Extend JavaScript Array prototype with method to iterate entries in reverse?

Question

Putting aside concerns about modifying JavaScript's Array.prototype for now, how would I go about writing a custom method on the Array prototype that returns an iterator in reverse? Specifically, I'd like to provide an entriesReverse() method that does exactly what Array.prototype.entries() does except in reverse order. I've got as far as a bespoke function:

const entriesReverse = (array) => ({
    [Symbol.iterator]() {
        let index = array.length;
        return {
            next: () => ({
                value: [--index, array[index]],
                done: index < 0
            })
        }
    }
})

but I'll be darned if I can figure out how to implement this as a method on the existing Array prototype. The problem seems to be trying to refer to 'this' inside the Symbol.iterator block - it doesn't refer to the array itself but to the iterator. Any thoughts?

Don't use an arrow function if you want to refer to the dynamic this argument. — Bergi
– Bergi, Commented Nov 19, 2021 at 0:16

CertainPerformance · Accepted Answer · 2021-11-19 00:56:51Z

0

It'll be easier to use a generator function. Call the original .entries() and turn it into an array of the entries, then iterate backwards over that array and yield each item.

Array.prototype.entriesReverse = function*() {
  const entries = [...this.entries()];
  for (let i = entries.length - 1; i >= 0; i--) {
    yield entries[i];
  }
};

for (const entry of ['a', 'b', 'c'].entriesReverse()) {
  console.log(entry);
}

Another option:

Array.prototype.entriesReverse = function*() {
  yield* [...this.entries()].reverse();
};

for (const entry of ['a', 'b', 'c'].entriesReverse()) {
  console.log(entry);
}

edited Nov 19, 2021 at 0:56

answered Nov 18, 2021 at 23:22

CertainPerformance

373k55 gold badges354 silver badges359 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Andrew Over a year ago

Huh. That works. Inspired by your use of a generator, I came up with something even more concise:

Array.prototype.entriesReverse = function*() {   for(let index = this.length - 1; index >= 0; index--) {     yield [index, this[index]];   } }

Which appears to work just fine. :)

Collectives™ on Stack Overflow

Extend JavaScript Array prototype with method to iterate entries in reverse?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related