undefined yields true).
As for the second part, map, as the name implies, maps from one array to a new one. It does so by traversing through the original array and invoking the mapping-function on each item.
All that's left is to convince you that the result of mapping-function is the index. The trick is to use the method named 'call'(*) which invokes a function with the small exception that the first param is set to be the 'this' context, and the second becomes the first param (and so on). Coincidentally, when the mapping-function is invoked, the second param is the index.
Last but not least, the method which is invoked is the Number "Class", and as we know in JS, a "Class" is simply a function, and this one (Number) expects the first param to be the value.
(*) found in Function's prototype (and Number is a function).
MASHAL