Develop Reference

Difference between fill and fill map

I came across some code which was filling an array of objects like so:
const getObj = () => {
return {a: 1, b: 2, c: 3};
}
const arr = Array(3).fill(null).map(getObj);
console.log(arr);
However, I'm wondering what the main purpose of fill(null).map(getObj) is? It seems redundant as I can simply write the following and get the same resulting array:
const getObj = () => {
return {a: 1, b: 2, c: 3};
}
const arr = Array(3).fill(getObj());
console.log(arr);
So, I'm wondering if these two lines of code do exactly the same thing or if there is something I'm missing?
The resulting arrays (top array first method with fill + map bottom array is only using map):

Array(3).fill(getObj()) will fill your array with references to the same object, Array(3).fill(null).map(getObj) will create object per element. See the example below:
const getObj = () => {
return {a: 1, b: 2, c: 3};
}
const arr = Array(3).fill(null).map(getObj);
arr[0].b=4;
console.log(JSON.stringify(arr));
const arr1 = Array(3).fill(getObj());
arr1[0].b=4;
console.log(JSON.stringify(arr1))

When it comes to Array.fill it is stated in the documentation that:
When fill gets passed an object, it will copy the reference and fill
the array with references to that object.
So using a Array.fill with objects has somewhat limited application unless you really want to have multiple objects pointing to the same reference. In more than few use cases however that would lead to bugs if not understood.
For the 2nd case where you do Array(3).fill(null).map(getObj) this is one of the ways to create a new array based on a given arbitrary size and at the same time fill it with new objects.
The real need for the fill(null) is due to the fact that calling Array(3) would only do one thing. Create a new array and set its length property to 3. That is it!
let arr = Array(3) // returns new array with its "length" property set to 3
console.log(arr) // [empty × 3] <-- browser console
So that array now has only length and bunch of empty elements. You can't do much with it until it actually has values. Therefore the need for fill so that you give it any value and then map through it to set the values you actually want. Using Array.map and calling each iteration your function guarantees you do not have same references. You could have skipped the fill step and done something like this:
const getObj = () => ({a: 1, b: 2, c: 3})
// using array destructuring
let arr = [...Array(3)].map(getObj)
arr[0].a = 3
console.log(arr)
// using Array.from
let arr2 = Array.from(Array(3)).map(getObj)
arr2[0].a = 3
console.log(arr2)
There are somewhat shorter and get you the exact same result of filling the array with specified length with objects and not references to the same object.
The trick here is that both would "fill" the array after it is defined with undefined values instead, after which the map would fill it with the values we want.

Is it possible to "overwrite a pointer" in JavaScript?

Is it possible to emulate the code below, in JavaScript?
var ref = new Reference();
var arr = [1, 2, ref];
ref.overwrite(3);
console.log(arr); // output: [1,2,3]
This code places a ref inside an array, then overwrites it, completely replacing its occurrence by 3. We could do something similar by storing the parent object and the index:
var arr = [1, 2, null];
var ref = {object: arr, index: 2};
ref.object[ref.index] = 3;
console.log(arr); // output: [1, 2, 3]
But this is not the same thing, since ref must keep track of all the parent objects where it is used. I'm interested in using ref as a placeholder, storing it in multiple places and then replacing all occurrences by something else, without keeping track of where it was used. We could also "almost" do this as such:
var ref = {value: null};
var arr = [1, 2, ref];
ref.value = 3;
console.log(arr); // output: [1, 2, {value: 3}]
But this doesn't completely get rid of the wrapping object.

No, JavaScript does not have references or pointers as values. You always need some wrapper object (which you can mutate however you want, but not replace), it's not possible to create a custom primitive value.

There is no way to really do what you want to do, but depending on how you are using it, you can fake it. You can use valueOf to return what you want. Example here shows using it to sum up an array of numbers and than altering it and running it again.
const sum = arr => arr.reduce((t, i) => t + i, 0)
function MyRef(n) {
this.number = n;
this.overWrite = n => this.number = n
}
MyRef.prototype.valueOf = function() {
return this.number;
};
const ref = new MyRef(100);
const myArray = [1, 2, ref]
console.log(sum(myArray))
ref.overWrite(3)
console.log(sum(myArray))

JavaScript Array.concat and push workaround

It's always tricky to think Array.concat thing. Often, I just want to use mutable Array.push because I simply add extra-data on the immutable data. So, I usually do:
array[array.length] = newData;
I've asked a question related got some answers here: How to store data of a functional chain
const L = (a) => {
const m = a => (m.list ? m.list : m.list = [])
.push(a) && m;
//use `concat` and refactor needed instead of `push`
//that is not immutable
return m(a); // Object construction!
};
console.log(L);
console.log(L(2));
console.log(L(1)(2)(3))
some outputs:
{ [Function: m] list: [ 2 ] }
{ [Function: m] list: [ 1, 2, 3 ] }
I feel that push should be replaced with using concat, but still, push makes the code elegant simply because we don't want to prepare another object here.
Basically, I want to do:
arr1 = arr1.concat(arr2);
but, is there any way to write
arr1[arr1.length] = arr2;
which ends up with a nested array, and does not work.

