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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.
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.
I have a nested array, consisting of a lot of objects (400 items). That objects have an ID property and an incomes property. That incomes property is an array of objects (40 items) that have value and date.Question is, how can I iterate through that main array to sum all my small array values to one summary?
[{id: 1-400, incomes:[{value: "String", date: "String"}]}]
And, for example, that summary will be the 3rd property of main array:
[{id: 1-400, incomes:[{value: "String", date: "String"}], summaryIncome: "number"}]
I can iterate for that 400 items with for in loop, but I don`t understand how can I iterate both for big 400 items and small 40 items arrays with one loop.
for(let i in data){
data[i].incomes[i].value
}
So i will be >40 and I`ll get undefined in incomes[i].value. Please help :)
var a = [
{id: 1, incomes: [{value:1}, {value: 2}]},
{id: 2, incomes: [{value:2}, {value: 3}]}];
var b = a.map(item => ({
...item,
summaryIncome: item.incomes.reduce((t, income) =>
t += income.value, 0)
}));
console.log(b);
This should print:
{id: 1, incomes: Array(2), summaryIncome: 3}
{id: 2, incomes: Array(2), summaryIncome: 5}
Sum Array of Objects
Iterate over each object and calculate the sum, assigning it to a new key:
let data = getData()
data.forEach(obj => {
obj.summaryIncome = obj.incomes.reduce((agg,{value}) => agg+=value, 0);
})
console.log(data);
function getData(){
return [
{id: 1, incomes: [{value:1, date:new Date()}, {value: 2, date:new Date()}]},
{id: 2, incomes: [{value:3, date:new Date()}, {value: 4, date:new Date()}]}
];
}
This assumes:
source data has a value
the source data is mutable (can store summaryIncome with it)
there is always an incomes and value key
it uses reduce to iterate over each value and collect an aggregate, agg, which will hold the item's sum
your browser supports ES6 or you have a transpiler for polyfills
Your Issues
400 items is not a lot of items. You should be able to easily iterate them; however, your number of items and the number of values are not a uniform array; it's not a 400x400, it's 400x40. You don't have the same number of ids as you do values for each id. So your i is fine for your item loop, but because you have fewer values than items, you will reach an end.
Even if they were the same number, it doesn't seem there is anything that binds the observation to the number of incomes, so it would be strongly discouraged to only use one loop and/or variable for tracking the index.
For an example of an implementation that uses two loops, refer to the previous section, which uses forEach and reduce to iterate both dimensions of your data. Likewise, below modifies your original code that to demonstrate using two for-loops:
for(let item of data){
let sum = 0;
for(let {value} of item.incomes){
sum += value;
}
item.summaryIncome = sum
}
MDN references JavaScript's Set collection abstraction. I've got an array of objects that I'd like to convert to a set so that I am able to remove (.delete()) various elements by name:
var array = [
{name: "malcom", dogType: "four-legged"},
{name: "peabody", dogType: "three-legged"},
{name: "pablo", dogType: "two-legged"}
];
How do I convert this array to a set? More specifically, is it possible to do this without iterating over the above array? The documentation is relatively lacking (sufficient for instantiated sets; not for conversions - if possible).
I may also be thinking of the conversion to a Map, for removal by key. What I am trying to accomplish is an iterable collection that can be accessed or modified via accessing the elements primarily via a key (as opposed to index).
Conversion from an array to the other being the ultimate goal.
Just pass the array to the Set constructor. The Set constructor accepts an iterable parameter. The Array object implements the iterable protocol, so its a valid parameter.
var arr = [55, 44, 65];
var set = new Set(arr);
console.log(set.size === arr.length);
console.log(set.has(65));
See here
If you start out with:
let array = [
{name: "malcom", dogType: "four-legged"},
{name: "peabody", dogType: "three-legged"},
{name: "pablo", dogType: "two-legged"}
];
And you want a set of, say, names, you would do:
let namesSet = new Set(array.map(item => item.name));
By definition "A Set is a collection of values, where each value may occur only once." So, if your array has repeated values then only one value among the repeated values will be added to your Set.
var arr = [1, 2, 3];
var set = new Set(arr);
console.log(set); // {1,2,3}
var arr = [1, 2, 1];
var set = new Set(arr);
console.log(set); // {1,2}
So, do not convert to set if you have repeated values in your array.
const categoryWithDuplicates =[
'breakfast', 'lunch',
'shakes', 'breakfast',
'lunch', 'shakes',
'breakfast', 'lunch',
'shakes'
]
const categoryWithoutDuplicates =[...new Set(categoryWithDuplicates)];
console.log(categoryWithoutDuplicates)
Output: [ 'breakfast', 'lunch', 'shakes' ]
What levi said about passing it into the constructor is correct, but you could also use an object.
