I'm trying to understand why Javascript prints an empty array when it should have at least one value inside it. Please, check this code:
detail = [];
detail.cat1=[];
detail.cat2=[];
detail.cat3=[];
detail.cat4=[];
var newEntry = {"cod":"01","dt":"2021-10-02 09:07:21.205-07:00"};
detail.cat2.push(newEntry);
console.log(detail);
console.log(detail.length);
console.log(detail.cat2);
The results are:
> Array []
> 0
> Array [Object { cod: "01", dt: "2021-10-02 09:07:21.205-07:00" }]
How could it print [] since I have one object inside? And how can the length be zero? I was using any Javascript online editor, and the result is the same.
Since an array is really an object, we can add methods/properties directly to an individual array.
Then if you want to see the keys in an object for instance, Object.keys won't see it, but Reflect.ownKeys() will.
The Reflect version of defineProperty will return success or fail, while Object's returns the object passed.
Remember Reflect.preventExtensions() will (like it says) stop you or others from extending them.
As far as question about length is there, ".length" gives length of values not properties so it still gives 0
Array is an indexed collection of values, means you can put element at particular index and get element from particular index.
In the below example, you can see that if there are already elements in an array then you can get its length using length property.
Remember: The length of an array is (highest integer index at which the element is present in array + 1)
const arr = [1, 2, 3, 4];
console.log(arr); // prints whole array
console.log(arr.length); // prints length of an array
console.log(arr[2]); // Element at index 2
If you set element at particular index let say arr[8] = "something" then its length will be 9
const arr = [];
arr[8] = "something";
console.log(arr.length) // 9
It is true that array's are objects, so you can set/get values to/from array. But if you set property that is not a numbered index then it will not increase its length but It will set the property to that array object as:
const arr = [];
arr["introduction"] = "I'm an array";
arr.property = "This is a property"
console.log(arr.length);
console.log(arr.introduction);
console.log(arr.property);
When displaying an array, the default Array.prototype.toString method is usually called. This actually calls Array.prototype.join with no arguments, so produces a comma separated string of the elements of the array.
The join method only returns properties that have a positive integer name (an array index, indicating an array element), other properties are ignored by join.
Similarly, only elements of the array affect the length property, ordinarly properties (i.e. those that don't have an array index for their name) don't affect length.
Was writing a script in JS to make some dummy data for testing my API and ran into an interesting quirk with the forEach loop in JS.
const dictionary = {};
const undefinedArray = Array(3); // [undefined, undefined, undefined]
undefinedArray.forEach((_, index) => {
console.log('Logging at index: ', index)
const someObject = { id: index };
if (!dictionary[someObject.id]) {
dictionary[someObject.id] = someObject
}
});
console.log(dictionary);
After checking the output of this snippet, you'll see that nothing inside the forEach loop is logged and the dictionary is still an empty object. I was talking with my coworker about this behaviour and he said he ran into this particular issue before and offered this as a solution.
const dictionary = {};
const undefinedArray = [...Array(3)]; // [undefined, undefined, undefined]
undefinedArray.forEach((_, index) => {
console.log('Logging at index: ', index)
const someObject = { id: index };
if (!dictionary[someObject.id]) {
dictionary[someObject.id] = someObject
}
});
console.log(dictionary);
By wrapping the Array constructor in square brackets and utilizing the spread operator, now the array is looped through and the object is built correctly. This fascinated me, so I went to the documentation for the Array object and found this:
arrayLength
If the only argument passed to the Array constructor is an integer between 0 and 2^32 - 1 (inclusive), this returns a new JavaScript array with its length property set to that number (Note: this implies an array of arrayLength empty slots, not slots with actual undefined values). If the argument is any other number, a RangeError exception is thrown.
So apparently it is not assigning each value undefined, but only setting its length property to whatever is passed in the constructor. This is not apparent when you log Array(n) to the console because it shows an array with n undefined values. I assume the toString method for the Array object is based on its length property and uses a normal for or for of loop to construct the string.