You could assign a new array with a default array for not given m.list.
const L = (a) => {
const m = a => (m.list = (m.list || []).concat(a), m);
return m(a);
};
console.log(L.list);
console.log(L(2).list);
console.log(L(1)(2)(3).list);

You can use multiple parameters in Array.push so:
var a = [];
a.push(3, 4) // a is now [3, 4]
Combined with the ES6 spread syntax:
var a = [1, 2];
var b = [3, 4]
a.push(...b); // a is now [1, 2, 3, 4]

arr1[arr1.length] represents a single value, the value at index arr1.length.
Imagine this array
[ 1 , 2 , 3 , 4 ] // arr of length 4
^0 ^1 ^2 ^3 // indexes
If we say arr1[arr1.length] = someThing
We ask javascript to put something right here, and only here, at index 4:
[ 1 , 2 , 3 , 4 , ] // arr of length 4
^0 ^1 ^2 ^3 ^4 // add someThing in index 4
So, if we want to add something strictly with arr1[arr1.length], then we need to keep doing that for each index. for each meaning any kind of loop. E.G:
// Not recommended to use
var arr1 = [1,2,3];
var arr2 = [3,4,5];
while (arr2.length){
arr1[arr1.length] = arr2.shift();
}
console.log(arr1); // [1,2,3,3,4,5]
console.log(arr2); // []
But, as you can see, this method, or any similar one, even if optimized, is not the right approach. You need a concatenation.
Since you mention a functional one, which returns the resulting array, you can simply replace the initial array and make use of spread operator:
var arr1 = [1,2,3];
var arr2 = [3,4,5];
console.log(arr1 = [...arr1,...arr2]); // [1,2,3,3,4,5]

Inserting an object so the array stays sorted

I have an enum of different Steps
export enum StepCategory {
START = 0,
POSITION_1 = 1,
TRANSPORT = 2,
RECEIVER = 3,
END = 4,
NO_STEP_MATCH = 5
}
This will later result in an array, where for every Step I have an object. The Problem is I won't load all the information at once, so i can'tdo a simple for-loop and push each item chronogically. I could be that I first load the value for Step 4, so my array would be:
var array = [{"END" , "POSITIVE"}]
Afterwards I would get the Info for Step 2, then I would have:
var array = [{"END", "POSITIVE"}, {"TRANSPORT", "POSITIVE"}]
But this is not sorted.
What should I do? Should I declare an array of 6 undefined values
var array = [undefined, undefined, undefined, undefined, undefined, undefined]
This way I wouldn't need any sorting-algorithm after each Update, and can just push the value at the right position.
Just some small Info: I use AngularJS 1, Typescript and Lodash

In plain Javascript, you could sort it with an object and the assigned values of the key.
var StepCategory = { START: 0, POSITION_1: 1, TRANSPORT: 2, RECEIVER: 3, END: 4, NO_STEP_MATCH: 5 },
array = [["END", "POSITIVE"], ["TRANSPORT", "POSITIVE"]];
array.sort(function (a, b) {
return StepCategory[a[0]] - StepCategory[b[0]];
});
console.log(array)

First of all, this is - as someone mentioned in the comments - not syntactically correct:
var array = [{"END", "POSITIVE"}, {"TRANSPORT", "POSITIVE"}]
I assume that this was just a typo writing this question. Additionally if you actually use the enum in your array as key and just left it out for demonstration purposes, I would expect your array of objects to look something like this:
var array = [{StepCategory.END: "POSITIVE"}, {StepCategory.TRANSPORT: "POSITIVE"}]
In order to sort this with LoDash you could use something like this:
var sortedArray = _.sortBy(array, i => Object.keys(i)[0]);
This sorts your array by the value of the first key of each object which refers to your enum value.

How to initialize an array's length in JavaScript?

