JavaScript function similar to Python range()

Question

For a very simple range in ES6:

let range = n => Array.from(Array(n).keys())

From bigOmega's comment, this can be shortened using Spread syntax:

let range = n => [...Array(n).keys()]

No, there is none, but you can make one.

JavaScript's implementation of Python's range()

Trying to emulate how it works in Python, I would create function similar to this:

function range(start, stop, step) {
    if (typeof stop == 'undefined') {
        // one param defined
        stop = start;
        start = 0;
    }

    if (typeof step == 'undefined') {
        step = 1;
    }

    if ((step > 0 && start >= stop) || (step < 0 && start <= stop)) {
        return [];
    }

    var result = [];
    for (var i = start; step > 0 ? i < stop : i > stop; i += step) {
        result.push(i);
    }

    return result;
};

See this jsfiddle for a proof.

Comparison between range() in JavaScript and Python

It works in the following way:

• range(4) returns [0, 1, 2, 3],
• range(3,6) returns [3, 4, 5],
• range(0,10,2) returns [0, 2, 4, 6, 8],
• range(10,0,-1) returns [10, 9, 8, 7, 6, 5, 4, 3, 2, 1],
• range(8,2,-2) returns [8, 6, 4],
• range(8,2) returns [],
• range(8,2,2) returns [],
• range(1,5,-1) returns [],
• range(1,5,-2) returns [],

and its Python counterpart works exactly the same way (at least in the mentioned cases):

>>> range(4)
[0, 1, 2, 3]
>>> range(3,6)
[3, 4, 5]
>>> range(0,10,2)
[0, 2, 4, 6, 8]
>>> range(10,0,-1)
[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
>>> range(8,2,-2)
[8, 6, 4]
>>> range(8,2)
[]
>>> range(8,2,2)
[]
>>> range(1,5,-1)
[]
>>> range(1,5,-2)
[]

So if you need a function to work similarly to Python's range(), you can use above mentioned solution.

2018: this answer keeps getting upvotes, so here's an update. The code below is obsolete, but luckily ES6 standardized generators and the yield keyword, and they are universally supported across platforms. An example of the lazy range() using yield can be found here.

---

In addition to what's already said, Javascript 1.7+ provides support for iterators and generators which can be used to create a lazy, memory-efficient version of range, simlar to xrange in Python2:

function range(low, high) {  
    return {
        __iterator__: function() {
            return {  
                next: function() {
                    if (low > high)
                        throw StopIteration;  
                    return low++;
                }
            }
        }
    }
}

for (var i in range(3, 5))  
  console.log(i); // 3,4,5

Fusing together both answers from @Tadeck and @georg, I came up with this:

function* range(start, stop, step = 1) {
    if (stop == null) {
        // one param defined
        stop = start;
        start = 0;
    }

    for (let i = start; step > 0 ? i < stop : i > stop; i += step) {
        yield i;
    }
}

To use it in a for loop you need the ES6/JS1.7 for-of loop:

for (let i of range(5)) {
    console.log(i);
}
// Outputs => 0 1 2 3 4

for (let i of range(0, 10, 2)) {
    console.log(i);
}
// Outputs => 0 2 4 6 8

for (let i of range(10, 0, -2)) {
    console.log(i);
}
// Outputs => 10 8 6 4 2