JavaScript object iterator and "..."

Question

Hello, I was looking for way how to make range function in js and I found this: "...Array(5).keys()"
I was trying to figure out what does it mean, so I loged keys() and it returned object iterator, so my question is, what is it and why it doesn't return array? Those 3 dots than make array from it

Re: those 3 dots iterate trough it

Answer 1

You're familiar with the range function from Python and want something like it in JavaScript, right?

JavaScript lets you define generators which work like generators from Python.

Here is a range function that is similar to Python's except calling this range like range(5) won't do what you expect from Python. You'd have to pass more parameters to this one for a similar effect.

function* range(start = 0, end = 100, step = 1) {
let iterationCount = 0;
for (let i = start; i < end; i += step) {
iterationCount++;
yield i;
}
return iterationCount;
}

for (var j of range(0, 5)) {
console.log(j);
}

You could use the spread operator(...) with this range generator too.

The Array(5) creates an Array with 5 empty elements in it. The keys method returns the keys from the Array. Since the 5 array indexes would be in the range of 0,1,2,3,4, keys() returns an iterator for [0,1,2,3,4]. keys() doesn't return an Array. An iterator is also what the above generator produces.

Answer 2

Thanks, and yeah, it's idea from python, I mostly use for (let i = 0; i < number; i++) which is quite effective and simple usable in any case