Most of the tutorials that I've read on arrays in JavaScript (including w3schools and devguru) suggest that you can initialize an array with a certain length by passing an integer to the Array constructor using the var test = new Array(4); syntax.
After using this syntax liberally in my js files, I ran one of the files through jsLint, and it freaked out:
Error: Problem at line 1 character 22: Expected ')' and instead saw '4'.
var test = new Array(4);
Problem at line 1 character 23: Expected ';' and instead saw ')'.
var test = new Array(4);
Problem at line 1 character 23: Expected an identifier and instead saw ')'.
After reading through jsLint's explanation of its behavior, it looks like jsLint doesn't really like the new Array() syntax, and instead prefers [] when declaring arrays.
So I have a couple questions:
First, why? Am I running any risk by using the new Array() syntax instead? Are there browser incompatibilities that I should be aware of?
And second, if I switch to the square bracket syntax, is there any way to declare an array and set its length all on one line, or do I have to do something like this:
var test = [];
test.length = 4;

Array(5) gives you an array with length 5 but no values, hence you can't iterate over it.
Array.apply(null, Array(5)).map(function () {}) gives you an array with length 5 and undefined as values, now it can be iterated over.
Array.apply(null, Array(5)).map(function (x, i) { return i; }) gives you an array with length 5 and values 0,1,2,3,4.
Array(5).forEach(alert) does nothing, Array.apply(null, Array(5)).forEach(alert) gives you 5 alerts
ES6 gives us Array.from so now you can also use Array.from(Array(5)).forEach(alert)
If you want to initialize with a certain value, these are good to knows...
Array.from('abcde'), Array.from('x'.repeat(5))
or Array.from({length: 5}, (v, i) => i) // gives [0, 1, 2, 3, 4]

With ES2015 .fill() you can now simply do:
// `n` is the size you want to initialize your array
// `0` is what the array will be filled with (can be any other value)
Array(n).fill(0)
Which is a lot more concise than Array.apply(0, new Array(n)).map(i => value)
It is possible to drop the 0 in .fill() and run without arguments, which will fill the array with undefined. (However, this will fail in Typescript)

Why do you want to initialize the length? Theoretically there is no need for this. It can even result in confusing behavior, because all tests that use the length to find out whether an array is empty or not will report that the array is not empty.
Some tests show that setting the initial length of large arrays can be more efficient if the array is filled afterwards, but the performance gain (if any) seem to differ from browser to browser.
jsLint does not like new Array() because the constructer is ambiguous.
new Array(4);
creates an empty array of length 4. But
new Array('4');
creates an array containing the value '4'.
Regarding your comment: In JS you don't need to initialize the length of the array. It grows dynamically. You can just store the length in some variable, e.g.
var data = [];
var length = 5; // user defined length
for(var i = 0; i < length; i++) {
data.push(createSomeObject());
}

[...Array(6)].map(x => 0);
// [0, 0, 0, 0, 0, 0]
OR
Array(6).fill(0);
// [0, 0, 0, 0, 0, 0]
Note: you can't loop empty slots i.e. Array(4).forEach(() => …)
OR
( typescript safe )
Array(6).fill(null).map((_, i) => i);
// [0, 1, 2, 3, 4, 5]
OR
Classic method using a function ( works in any browser )
function NewArray(size) {
var x = [];
for (var i = 0; i < size; ++i) {
x[i] = i;
}
return x;
}
var a = NewArray(10);
// [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Creating nested arrays
When creating a 2D array with the fill intuitively should create new instances. But what actually going to happen is the same array will be stored as a reference.
var a = Array(3).fill([6]);
// [ [6], [6], [6] ]
a[0].push(9);
// [ [6, 9], [6, 9], [6, 9] ]
Solution
var a = [...Array(3)].map(x => []);
a[0].push(4, 2);
// [ [4, 2], [], [] ]
So a 3x2 Array will look something like this:
[...Array(3)].map(x => Array(2).fill(0));
// [ [0, 0], [0, 0], [0, 0] ]
N-dimensional array
function NArray(...dimensions) {
var index = 0;
function NArrayRec(dims) {
var first = dims[0], next = dims.slice().splice(1);
if(dims.length > 1)
return Array(dims[0]).fill(null).map((x, i) => NArrayRec(next ));
return Array(dims[0]).fill(null).map((x, i) => (index++));
}
return NArrayRec(dimensions);
}
var arr = NArray(3, 2, 4);
// [ [ [ 0, 1, 2, 3 ] , [ 4, 5, 6, 7] ],
// [ [ 8, 9, 10, 11] , [ 12, 13, 14, 15] ],
// [ [ 16, 17, 18, 19] , [ 20, 21, 22, 23] ] ]
Initialize a chessboard
var Chessboard = [...Array(8)].map((x, j) => {
return Array(8).fill(null).map((y, i) => {
return `${String.fromCharCode(65 + i)}${8 - j}`;
});
});
// [ [A8, B8, C8, D8, E8, F8, G8, H8],
// [A7, B7, C7, D7, E7, F7, G7, H7],
// [A6, B6, C6, D6, E6, F6, G6, H6],
// [A5, B5, C5, D5, E5, F5, G5, H5],
// [A4, B4, C4, D4, E4, F4, G4, H4],
// [A3, B3, C3, D3, E3, F3, G3, H3],
// [A2, B2, C2, D2, E2, F2, G2, H2],
// [A1, B1, C1, D1, E1, F1, G1, H1] ]
Math filled values
handy little method overload when working with math
function NewArray( size , method, linear )
{
method = method || ( i => i );
linear = linear || false;
var x = [];
for( var i = 0; i < size; ++i )
x[ i ] = method( linear ? i / (size-1) : i );
return x;
}
NewArray( 4 );
// [ 0, 1, 2, 3 ]
NewArray( 4, Math.sin );
// [ 0, 0.841, 0.909, 0.141 ]
NewArray( 4, Math.sin, true );
// [ 0, 0.327, 0.618, 0.841 ]
var pow2 = ( x ) => x * x;
NewArray( 4, pow2 );
// [ 0, 1, 4, 9 ]
NewArray( 4, pow2, true );
// [ 0, 0.111, 0.444, 1 ]

