javascript - get index in Map.prototype.forEach()

Question

Map.prototype.forEach takes callback with two params: value and key. Is it possible to get the index of each entry, similar to Array.prototype.forEach((value, index) => {})

Answer 1

Map.prototype.forEach takes callback with two params: value and key.

No, it's invoked with three arguments, just like Array#forEach. The third is the map.

Is it possible to get the index of each entry, similar to Array.prototype.forEach(functcion(value, index) => {})

(Fairly sure the functcion part of that wasn't meant to be there.)

That's what key is. There is no separate "index."

Map's iteration order is defined for various iteration operations, but there's no direct iteration construct that gives you the index (as opposed to key) of an entry in that order. The order is original key insertion order, so for instance:

const m = new Map();
m.set("x", 1);
m.set("q", 2);
m.set("z"), 3);
m.set("x", "one");

If we loop through that map, we'll see the keys in the order the key was first added, so "x", "q", "z". Note that changing the value of the entry with the ke "x" didn't move it to the end.

Map#forEach (and for-of and anything else using the map's iterator) follows the iteration order, so if you're thinking of the index in terms of that order, you could track the index yourself:

const m = new Map();
m.set("one", "uno");
m.set("two", "due");
m.set("three", "tre");
let index = 0;
m.forEach((value, key) => {
  console.log(index++, key, value);
});

Alternately, you can get an array of the map entries (as [key, value] arrays):

let entries = Array.from(m.entries());

...and use the indexes of that array:

I'm just not sure what it buys you. :-)

Answer 2

I don't think so. Map object is a key/value map. To me at least the idea of an index doesn't make much sense, given that the order of the key/value pairs should not be guaranteed in a Map. If you are just curious how many pairs you have iterated on so far, you could just keep a counter that you mutate on each call, something like:

mapObj.forEach(() => {
    let i=0;
    return (key, value) => {
        // whatever you are doing with each key/value
        i += 1;
    }
});

It is not as elegant as an index as parameter, but would at least meet your needs.