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Javascript Array(): WAT?

jacoby profile image Dave Jacoby Originally published at jacoby.github.io on ・3 min read

This start with me trying to get something like a range() function in Javascript. Perl, my language of choice, allows me to create an array with my @hedgehog = 0..4, giving me 0,1,2,3,4 as an array.

The long way is with a for loop:

function range(min, max) {
  let x = [];
  for (let i = min; i <= max; i++) {
    x.push(i);
  }
  return x;
}

And if you like recursion, Jason Yu on Dev.to has you covered:

function range(start, end) {
  if(start === end) return [start];
  return [start, ...range(start + 1, end)];
}

Knowing this, I started to think that there must be a way to do something like this with arrow functions.

If we already have an array, we can reset it to a range starting with zero easily:

let hedgehog = [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29];
let shrews = hedgehogs.map((n,i)=>i);
// [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]

So, I start thinking that we can create arrays of specific sizes, like Array(5), so what if we make an array of nulls and map that into a range?

hedgehogs = Array(5);
shrews = hedgehogs.map((n,i)=>i);
console.log(JSON.stringify(shrews));
console.log(JSON.stringify(hedgehogs));
// [null,null,null,null,null]
// [null,null,null,null,null]

So, that’s a thing.

But what thing is it?

map calls a provided callback function once for each element in an array, in order, and constructs a new array from the results. callback is invoked only for indexes of the array which have assigned values, including undefined. It is not called for missing elements of the array (that is, indexes that have never been set, which have been deleted or which have never been assigned a value). (Emphasis Mozilla)

So, Array(5) is an array of nulls that are not just nulls, but never been set nulls, which map will not touch. What to do, what to do…

Fill them.

fill them.

hedgehogs = Array(5);
shrews = hedgehogs.fill().map((n,i)=>i);
console.log(JSON.stringify(shrews));
console.log(JSON.stringify(hedgehogs));
// [0,1,2,3,4]
// [null,null,null,null,null]

I get the default behavior. You can create a big but sparse array and then map over the few elements in it. Why go someplace where there’s no data?

hedgehogs = Array(5);
hedgehogs[2] = 1;
hedgehogs[4] = 2;
hedgehogs.map((n,i)=>{ console.log([i,n].join("|"))})
// 2|1
// 4|2

But it isn’t the behavior I expected.

And, while I normally use arrow functions within map and filter, you don’t have to.

const range3 = (n, i) => i;
hedgehogs = Array(5);
shrews = hedgehogs.fill().map(range3);
console.log(JSON.stringify(shrews));
console.log(JSON.stringify(hedgehogs));
console.log(range3(1,4)) // because they aren't just for arrays
// [0,1,2,3,4]
// [null,null,null,null,null]
// 4

The next step is to make a way to set the first value before creating. But not today.

If you have any questions or comments, I would be glad to hear it. Ask me on Twitter or make an issue on my blog repo.

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jacoby profile

Dave Jacoby

@jacoby

Long time computer user. Long time programmer.

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