I'm looking for any alternatives to the below for creating a JavaScript array containing 1 through to N where N is only known at runtime.
var foo = []; for (var i = 1; i <= N; i++) { foo.push(i); } To me it feels like there should be a way of doing this without the loop.
In ES6 using Array from() and keys() methods.
Array.from(Array(10).keys()) //=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] Shorter version using spread operator.
[...Array(10).keys()] //=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] Start from 1 by passing map function to Array from(), with an object with a length property:
Array.from({length: 10}, (_, i) => i + 1) //=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] 
map to the array to adjust the values ([...Array(10).keys()].map(x => x++);) to start at 1
map(x => x++) to map(x => ++x) due to precedence increment happens after the value return :)
map when you can simply slice? [...Array(N+1).keys()].slice(1)
keys and only 1 map -> Array.from(Array(10)).map((e,i)=>i+1)
from Array.from(Array(10), (e,i)=>i+1)You can do so:
var N = 10; Array.apply(null, {length: N}).map(Number.call, Number) result: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
or with random values:
Array.apply(null, {length: N}).map(Function.call, Math.random) result: [0.7082694901619107, 0.9572225909214467, 0.8586748542729765, 0.8653848143294454, 0.008339877473190427, 0.9911756622605026, 0.8133423360995948, 0.8377588465809822, 0.5577575915958732, 0.16363654541783035]
First, note that Number.call(undefined, N) is equivalent to Number(N), which just returns N. We'll use that fact later.
Array.apply(null, [undefined, undefined, undefined]) is equivalent to Array(undefined, undefined, undefined), which produces a three-element array and assigns undefined to each element.
How can you generalize that to N elements? Consider how Array() works, which goes something like this:
function Array() { if ( arguments.length == 1 && 'number' === typeof arguments[0] && arguments[0] >= 0 && arguments && arguments[0] < 1 << 32 ) { return [ … ]; // array of length arguments[0], generated by native code } var a = []; for (var i = 0; i < arguments.length; i++) { a.push(arguments[i]); } return a; } Since ECMAScript 5, Function.prototype.apply(thisArg, argsArray) also accepts a duck-typed array-like object as its second parameter. If we invoke Array.apply(null, { length: N }), then it will execute
function Array() { var a = []; for (var i = 0; i < /* arguments.length = */ N; i++) { a.push(/* arguments[i] = */ undefined); } return a; } Now we have an N-element array, with each element set to undefined. When we call .map(callback, thisArg) on it, each element will be set to the result of callback.call(thisArg, element, index, array). Therefore, [undefined, undefined, …, undefined].map(Number.call, Number) would map each element to (Number.call).call(Number, undefined, index, array), which is the same as Number.call(undefined, index, array), which, as we observed earlier, evaluates to index. That completes the array whose elements are the same as their index.
Why go through the trouble of Array.apply(null, {length: N}) instead of just Array(N)? After all, both expressions would result an an N-element array of undefined elements. The difference is that in the former expression, each element is explicitly set to undefined, whereas in the latter, each element was never set. According to the documentation of .map():
callbackis invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values.
Therefore, Array(N) is insufficient; Array(N).map(Number.call, Number) would result in an uninitialized array of length N.
Since this technique relies on behaviour of Function.prototype.apply() specified in ECMAScript 5, it will not work in pre-ECMAScript 5 browsers such as Chrome 14 and Internet Explorer 9.
Function.prototype.call's first param is the this object to map directly over Array.prototype.map's iterator parameter has a certain brilliance to it.