I think what Veverke is trying to say is that you could easily use the delete keyword on an object to achieve the same effect.
I think you're confused by the terminology; properties are components of the object that you can use as named indices (if you want to think of it that way).
Try something like this:
var obj = {
"bob": "dole",
"mr.": "peabody",
"darkwing": "duck"
};
Then, you could just do this:
delete obj["bob"];
The structure of the object would then be this:
{
"mr.": "peabody",
"darkwing": "duck"
}
Which has the same effect.
var yourSetAsArray = Array.from(new Set([1, 2, 2, 3, 3, 4, 4]));
// returns [1, 2, 3, 4]
I would like to know as much as possible as to how this is working - especially as it relates to the usage of the ternary and the object argument containing two spreads.
rows = rows.map(row => (changed[row.ID] ? { ...row, ...changed[row.ID] } : row));
First - the objects being passed into the map are structured like this:
changed is shaped like this {"75864":{"ActType":"DEADLINE"}}
rows is formatted like this (for example):
[{
"ID": 75864,
"NextDate": "2018-03-02T00:00:00",
"NextTime": "1030am",
"MatterID": 14116,
"Descr": " Responses to pending discovery",
"StatusID": 19,
"Actor_s_": null,
"Accrued": 0,
"Go": "",
"AspNetUserID": null,
"DomainID": 2,
"UserID": 1,
"StatusType": "Pending",
"ActTypeID": 50,
"ActType": "DEADLINE",
"MatterName": "WYNBAS "
This is "merging" row and changed[row.ID] into a single object. Let's look at what happens when row is the one with the ID "75864":
// row: {"ID": 75864, "ActType": "DEADLINE", (more properties)}
// changed: {"75864": {"ActType": "OTHER ACTION"}}
// (Note - I changed `changed` so that the ActType specified is different from
// what's already in the row object, otherwise it's really difficult for me to
// demonstrate exactly what's happening here.)
// This is the body of the arrow function:
return changed[row.ID] ? { ...row, ...changed[row.ID] } : row
// Well, changed[row.ID] does exist:
// changed[row.ID]: {"ActType": "OTHER ACTION"}
// So we take this branch of the ternary:
return { ...row, ...changed[row.ID] }
// Basically, this expression works like an array spread, but for objects.
// As a refresher, here's what an array spread might look like:
//
// a = [1, 2, 3]
// b = ['cat', 'dog', 'armadillo']
// c = [...a, ...b]
// c: [1, 2, 3, 'cat', 'dog', 'armadillo']
//
// The array spread works by creating a completely new, empty array. Then
// it adds the items of each array that's spread into it; so first it adds
// all the items of a (1, 2, 3), then all the items of b (cat, dog, armadillo).
// Object spread works pretty much the same way. First we create a completely
// new object: {}.
// Then we add all the properties of row: {ID: 75864, ActType: "DEADLINE",
// "MatterID": 14116, (more properties)}.
// Then it adds the the properties of changed[row.ID]. This is the important
// part, because changed[row.ID] actually *overwrites* any properties that
// we've already added from "row". This makes the result look like this:
return {ID: 75864, ActType: "OTHER ACTION", MatterID: 14116, (more properties)}
// Note that the return value's ActType property is OTHER ACTION, not DEADLINE!
Note that object spread is essentially the same as using Object.assign with an empty object as the first argument. (Object.assign takes all the properties from the second, third, etc arguments and sets them on the first argument. That means it actually changes - mutates - its first argument; and here, we aren't mutating row, we're returning a totally new object based on row (and changed[row.ID]).) So writing your code with Object.assign would look like this:
return Object.assign({}, row, changed[row.ID])
IF the row.ID value matches a key which has "truthy" value in the changed object,
THEN you return a new object, with all values of the row (you spread the row), and you add to this new object the values of the matching changed object (you spread the truthy value of changed).
ELSE you simply return the given row.
Is it the answer your are looking for?