Discussion on: Why is using javascript “for loop” for array iteration a bad idea?

[Adam Crockett 🌀]

But our for loop iterates over the empty slot, that is bad for our software performance.

This is a code smell it's not much to do with for loops being bad they are doing exactly what you would expect.

It's better to sanitise your inputs such as

const arr = new Array(10); // even making an array like this is broken in JavaScript

arr.length // 10
arr.push('hi');
const cleanArr = (arr.filter(Boolean)).length // 1

// Then a loop is fine either way

Or better, don't make 10 empty slots ☺️

[peerreynders]

It doesn't matter what function you pass to filter() - it skips holes.

const list = [, , { level: { deep: 3 } }, ,];
const predicate = (o) => o.level.deep > 0;

console.log(list.filter(predicate));
// [{ level: { deep: 3}}]

ECMAScript 6: holes in Arrays.

[Adam Crockett 🌀]

I imagine you misunderstood my code

[peerreynders]

I imagine that you were trying to use

console.log(Boolean(undefined)); // false

to sanitize the array. However observe

// sparse array - i.e. has holes
const a = [, , 2, , ,];
// dense array - no holes
const b = [undefined, undefined, 2, undefined, undefined];

console.log(`a: ${a.length}`); // "a: 5" i.e. last comma is treated as a trailing comma
console.log(`b: ${b.length}`); // "b: 5"

function predicate(value, index) {
  console.log(index);
  return Boolean(value);
}

const af = a.filter(predicate); // 2
const bf = b.filter(predicate); // 0, 1, 2, 3, 4

console.log(af); // [2]
console.log(bf); // [2]

i.e. holes are skipped, actual undefined values are not.

So the function passed to filter() never gets to make a decision about the holes.

sparse arrays vs. dense arrays

[Adam Crockett 🌀]

The intent is to remove All falsey values which I admit is a bit of a hammer and yes I can see your point, filter ignores empties, but it still cleans an array which is my point.

To anyone not aware, This is a well known trick which removes ALL falsey value.

If you want to handle falsey valuess such as 0, -n, false, null, undefined, write a better predicate function, my demo is just that, a quick demo of principal, prepare your inputs.

[Prosen Ghosh]

You are right Adam,

const array = new Array(10); // JUST FOR EXAMPLE, WE DON'T DO IT IN REAL LIFE

no one going to create an empty slot for an array, it can happen accidentally/unintentionally and it can be bad for our application performance that is the whole point of this post.

i have updated my post using a relevant code example. Please take a look.

Thanks for your feedback.

[Adam Crockett 🌀]

Graciously I admit I did learn more foibles about the forEach method in this post, not iterating over empties is not what I expected at all, thanks for bringing it up!