map on unassigned values, in my opinion. Another version (and possibly slightly clearer, albeit longer) is: Array.apply(null, { length: N }).map(function(element, index) { return index; })
Array.apply(null, new Array(N)).map(function(_,i) { return i; }) or, in case of es6 and arrow functions, even shorter: Array.apply(null, new Array(N)).map((_,i) => i)
...) and keys method[ ...Array(N).keys() ].map( i => i+1); Array(N).fill().map((_, i) => i+1); Array.from(Array(N), (_, i) => i+1) { length: N } hackArray.from({ length: N }, (_, i) => i+1) All the forms above can produce arrays initialised to pretty much any desired values by changing i+1 to expression required (e.g. i*2, -i, 1+i*2, i%2 and etc). If expression can be expressed by some function f then the first form becomes simply
[ ...Array(N).keys() ].map(f) Array.from({length: 5}, (v, k) => k+1); // [1,2,3,4,5] Since the array is initialized with undefined on each position, the value of v will be undefined
let demo= (N) => { console.log( [ ...Array(N).keys() ].map(( i) => i+1), Array(N).fill().map((_, i) => i+1) , Array.from(Array(N), (_, i) => i+1), Array.from({ length: N }, (_, i) => i+1) ) } demo(5)f i.e.[ ...Array(N).keys() ].map((i) => f(i)) or even simpler
[ ...Array(N).keys() ].map(f) let demo= (N,f) => { console.log( [ ...Array(N).keys() ].map(f), Array(N).fill().map((_, i) => f(i)) , Array.from(Array(N), (_, i) => f(i)), Array.from({ length: N }, (_, i) => f(i)) ) } demo(5, i=>2*i+1)
k++, use ++k.
Array.from({length: 5}, (v, k) => k+1);, which does indeed only iterate once.If I get what you are after, you want an array of numbers 1..n that you can later loop through.
If this is all you need, can you do this instead?
var foo = new Array(45); // create an empty array with length 45 then when you want to use it... (un-optimized, just for example)
for(var i = 0; i < foo.length; i++){ document.write('Item: ' + (i + 1) + ' of ' + foo.length + '<br/>'); } e.g. if you don't need to store anything in the array, you just need a container of the right length that you can iterate over... this might be easier.
See it in action here: http://jsfiddle.net/3kcvm/
var n = 45; and then looping from 1..n would do.
foo.length = M --- The cut off info is lost. See it in action ==> jsfiddle.net/ACMXp
var n = 45;.
new Array(45); does not "create a 45 element array" (in same meaning as [undefined,undefined,..undefined] does). It rather "creates empty array with length = 45" ([undefined x 45]), same as var foo = []; foo.length=45;. That's why forEach, and map will not apply in this case.Arrays innately manage their lengths. As they are traversed, their indexes can be held in memory and referenced at that point. If a random index needs to be known, the indexOf method can be used.
This said, for your needs you may just want to declare an array of a certain size:
var foo = new Array(N); // where N is a positive integer /* this will create an array of size, N, primarily for memory allocation, but does not create any defined values foo.length // size of Array foo[ Math.floor(foo.length/2) ] = 'value' // places value in the middle of the array */ Making use of the spread operator (...) and keys method, enables you to create a temporary array of size N to produce the indexes, and then a new array that can be assigned to your variable:
var foo = [ ...Array(N).keys() ]; You can first create the size of the array you need, fill it with undefined and then create a new array using map, which sets each element to the index.
var foo = Array(N).fill().map((v,i)=>i); This should be initializing to length of size N and populating the array in one pass.
Array.from({ length: N }, (v, i) => i)
In lieu of the comments and confusion, if you really wanted to capture the values from 1..N in the above examples, there are a couple options:
++i). in cases where index is not used -- and possibly a more efficient way -- is to create your array but make N represent N+1, then shift off the front.
So if you desire 100 numbers:
let arr; (arr=[ ...Array(101).keys() ]).shift()
[0,..,n-1] whereas question is for [1,...,n]
In ES6 you can do:
Array(N).fill().map((e,i)=>i+1);
http://jsbin.com/molabiluwa/edit?js,console
Edit: Changed Array(45) to Array(N) since you've updated the question.
console.log( Array(45).fill(0).map((e,i)=>i+1) );
.join.splitversion - but I still think the humble loop is better.const gen = N => [...(function*(){let i=0;while(i<N)yield i++})()] 
.fill()
.fill() is necessary. I see that it is when I test on node's repl, but since Array(1)[0] === undefined, what difference does the call to fill() in Array(1).fill(undefined) make?Use the very popular Underscore _.range method
// _.range([start], stop, [step]) _.range(10); // => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] _.range(1, 11); // => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] _.range(0, 30, 5); // => [0, 5, 10, 15, 20, 25] _.range(0, -10, -1); // => [0, -1, -2, -3, -4, -5, -6, -7, -8, -9] _.range(0); // => [] 
Simply, this worked for me:
[...Array(5)].map(...) 
