Create a JavaScript array containing 1...N
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.
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/>');
}
eg 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/
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]
Explanation
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() :
callback is 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.
Compatibility
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.
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]
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