It does begin to make a little bit more sense, however, when you explicitly set an index of the newly defined array. In the snippet below, the same array is initialized, but the zero index is explicitly assigned undefined as a value. Since this is an "actual undefined value" in Mozilla's words, the forEach loop exectues at index zero.
const dictionary = {};
const undefinedArray = Array(3); // [undefined, undefined, undefined]
undefinedArray[0] = undefined
undefinedArray.forEach((_, index) => {
console.log('Logging at index: ', index)
const someObject = { id: index };
if (!dictionary[someObject.id]) {
dictionary[someObject.id] = someObject
}
});
console.log(dictionary);
Array.map() behaves the same way. So I guess my main question would be, are there other ways to execute forEach and map without filling the array or by using the quirky hack I mentioned earlier?
To recap: these are the two work arounds I've found for this particular use case:
[...Array(n)] OR Array(n).fill(). Both of these mutations to the array will allow a forEach loop to iterate over all values in the array.
So apparently it is not assigning each value undefined, but only setting its length property to whatever is passed in the constructor.
Correct. (Provided you pass only a single argument and it's a number. If you pass a non-number, or pass more than one argument, they're used as elements for the array. So Array("3") results in ["3"]; Array(3, 4) results in [3, 4].)
This is not apparent when you log Array(n) to the console because it shows an array with n undefined values.
It depends on what console you use. The devtools in Chromium browsers show (3) [empty x 3] for exactly that reason, to differentiate between empty array slots and ones containing the value undefined.
So I guess my main question would be, are there other ways to execute forEach and map without filling the array or by using the quirky hack I mentioned earlier?
If you want forEach and map to visit elements of the array, they have to actually exist. Those methods (and several others) are defined such that they don't call your callback for empty slots in sparse arrays. If by "quirky hack" you mean [...Array(3)], that's also filling the array (and is fully-specified behavior: [...x] uses the iterator x provides, and the array iterator is defined that it yields undefined for empty slots rather than skipping them as forEach, map, and similar do). Doing that (spreading the sparse array) is one way to create an array filled with undefined (not empty) elements. Array.fill is another. Here's a third: Array.from({length: 3})
const a = Array.from({length: 3});
a.forEach(value => {
console.log(`value = ${value}`);
});
Which you use is up to you. Array.from is very simple and direct. Similarly Array(3).fill(). I probably wouldn't use the spread version (just because I think it's fairly unclear to people who don't have a deep knowledge of how the array iterator works), but it's a matter of style.
How to initialize a string array (size<100 items) in javascript whose indexes are scattered over entire integer range, with data items.
If I do like this:
array1 = ["string1","string2","string3","string4"];
then I get array of length 4 with indices ranging 0 to 3
But in my case i want to keep my own indices, so that the array could be used like a high performance int-string hash table.
I'm preferably looking out for a single statement initialization.
The items of the array should be accessible like this: array1[23454]
Update From Comments
I'm restricted to initialize the array as a single statement since a dynamically prepared array initialization string is appended from server side like this: var array = <string from server here>
To create an array with a set number of indexes you can use
// Creates an array with 12 indexes
var myArray = new Array(12);
This isn't needed in javascript due to the way its array's work. There isn't an upper-bound for arrays. If you try to reference an item index in the array that doesn't exist, undefined is returned but no error is thrown
To create an array with perscribed indexes you can use something like array['index'] = value though this would force you to use multiple statements. Javascript doesn't have an array initalizer to allow for you to specify indexes and values all in a single statement though you can create a function to do as such
function indexArray(param) {
var a = [], i;
for (i=0; i<param.length; i+=1) {
a[param[i].index] = param[i].value;
}
return a;
}
var myArray = indexArray([
{ index: 123456, value : "bananas" },
{ index: 12, value : "grapes" },
{ index: 564, value : "monkeys" }
]);
var array1 = []
array1[23454] = 2
Just doing this should be fine. There's no set array size for javascript in the way there is for java.