[...Array(5)].map((a)=>7) gives an array of sevens while Array(5).map((a)=>7) doesn't. The slitting of the array using ... is still producing an array of undefined values so why would it make difference?i still works.i here, just the item a (be it empty, null or whatever else). Are you saying that we actually don't iterate the array for empty values? Because if we so, then I'd somehow expect 7 to be placed in each spot when an empty value resides. Help me understand.function range(start, end) { var foo = []; for (var i = start; i <= end; i++) { foo.push(i); } return foo; } Then called by
var foo = range(1, 5); There is no built-in way to do this in Javascript, but it's a perfectly valid utility function to create if you need to do it more than once.
Edit: In my opinion, the following is a better range function. Maybe just because I'm biased by LINQ, but I think it's more useful in more cases. Your mileage may vary.
function range(start, count) { if(arguments.length == 1) { count = start; start = 0; } var foo = []; for (var i = 0; i < count; i++) { foo.push(start + i); } return foo; } 
Array instead of Array.prototype as there is no reason (it might even be considered rather dumb) to have this method on every array.
Array.range(1, 5) would probably be more appropriate, but there is something kind of cool about writing [].range(1, 5).
foo = [1, 2, 3]; bar = foo.range(0, 10);. But that's just...confusing. bar = Array.range(0, 10) is a lot more clear and explicit. The range has nothing to do with the instance, so there's no reason to make it an instance method.Today 2020.12.11 I performed tests on macOS HighSierra 10.13.6 on Chrome v87, Safari v13.1.2 and Firefox v83 for chosen solutions.
For all browsers
while) is the fastest (except Firefox for big N - but it's fast there)
I perform 2 tests cases:
Below snippet presents all tested solutions A B C D E F G H I J K L M N O P Q R S T U V W X
function A(N) { return Array.from({length: N}, (_, i) => i + 1) } function B(N) { return Array(N).fill().map((_, i) => i+1); } function C(N) { return Array(N).join().split(',').map((_, i) => i+1 ); } function D(N) { return Array.from(Array(N), (_, i) => i+1) } function E(N) { return Array.from({ length: N }, (_, i) => i+1) } function F(N) { return Array.from({length:N}, Number.call, i => i + 1) } function G(N) { return (Array(N)+'').split(',').map((_,i)=> i+1) } function H(N) { return [ ...Array(N).keys() ].map( i => i+1); } function I(N) { return [...Array(N).keys()].map(x => x + 1); } function J(N) { return [...Array(N+1).keys()].slice(1) } function K(N) { return [...Array(N).keys()].map(x => ++x); } function L(N) { let arr; (arr=[ ...Array(N+1).keys() ]).shift(); return arr; } function M(N) { var arr = []; var i = 0; while (N--) arr.push(++i); return arr; } function N(N) { var a=[],b=N;while(b--)a[b]=b+1; return a; } function O(N) { var a=Array(N),b=0; while(b<N) a[b++]=b; return a; } function P(N) { var foo = []; for (var i = 1; i <= N; i++) foo.push(i); return foo; } function Q(N) { for(var a=[],b=N;b--;a[b]=b+1); return a; } function R(N) { for(var i,a=[i=0];i<N;a[i++]=i); return a; } function S(N) { let foo,x; for(foo=[x=N]; x; foo[x-1]=x--); return foo; } function T(N) { return new Uint8Array(N).map((item, i) => i + 1); } function U(N) { return '_'.repeat(5).split('').map((_, i) => i + 1); } function V(N) { return _.range(1, N+1); } function W(N) { return [...(function*(){let i=0;while(i<N)yield ++i})()] } function X(N) { function sequence(max, step = 1) { return { [Symbol.iterator]: function* () { for (let i = 1; i <= max; i += step) yield i } } } return [...sequence(N)]; } [A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X].forEach(f=> { console.log(`${f.name} ${f(5)}`); })<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.20/lodash.min.js" integrity="sha512-90vH1Z83AJY9DmlWa8WkjkV79yfS2n2Oxhsi2dZbIv0nC4E6m5AbH8Nh156kkM7JePmqD6tcZsfad1ueoaovww==" crossorigin="anonymous"> </script> This snippet only presents functions used in performance tests - it does not perform tests itself!And here are example results for chrome
the fastest way to fill an Array in v8 is:
[...Array(5)].map((_,i) => i); result will be: [0, 1, 2, 3, 4]
This question has a lot of complicated answers, but a simple one-liner:
[...Array(255).keys()].map(x => x + 1) Also, although the above is short (and neat) to write, I think the following is a bit faster (for a max length of:
127, Int8,
255, Uint8,
32,767, Int16,
65,535, Uint16,
2,147,483,647, Int32,
4,294,967,295, Uint32.
