Generating a Range of Numbers in JavaScript

May 20, 2018

Have you ever wanted to generate a range of numbers in JavaScript from 0 to a specified bound?

Let's say you wanted an array containing numbers from 0 to 3. In other languages, you may be able to do something similar to the following:

range(0, 3)

Unfortunately, JavaScript does not natively support a similar construct. However, we can use can use a library like Lodash or write our own implementation. In this post, I will go over implementing it ourselves.

Keys and Spread

const numbers = [...Array(4).keys()]

console.log(numbers); // [0, 1, 2, 3]

Let's dissect this.

Array(4) creates a new array with 4 empty slots. Calling keys() on the new array will return an array iterator that will yield the key at each index. In an array, the key will be the index position.

Afterwards, we "consume" the iterator using the spread operator.

There are a few caveats to this method that you need to be aware of. First, this method will always start at 0 so you cannot specifiy the lower bound without more manipulation. Similarly, the number you pass into Array() is inclusive; meaning that it is also included in the range.

A Cool Trick

What if you could just do this?

const numbers = [...3]

console.log(numbers); // [0, 1, 2, 3]

Well, you can! Here's how:

Number.prototype[Symbol.iterator] = function *range() {
  for (let i = 0; i <= this; i++) {
    yield i;
  }
}

Here, we are modifying the iterator property on the Number prototype. We are assigning our own generator to the iterable property that will yield an iterator that counts from 0 to the specified number.

Check out the resources in the previous section because we are incorporating all those concepts in this example.

I have to mention that modifying a primitive's prototype is not recommended. I would probably not use this in production but it is a pretty awesome trick to show at parties.

Do not ask what kind of parties I attend. 🙂

Closing Thoughts

I am certain that there are more ways of generating a range of numbers out there. In my opinion, this is one of the beauties of programming. There are many ways of approaching a problem and you are able to craft your own awesome solution!

Copyright © 2017 - 2018 David Tran