If you really want to do this all in a single statement, you can make an object instead like this:
var object1 = {
"23454":2,
"123":1,
"50":3
};
and then retrieve the numbers like this:
object1["23454"] //2
I don't really recommend this though. The array method is a cleaner way of doing it even if it takes multiple lines since it doesn't require string conversion. I don't know enough about how these are implemented in browsers to comment on the performance impact.
Update
Since the 1 line requirement is based on something being passed to the server, I would recommend passing a JSON object to the server in the form:
"{"23454":2,"123":1,"50":3}"
then this code will parse it to an object:
var object1 = JSON.parse(jsonstringfromserver);
and if you like you can always convert that to an array by enumerating over the properties with a for in loop:
var array1 = []
for ( num in object1){
array1[num] = object1[num];
That is probably unnecessary though since object1[123] will already return 1. You only need this if you plan on doing array specific operations.
You don't have to pre-define the size of an array before you assign to it. For example:
var _array = [];
_array[0] = "foo";
_array[1000] = "bar"; // _array.length => 1001
_array[1] //undefined
No need to initialise the appropriate number of array elements before you assign to them.
Update
It already has been pointed out that you can use an object rather than an array. However, if you want to take advantage of array methods then this is still possible. Let me give you an example:
var obj = {
0: 15,
1: 10,
2: 5,
length: 3
};
If the object contains a length property then it can be treated as an array-like object. Although you can't call array methods directly from these objects you can use array methods.
Array.prototype.join.call( obj ); // 15,10,5
In fact using the ECMAScript 5 map function you can easily convert the above object to an array.
var _array = Array.prototype.map.call( obj, function( x ) { return x; } );
The map function does not exist in all browsers but you can use the following function if it doesn't.
Array.map = Array.map || function(a, f, thisArg) {
return Array.prototype.map.call(a, f, thisArg);
}
You can do what you want with an Object in this way:
var o = {23454: 'aaaa', 23473: 'bbb'};
You will lose the array methods/fields, e.g. length, but you will gain what you said you are looking for, and you will be able to add/remove members easily.
What's the real difference between declaring an array like this:
var myArray = new Array();
and
var myArray = [];
There is a difference, but there is no difference in that example.
Using the more verbose method: new Array() does have one extra option in the parameters: if you pass a number to the constructor, you will get an array of that length:
x = new Array(5);
alert(x.length); // 5
To illustrate the different ways to create an array:
var a = [], // these are the same
b = new Array(), // a and b are arrays with length 0
c = ['foo', 'bar'], // these are the same
d = new Array('foo', 'bar'), // c and d are arrays with 2 strings
// these are different:
e = [3] // e.length == 1, e[0] == 3
f = new Array(3), // f.length == 3, f[0] == undefined
;
Another difference is that when using new Array() you're able to set the size of the array, which affects the stack size. This can be useful if you're getting stack overflows (Performance of Array.push vs Array.unshift) which is what happens when the size of the array exceeds the size of the stack, and it has to be re-created. So there can actually, depending on the use case, be a performance increase when using new Array() because you can prevent the overflow from happening.
As pointed out in this answer, new Array(5) will not actually add five undefined items to the array. It simply adds space for five items. Be aware that using Array this way makes it difficult to rely on array.length for calculations.
The difference between creating an array with the implicit array and the array constructor is subtle but important.
When you create an array using
var a = [];
You're telling the interpreter to create a new runtime array. No extra processing necessary at all. Done.
If you use:
var a = new Array();
You're telling the interpreter, I want to call the constructor "Array" and generate an object. It then looks up through your execution context to find the constructor to call, and calls it, creating your array.
You may think "Well, this doesn't matter at all. They're the same!". Unfortunately you can't guarantee that.