(based on the max integer values), also here's more on Typed Arrays):
(new Uint8Array(255)).map(($,i) => i + 1); Although this solution is also not so ideal, because it creates two arrays, and uses the extra variable declaration "$" (not sure any way to get around that using this method). I think the following solution is the absolute fastest possible way to do this:
for(var i = 0, arr = new Uint8Array(255); i < arr.length; i++) arr[i] = i + 1; Anytime after this statement is made, you can simple use the variable "arr" in the current scope;
If you want to make a simple function out of it (with some basic verification):
function range(min, max) { min = min && min.constructor == Number ? min : 0; !(max && max.constructor == Number && max > min) && // boolean statements can also be used with void return types, like a one-line if statement. ((max = min) & (min = 0)); //if there is a "max" argument specified, then first check if its a number and if its graeter than min: if so, stay the same; if not, then consider it as if there is no "max" in the first place, and "max" becomes "min" (and min becomes 0 by default) for(var i = 0, arr = new ( max < 128 ? Int8Array : max < 256 ? Uint8Array : max < 32768 ? Int16Array : max < 65536 ? Uint16Array : max < 2147483648 ? Int32Array : max < 4294967296 ? Uint32Array : Array )(max - min); i < arr.length; i++) arr[i] = i + min; return arr; } //and you can loop through it easily using array methods if you want range(1,11).forEach(x => console.log(x)); //or if you're used to pythons `for...in` you can do a similar thing with `for...of` if you want the individual values: for(i of range(2020,2025)) console.log(i); //or if you really want to use `for..in`, you can, but then you will only be accessing the keys: for(k in range(25,30)) console.log(k); console.log( range(1,128).constructor.name, range(200).constructor.name, range(400,900).constructor.name, range(33333).constructor.name, range(823, 100000).constructor.name, range(10,4) // when the "min" argument is greater than the "max", then it just considers it as if there is no "max", and the new max becomes "min", and "min" becomes 0, as if "max" was never even written );so, with the above function, the above super-slow "simple one-liner" becomes the super-fast, even-shorter:
range(1,14000); 
fill method: Array(255).fill(0,0,255)Solution for empty array and with just number in array
const arrayOne = new Array(10); console.log(arrayOne); const arrayTwo = [...Array(10).keys()]; console.log(arrayTwo); var arrayThree = Array.from(Array(10).keys()); console.log(arrayThree); const arrayStartWithOne = Array.from(Array(10).keys(), item => item + 1); console.log(arrayStartWithOne)Using ES2015/ES6 spread operator
[...Array(10)].map((_, i) => i + 1) console.log([...Array(10)].map((_, i) => i + 1))
You can use this:
new Array(/*any number which you want*/) .join().split(',') .map(function(item, index){ return ++index;}) for example
new Array(10) .join().split(',') .map(function(item, index){ return ++index;}) will create following array:
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10] 
join, once to split, and once for the thing you actually want to do) is just not nice - I know they seem to have fallen out of favor for some reason, but it would be far better to simply use a good old fashioned loop!If you happen to be using d3.js in your app as I am, D3 provides a helper function that does this for you.
So to get an array from 0 to 4, it's as easy as:
d3.range(5) [0, 1, 2, 3, 4] and to get an array from 1 to 5, as you were requesting:
d3.range(1, 5+1) [1, 2, 3, 4, 5] Check out this tutorial for more info.
