# Tersest way to create an array of integers from 1..20 in JavaScript

What would be the tersest way to create this array:

var x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
11, 12, 13, 14, 15, 16, 17, 18, 19, 20];

For example, a for loop:

var x = [];
for (var i=1;i<=20;i++) {
x.push(i);
}

Or a while loop:

var x = [], i = 1, endInt = 20;
while (i <= endInt) {
x.push(i);
i++;
}

Would there be other examples that would be terser — in other words — less code? I’m thinking of things like in Ruby where the equivalent code I believe would be as simple as 1..20. I’m not aware of syntax like that in JavaScript but I’m wondering if there are shorter ways to do that same thing.

UPDATE: I wasn’t thinking of removing semicolons or var for answers in the question, but I have to admit the question implies that. I am more curious about algorithms than shaving bytes. Sorry if I was unclear! Also, making it into a function is simple enough, just slap function range(start, end) { /* guts here */ } around it and you’re there. The question is are there novel approaches to the “guts.”

After thinking about it a bit, this is the shortest implementation of the standard range(N) function in JavaScript I could come up with:

function range1(i){return i?range1(i-1).concat(i):[]}

Note: Do not use this in production; it’s O(N^2)

function range1(i){var x=[];var i=1;while(x.push(i++)<i){};return x}

Example:

> range1(5)
[1, 2, 3, 4, 5]

This is like the poster child for recursion, though I was expecting it to be longer until I thought of ternary-if-statement, which brings it down to 42 necessary characters.

Note that the “standard” range function returning [start,end) can be written by doing .concat(i-1).

Update: Ooh, I discovered an incredibly short version with ugly imperative syntax by abusing for loops, reverse-ordering, the fact that assignments return a value: for(y=[],i=20;y[--i]=i;){} consisting of only 25 characters (though you will want var y which you can insert into a for loop, and +1 if you don’t want 0…19). While it is not shorter if you need to define a function, it is shorter than i?r(i-1).concat(i):[] if you do not need to make a function.

## Favorite method

Update Sep13,2015:

Just came up with this new method which works with browsers which support the ES6 standard:

> Array(5).fill().map((x,i)=>i)
[0, 1, 2, 3, 4]

Also this:

> Array.from(Array(5),(x,i)=>i)
[0, 1, 2, 3, 4]

Added some performance profiling testcases: it seems that everything besides a standard in-order for-loop is 10x slower, at least on V8. https://jsperf.com/array-range-in-javascript (Of course, none of this matters if you’re programming in a functional style anyway and would hit every element with a function call anyway.)