Take the following example:
function Array() {
this.is = 'SPARTA';
}
var a = new Array();
var b = [];
alert(a.is); // => 'SPARTA'
alert(b.is); // => undefined
a.push('Woa'); // => TypeError: a.push is not a function
b.push('Woa'); // => 1 (OK)
In the above example, the first call will alert 'SPARTA' as you'd expect. The second will not. You will end up seeing undefined. You'll also note that b contains all of the native Array object functions such as push, where the other does not.
While you may expect this to happen, it just illustrates the fact that [] is not the same as new Array().
It's probably best to just use [] if you know you just want an array. I also do not suggest going around and redefining Array...
There is an important difference that no answer has mentioned yet.
From this:
new Array(2).length // 2
new Array(2)[0] === undefined // true
new Array(2)[1] === undefined // true
You might think the new Array(2) is equivalent to [undefined, undefined], but it's NOT!
Let's try with map():
[undefined, undefined].map(e => 1) // [1, 1]
new Array(2).map(e => 1) // "(2) [undefined × 2]" in Chrome
See? The semantics are totally different! So why is that?
According to ES6 Spec 22.1.1.2, the job of Array(len) is just creating a new array whose property length is set to the argument len and that's it, meaning there isn't any real element inside this newly created array.
Function map(), according to spec 22.1.3.15 would firstly check HasProperty then call the callback, but it turns out that:
new Array(2).hasOwnProperty(0) // false
[undefined, undefined].hasOwnProperty(0) // true
And that's why you can not expect any iterating functions working as usual on arrays created from new Array(len).
BTW, Safari and Firefox have a much better "printing" to this situation:
// Safari
new Array(2) // [](2)
new Array(2).map(e => 1) // [](2)
[undefined, undefined] // [undefined, undefined] (2)
// Firefox
new Array(2) // Array [ <2 empty slots> ]
new Array(2).map(e => 1) // Array [ <2 empty slots> ]
[undefined, undefined] // Array [ undefined, undefined ]
I have already submitted an issue to Chromium and ask them to fix this confusing printing:
https://bugs.chromium.org/p/chromium/issues/detail?id=732021
UPDATE: It's already fixed. Chrome now printed as:
new Array(2) // (2) [empty × 2]
Oddly enough, new Array(size) is almost 2x faster than [] in Chrome, and about the same in FF and IE (measured by creating and filling an array). It only matters if you know the approximate size of the array. If you add more items than the length you've given, the performance boost is lost.
More accurately: Array( is a fast constant time operation that allocates no memory, wheras [] is a linear time operation that sets type and value.
For more information, the following page describes why you never need to use new Array()
You never need to use new Object() in
JavaScript. Use the object literal {}
instead. Similarly, don’t use new Array(),
use the array literal []
instead. Arrays in JavaScript work
nothing like the arrays in Java, and
use of the Java-like syntax will
confuse you.
Do not use new Number, new String, or
new Boolean. These forms produce
unnecessary object wrappers. Just use
simple literals instead.
Also check out the comments - the new Array(length) form does not serve any useful purpose (at least in today's implementations of JavaScript).
In order to better understand [] and new Array():
> []
[]
> new Array()
[]
> [] == []
false
> [] === []
false
> new Array() == new Array()
false
> new Array() === new Array()
false
> typeof ([])
"object"
> typeof (new Array())
"object"
> [] === new Array()
false
> [] == new Array()
false
The above result is from Google Chrome console on Windows 7.
The first one is the default object constructor call. You can use it's parameters if you want.
var array = new Array(5); //initialize with default length 5
The second one gives you the ability to create not empty array:
var array = [1, 2, 3]; // this array will contain numbers 1, 2, 3.
I can explain in a more specific way starting with this example that's based on Fredrik's good one.
var test1 = [];
test1.push("value");
test1.push("value2");
var test2 = new Array();
test2.push("value");
test2.push("value2");
alert(test1);
alert(test2);
alert(test1 == test2);
alert(test1.value == test2.value);
I just added another value to the arrays, and made four alerts:
The first and second are to give us the value stored in each array, to be sure about the values. They will return the same!