This is probably the fastest way to generate an array of numbers
Shortest
var a=[],b=N;while(b--)a[b]=b+1; Inline
var arr=(function(a,b){while(a--)b[a]=a;return b})(10,[]); //arr=[0,1,2,3,4,5,6,7,8,9] If you want to start from 1
var arr=(function(a,b){while(a--)b[a]=a+1;return b})(10,[]); //arr=[1,2,3,4,5,6,7,8,9,10] Want a function?
function range(a,b,c){c=[];while(a--)c[a]=a+b;return c}; //length,start,placeholder var arr=range(10,5); //arr=[5,6,7,8,9,10,11,12,13,14] WHY?
while is the fastest loop
Direct setting is faster than push
[] is faster than new Array(10)
it's short... look the first code. then look at all other functions in here.
If you like can't live without for
for(var a=[],b=7;b>0;a[--b]=b+1); //a=[1,2,3,4,5,6,7] or
for(var a=[],b=7;b--;a[b]=b+1); //a=[1,2,3,4,5,6,7] 
console.time() to get an intuition, but for a proof, you need jsperf.com AND it shows you cross-browser results from other people (different hardware etc)
var a=[],b=N;while(b--){a[b]=a+1};
If you are using lodash, you can use _.range:
_.range([start=0], end, [step=1])Creates an array of numbers (positive and/or negative) progressing from start up to, but not including, end. A step of -1 is used if a negative start is specified without an end or step. If end is not specified, it's set to start with start then set to 0.
Examples:
_.range(4); // ➜ [0, 1, 2, 3] _.range(-4); // ➜ [0, -1, -2, -3] _.range(1, 5); // ➜ [1, 2, 3, 4] _.range(0, 20, 5); // ➜ [0, 5, 10, 15] _.range(0, -4, -1); // ➜ [0, -1, -2, -3] _.range(1, 4, 0); // ➜ [1, 1, 1] _.range(0); // ➜ [] 
ifthe new way to filling Array is:
const array = [...Array(5).keys()] console.log(array)result will be: [0, 1, 2, 3, 4]
with ES6 you can do:
// `n` is the size you want to initialize your array // `null` is what the array will be filled with (can be any other value) Array(n).fill(null) 
map and forEach will work.Very simple and easy to generate exactly 1 - N
const [, ...result] = Array(11).keys(); console.log('Result:', result);Final Summary report .. Drrruummm Rolll -
This is the shortest code to generate an Array of size N (here 10) without using ES6. Cocco's version above is close but not the shortest.
(function(n){for(a=[];n--;a[n]=n+1);return a})(10) But the undisputed winner of this Code golf(competition to solve a particular problem in the fewest bytes of source code) is Niko Ruotsalainen . Using Array Constructor and ES6 spread operator . (Most of the ES6 syntax is valid typeScript, but following is not. So be judicious while using it)
[...Array(10).keys()] 
a. The loop should be for(var a=[];n--;a[n]=n+1)Code golf(competition to solve a particular problem in the fewest bytes of source code) part in the answer :)https://stackoverflow.com/a/49577331/8784402
[...Array(N)].map((v, i) => from + i * step); Examples and other alternatives
Array.from(Array(10).keys()).map(i => 4 + i * 2); //=> [4, 6, 8, 10, 12, 14, 16, 18, 20, 22] [...Array(10).keys()].map(i => 4 + i * -2); //=> [4, 2, 0, -2, -4, -6, -8, -10, -12, -14] Array(10).fill(0).map((v, i) => 4 + i * 2); //=> [4, 6, 8, 10, 12, 14, 16, 18, 20, 22] Array(10).fill().map((v, i) => 4 + i * -2); //=> [4, 2, 0, -2, -4, -6, -8, -10, -12, -14] [...Array(10)].map((v, i) => 4 + i * 2); //=> [4, 6, 8, 10, 12, 14, 16, 18, 20, 22] const range = (from, to, step) => [...Array(Math.floor((to - from) / step) + 1)].map((_, i) => from + i * step); range(0, 9, 2); //=> [0, 2, 4, 6, 8] // can also assign range function as static method in Array class (but not recommended ) Array.range = (from, to, step) => [...Array(Math.floor((to - from) / step) + 1)].map((_, i) => from + i * step); Array.range(2, 10, 2); //=> [2, 4, 6, 8, 10] Array.range(0, 10, 1); //=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] Array.range(2, 10, -1); //=> [] Array.range(3, 0, -1); //=> [3, 2, 1, 0] class Range { constructor(total = 0, step = 1, from = 0) { this[Symbol.iterator] = function* () { for (let i = 0; i < total; yield from + i++ * step) {} }; } } [...new Range(5)]; // Five Elements //=> [0, 1, 2, 3, 4] [...new Range(5, 2)]; // Five