The shortest:
let arr = [...Array(10)];
console.log(arr);

ES6 introduces Array.from which lets you create an Array from any "array-like" or iterables objects:
Array.from({length: 10}, (x, i) => i);
// [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
In this case {length: 10} represents the minimal definition of an "array-like" object: an empty object with just a length property defined.
Array.from allows for a second argument to map over the resulting array.

Sparse arrays are here! 🥳 [2021]
In modern JS engines, sparse arrays are fully supported. You can use [] or new Array(len) in any way you like, even with random access. Dictionary mode seems to be a thing of the past.
In current Chrome (and I guess any V8 environment), Arrays can have a length of up to 2^32-1 and allocation is sparse (meaning empty chunks don't use up any memory):
However, there is a catch
On the one hand, for loops work as intended, however, Array's builtin higher order functions (such as map, filter, find, some etc.) ignore unassigned elements. They require fill (or some other method of population) first:
const a = new Array(10);
const b = new Array(10).fill(0);
a.forEach(x => console.log(x)); // does nothing
b.forEach(x => console.log(x)); // works as intended
Old Version
(I removed most of the old version.) The gist was that creating a large array using new Array(largeNumber) or random accessing an array in places that have not yet been allocated would tumble it into "dictionary mode". Meaning you are using an array with indexes, but under the hood it would use a dictionary to store the values, thus messing with performance, and also with iteration behavior. Luckily that is a thing of the past.

This will initialize the length property to 4:
var x = [,,,,];

I'm surprised there hasn't been a functional solution suggested that allows you to set the length in one line. The following is based on UnderscoreJS:
var test = _.map(_.range(4), function () { return undefined; });
console.log(test.length);
For reasons mentioned above, I'd avoid doing this unless I wanted to initialize the array to a specific value. It's interesting to note there are other libraries that implement range including Lo-dash and Lazy, which may have different performance characteristics.

Here is another solution
var arr = Array.apply( null, { length: 4 } );
arr; // [undefined, undefined, undefined, undefined] (in Chrome)
arr.length; // 4
The first argument of apply() is a this object binding, which we don't care about here, so we set it to null.
Array.apply(..) is calling the Array(..) function and spreading out the { length: 3 } object value as its arguments.