Now try the third one, it returns false, that's because
JS treats test1 as a VARIABLE with a data type of array, and it treats test2 as an OBJECT with the functionality of an array, and
there are few slight differences here.
The first difference is when we call test1 it calls a variable without thinking, it just returns the values that are stored in this variable disregarding its data type!
But, when we call test2 it calls the Array() function and then it stores our "Pushed" values in its "Value" property, and the same happens when we alert test2, it returns the "Value" property of the array object.
So when we check if test1 equals test2 of course they will never return true, one is a function and the other is a variable (with a type of array), even if they have the same value!
To be sure about that, try the 4th alert, with the .value added to it; it will return true. In this case we tell JS "Disregarding the type of the container, whether was it function or variable, please compare the values that are stored in each container and tell us what you've seen!" that's exactly what happens.
I hope I said the idea behind that clearly, and sorry for my bad English.
There is no difference when you initialise array without any length. So var a = [] & var b = new Array() is same.
But if you initialise array with length like var b = new Array(1);, it will set array object's length to 1. So its equivalent to var b = []; b.length=1;.
This will be problematic whenever you do array_object.push, it add item after last element & increase length.
var b = new Array(1);
b.push("hello world");
console.log(b.length); // print 2
vs
var v = [];
a.push("hello world");
console.log(b.length); // print 1
There's more to this than meets the eye. Most other answers are correct BUT ALSO..
new Array(n)
Allows engine to reallocates space for n elements
Optimized for array creation
Created array is marked sparse which has the least performant array operations, that's because each index access has to check bounds, see if value exists and walk the prototype chain
If array is marked as sparse, there's no way back (at least in V8), it'll always be slower during its lifetime, even if you fill it up with content (packed array) 1ms or 2 hours later, doesn't matter
[1, 2, 3] || []
Created array is marked packed (unless you use delete or [1,,3] syntax)
Optimized for array operations (for .., forEach, map, etc)
Engine needs to reallocate space as the array grows
This probably isn't the case for older browser versions/browsers.
The first one is the default object constructor call.mostly used for dynamic values.
var array = new Array(length); //initialize with default length
the second array is used when creating static values
var array = [red, green, blue, yellow, white]; // this array will contain values.
The difference of using
var arr = new Array(size);
Or
arr = [];
arr.length = size;
As been discussed enough in this question.
I would like to add the speed issue - the current fastest way, on google chrome is the second one.
But pay attention, these things tend to change a lot with updates. Also the run time will differ between different browsers.
For example - the 2nd option that i mentioned, runs at 2 million [ops/second] on chrome, but if you'd try it on mozilla dev. you'd get a surprisingly higher rate of 23 million.
Anyway, I'd suggest you check it out, every once in a while, on different browsers (and machines), using site as such
As I know the diference u can find the slice(or the other funcitons of Array) like code1.and code2 show u Array and his instances:
code1:
[].slice; // find slice here
var arr = new Array();
arr.slice // find slice here
Array.prototype.slice // find slice here
code2:
[].__proto__ == Array.prototype; // true
var arr = new Array();
arr.__proto__ == Array.prototype; // true
conclusion:
as u can see [] and new Array() create a new instance of Array.And they all get the prototype functions from Array.prototype
They are just different instance of Array.so this explain why
[] != []
:)
There is no big difference, they basically do the same thing but doing them in different ways, but read on, look at this statement at W3C:
var cars = ["Saab", "Volvo","BMW"];
and
var cars = new Array("Saab", "Volvo", "BMW");
The two examples above do exactly the same. There is no need to use
new Array(). For simplicity, readability and execution speed, use the
first one (the array literal method).
But at the same time, creating new array using new Array syntax considered as a bad practice:
Avoid new Array()
There is no need to use the JavaScript's built-in array constructor
new Array().