Elements With Step 2 //=> [0, 2, 4, 6, 8] [...new Range(5, -2, 10)]; // Five Elements With Step -2 From 10 //=>[10, 8, 6, 4, 2] [...new Range(5, -2, -10)]; // Five Elements With Step -2 From -10 //=> [-10, -12, -14, -16, -18] // Also works with for..of loop for (i of new Range(5, -2, 10)) console.log(i); // 10 8 6 4 2 const Range = function* (total = 0, step = 1, from = 0) { for (let i = 0; i < total; yield from + i++ * step) {} }; Array.from(Range(5, -2, -10)); //=> [-10, -12, -14, -16, -18] [...Range(5, -2, -10)]; // Five Elements With Step -2 From -10 //=> [-10, -12, -14, -16, -18] // Also works with for..of loop for (i of Range(5, -2, 10)) console.log(i); // 10 8 6 4 2 // Lazy loaded way const number0toInf = Range(Infinity); number0toInf.next().value; //=> 0 number0toInf.next().value; //=> 1 // ... class Range2 { constructor(to = 0, step = 1, from = 0) { this[Symbol.iterator] = function* () { let i = 0, length = Math.floor((to - from) / step) + 1; while (i < length) yield from + i++ * step; }; } } [...new Range2(5)]; // First 5 Whole Numbers //=> [0, 1, 2, 3, 4, 5] [...new Range2(5, 2)]; // From 0 to 5 with step 2 //=> [0, 2, 4] [...new Range2(5, -2, 10)]; // From 10 to 5 with step -2 //=> [10, 8, 6] const Range2 = function* (to = 0, step = 1, from = 0) { let i = 0, length = Math.floor((to - from) / step) + 1; while (i < length) yield from + i++ * step; }; [...Range2(5, -2, 10)]; // From 10 to 5 with step -2 //=> [10, 8, 6] let even4to10 = Range2(10, 2, 4); even4to10.next().value; //=> 4 even4to10.next().value; //=> 6 even4to10.next().value; //=> 8 even4to10.next().value; //=> 10 even4to10.next().value; //=> undefined class _Array<T> extends Array<T> { static range(from: number, to: number, step: number): number[] { return Array.from(Array(Math.floor((to - from) / step) + 1)).map( (v, k) => from + k * step ); } } _Array.range(0, 9, 1); This solution is probably fastest it is inspired by lodash _.range function (but my is simpler and faster)
let N=10, i=0, a=Array(N); while(i<N) a[i++]=i; console.log(a);Performance advantages over current (2020.12.11) existing answers based on while/for
a=Array(N)i++ is used - which looks is about 30% faster than decreasing index i-- (probably because CPU cache memory faster in forward direction)Speed tests with more than 20 other solutions was conducted in this answer
Array(N) constructor creates an array with "holes", which may be less performant when it is actually used later. Engines use different internal representations for arrays depending on what's in them and whether they have "holes"; the internal representation can be downgraded if a hole is created, but is not upgraded when all holes are removed. In V8's terminology, this solution creates a HOLEY_SMI_ELEMENTS array, where a PACKED_SMI_ELEMENTS array would be preferable. v8.dev/blog/elements-kinds#the-elements-kind-lattice The V8 devs' advice is to use push instead.There is another way in ES6, using Array.from which takes 2 arguments, the first is an arrayLike (in this case an object with length property), and the second is a mapping function (in this case we map the item to its index)
Array.from({length:10}, (v,i) => i) this is shorter and can be used for other sequences like generating even numbers
Array.from({length:10}, (v,i) => i*2) Also this has better performance than most other ways because it only loops once through the array. Check the snippit for some comparisons
// open the dev console to see results count = 100000 console.time("from object") for (let i = 0; i<count; i++) { range = Array.from({length:10}, (v,i) => i ) } console.timeEnd("from object") console.time("from keys") for (let i =0; i<count; i++) { range = Array.from(Array(10).keys()) } console.timeEnd("from keys") console.time("apply") for (let i = 0; i<count; i++) { range = Array.apply(null, { length: 10 }).map(function(element, index) { return index; }) } console.timeEnd("apply")
Array.from({length:N}, (v,i) => i+1)
Using new Array methods and => function syntax from ES6 standard (only Firefox at the time of writing).