Please people don't give up your old habits just yet.
There is a large difference in speed between allocating memory once then working with the entries in that array (as of old), and allocating it many times as an array grows (which is inevitably what the system does under the hood with other suggested methods).
None of this matters of course, until you want to do something cool with larger arrays. Then it does.
Seeing as there still seems to be no option in JS at the moment to set the initial capacity of an array, I use the following...
var newArrayWithSize = function(size) {
this.standard = this.standard||[];
for (var add = size-this.standard.length; add>0; add--) {
this.standard.push(undefined);// or whatever
}
return this.standard.slice(0,size);
}
There are tradeoffs involved:
This method takes as long as the others for the first call to the function, but very little time for later calls (unless asking for a bigger array).
The standard array does permanently reserve as much space as the largest array you have asked for.
But if it fits with what you're doing there can be a payoff.
Informal timing puts
for (var n=10000;n>0;n--) {var b = newArrayWithSize(10000);b[0]=0;}
at pretty speedy (about 50ms for the 10000 given that with n=1000000 it took about 5 seconds), and
for (var n=10000;n>0;n--) {
var b = [];for (var add=10000;add>0;add--) {
b.push(undefined);
}
}
at well over a minute (about 90 sec for the 10000 on the same chrome console, or about 2000 times slower).
That won't just be the allocation, but also the 10000 pushes, for loop, etc..

(this was probably better as a comment, but got too long)
So, after reading this I was curious if pre-allocating was actually faster, because in theory it should be. However, this blog gave some tips advising against it http://www.html5rocks.com/en/tutorials/speed/v8/.
So still being unsure, I put it to the test. And as it turns out it seems to in fact be slower.
var time = Date.now();
var temp = [];
for(var i=0;i<100000;i++){
temp[i]=i;
}
console.log(Date.now()-time);
var time = Date.now();
var temp2 = new Array(100000);
for(var i=0;i<100000;i++){
temp2[i] = i;
}
console.log(Date.now()-time);
This code yields the following after a few casual runs:
$ node main.js
9
16
$ node main.js
8
14
$ node main.js
7
20
$ node main.js
9
14
$ node main.js
9
19

var arr=[];
arr[5]=0;
alert("length="+arr.length); // gives 6

The simplest form is to use
Array.from({ length: 3 });
// gives you
[undefined, undefined, undefined]
Unlike Array(3) which will give you an array you can't iterate over. Array.from({ length }) gives you an array you can iterate easily.
Array.from({ length: 3 }).map((e, idx) => `hi ${idx}`);
// ['hi 1', 'hi 2', 'hi 3']

Assuming that Array's length is constant. In Javascript, This is what we do:
const intialArray = new Array(specify the value);

The array constructor has an ambiguous syntax, and JSLint just hurts your feelings after all.
Also, your example code is broken, the second var statement will raise a SyntaxError. You're setting the property length of the array test, so there's no need for another var.
As far as your options go, array.length is the only "clean" one. Question is, why do you need to set the size in the first place? Try to refactor your code to get rid of that dependency.

In addition to the answers of others, another clever way is to use Float32Array to create an array and iterate on it.
For this purpose, create an instance from Float32Array with your desired length like this:
new Float32Array(5)
This code returns an array-like that you can convert it to an array with Array.from():
Array.from(new Float32Array(5)) // [0, 0, 0, 0, 0]
You can also use fill() to change the value of items:
Array.from(new Float32Array(5).fill(2)) // [2, 2, 2, 2, 2]
And of course you can iterate on it:
Array.from(new Float32Array(5)).map(item => /* ... */ )

In most answers it is recommended to fill the array because otherwise "you can't iterate over it", but this is not true. You can iterate an empty array, just not with forEach. While loops, for of loops and for i loops work fine.
const count = Array(5);
Does not work.
console.log('---for each loop:---');
count.forEach((empty, index) => {
console.log(`counting ${index}`);
});
These work:
console.log('---for of loop:---');
for (let [index, empty] of count.entries()) {
console.log(`counting for of loop ${index}`);
}
console.log('---for i loop:---');
for (let i = 0, il = count.length; i < il; ++i) {
console.log(`counting for i loop ${i}`);
}
console.log('---while loop:---');
let index = 0;
while (index < count.length) {
console.log(`counting while loop ${index}`);
index++;
}
Check this fiddle with the above examples.
Also angulars *ngFor works fine with an empty array:
<li *ngFor="let empty of count; let i = index" [ngClass]="
<span>Counting with *ngFor {{i}}</span>
</li>

You can set the array length by using array.length = youValue
So it would be
var myArray = [];
myArray.length = yourValue;

The reason you shouldn't use new Array is demonstrated by this code:
var Array = function () {};
var x = new Array(4);
alert(x.length); // undefined...
Some other code could mess with the Array variable. I know it's a bit far fetched that anyone would write such code, but still...
Also, as Felix King said, the interface is a little inconsistent, and could lead to some very difficult-to-track-down bugs.
If you wanted an array with length = x, filled with undefined (as new Array(x) would do), you could do this:
var x = 4;
var myArray = [];
myArray[x - 1] = undefined;
alert(myArray.length); // 4