Use [] instead.
These two different statements both create a new empty array named
points:
var points = new Array(); // Bad
var points = []; // Good
These two different statements both create a new array containing 6
numbers:
var points = new Array(40, 100, 1, 5, 25, 10); // Bad
var points = [40, 100, 1, 5, 25, 10]; // Good
The new keyword only complicates the code. It can also produce some
unexpected results:
var points = new Array(40, 100); // Creates an array with two elements (40 and 100)
What if I remove one of the elements?
var points = new Array(40); // Creates an array with 40 undefined elements !!!!!
So basically not considered as the best practice, also there is one minor difference there, you can pass length to new Array(length) like this, which also not a recommended way.
I've incurred in a weird behaviour using [].
We have Model "classes" with fields initialised to some value. E.g.:
require([
"dojo/_base/declare",
"dijit/_WidgetBase",
], function(declare, parser, ready, _WidgetBase){
declare("MyWidget", [_WidgetBase], {
field1: [],
field2: "",
function1: function(),
function2: function()
});
});
I found that when the fields are initialised with [] then it would be shared by all Model objects. Making changes to one affects all others.
This doesn't happen initialising them with new Array(). Same for the initialisation of Objects ({} vs new Object())
TBH I am not sure if its a problem with the framework we were using (Dojo)
Well, var x = new Array() is different than var x = [] is different in some features I'll just explain the most useful two (in my opinion) of them.
Before I get into expalining the differences, I will set a base first; when we use x = [] defines a new variable with data type of Array, and it inherits all the methods that belong to the array prototype, something pretty similar (but not exactly) to extending a class. However, when we use x = new Array() it initilizes a clone of the array prototype assigned to the variable x.
Now let's see what are the difference
The First Difference is that using new Array(x) where x is an integer, initilizes an array of x undefined values, for example new Array(16) will initialize an array with 16 items all of them are undefined. This is very useful when you asynchronously fill an array of a predefined length.
For example (again :) ) let's say you are getting the results of 100 competitiors, and you're receiving them asynchronously from a remote system or db, then you'll need to allocate them in the array according to the rank once you receive each result. In this very rare case you will do something like myArray[result.rank - 1] = result.name, so the rank 1 will be set to the index 0 and so on.
The second difference is that using new Array() as you already know, instanciates a whole new clone of the array prototype and assigns it to your variable, that allows you to do some magic (not recommended btw). This magic is that you can overwrite a specific method of the legacy array methods. So, for example you can set the Array.push method to push the new value to the beginning of the array instead of the end, and you can also add new methods (this is better) to this specific clone of the Array Prototype. That will allow you to define more complex types of arrays throughout your project with your own added methods and use it as a class.
Last thing, if you're from the very few people (that I truly love) that care about processing overhead and memory consumption of your app, you'd never tough new Array() without being desperate to use it :).
I hope that has explained enough about the beast new Array() :)
I found a difference while using promises. While using array of promises (say arr, initialised as arr=[]), got an error in Promise.all(arr). Whereas when declared as arr = Array(), did not get compilation issues. Hope this helps.
I've found one difference between the two constructions that bit me pretty hard.
Let's say I have:
function MyClass(){
this.property1=[];
this.property2=new Array();
};
var MyObject1=new MyClass();
var MyObject2=new MyClass();
In real life, if I do this:
MyObject1.property1.push('a');
MyObject1.property2.push('b');
MyObject2.property1.push('c');
MyObject2.property2.push('d');
What I end up with is this:
MyObject1.property1=['a','c']
MyObject1.property2=['b']
MyObject2.property1=['a','c']
MyObject2.property2=['d']
I don't know what the language specification says is supposed to happen, but if I want my two objects to have unique property arrays in my objects, I have to use new Array().
Using the Array constructor makes a new array of the desired length and populates each of the indices with undefined, the assigned an array to a variable one creates the indices that you give it info for.