By filling holes with undefined:
Array(N).fill().map((_, i) => i + 1); Array.from turns "holes" into undefined so Array.map works as expected:
Array.from(Array(5)).map((_, i) => i + 1) 
Array.from({length: N}, (v, k) => k).
Array.prototype.map.call, e.g. for NodeLists returned from document.querySelectorAll. developer.mozilla.org/en/docs/Web/JavaScript/Reference/…
Array.from which turns the sparse values into undefined. Rather Array(5) is called as an arguments object which in turn interprets the sparse values as undefined values :)Array.from({length: 1000}, (_, i) => i).slice(1); or better yet (without the extra variable _ and without the extra slice call):
Array.from({length:1000}, Number.call, i => i + 1) Or for slightly faster results, you can use Uint8Array, if your list is shorter than 256 results (or you can use the other Uint lists depending on how short the list is, like Uint16 for a max number of 65535, or Uint32 for a max of 4294967295 etc. Officially, these typed arrays were only added in ES6 though). For example:
Uint8Array.from({length:10}, Number.call, i => i + 1) Array.apply(0, {length: 1000}).map(function(){return arguments[1]+1}); Alternatively, in ES5, for the map function (like second parameter to the Array.from function in ES6 above), you can use Number.call
Array.apply(0,{length:1000}).map(Number.call,Number).slice(1) Or, if you're against the .slice here also, you can do the ES5 equivalent of the above (from ES6), like:
Array.apply(0,{length:1000}).map(Number.call, Function("i","return i+1")) Array(...Array(9)).map((_, i) => i); console.log(Array(...Array(9)).map((_, i) => i))
[...Array(9)].map((_, i) => i)for(var i,a=[i=0];i<10;a[i++]=i); a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Generate 9 items with a step size of 1, starting with 1:
Array(9).fill(1).map((e,i)=>e+(i*1)); or
Array(9).fill().map((e,i)=>1+(i*1)); Result:
[ 1, 2, 3, 4, 5, 6, 7, 8, 9 ] Generate 9 items with a step size of 5, starting with 5:
Array(9).fill(5).map((e,i)=>e+(i*5)); or
Array(9).fill().map((e,i)=>5+(i*5)); Result:
[ 5, 10, 15, 20, 25, 30, 35, 40, 45 ] Generate 9 items with a step size of 5, starting with -20:
Array(9).fill(-20).map((e,i)=>e+(i*5)); or
Array(9).fill().map((e,i)=>-20+(i*5)); Result:
[ -20, -15, -10, -5, 0, 5, 10, 15, 20 ] Execution, user and system time:
Arr size: 1 Execution Time: 3.731ms User CPU time: 0.033ms System CPU time: 0.013ms Arr size: 10 Execution Time: 3.704ms User CPU time: 0.031ms System CPU time: 0.01ms Arr size: 100 Execution Time: 4.001ms User CPU time: 0.038ms System CPU time: 0.013ms Arr size: 1000 Execution Time: 3.735ms User CPU time: 0.058ms System CPU time: 0.014ms Arr size: 10000 Execution Time: 5.185ms User CPU time: 0.262ms System CPU time: 0.098ms Arr size: 100000 Execution Time: 6.937ms User CPU time: 3.936ms System CPU time: 0.363ms Arr size: 1000000 Execution Time: 24.848ms User CPU time: 17.657ms System CPU time: 4.185ms 
Array(9).fill().map((e,i)=>i+1)?
Array(9).fill().map((e,i)=>(i+1)*5) - it's the same thing I guess, just in reverse. I wonder if there's any benefit to doing it one way or the other. Probably not.