Find a value in a Javascript Map

Question

How do I find the first occurrence of a particular value in a Javascript Map? For example:

const m = new Map();
m.set("a", "b");
m.set("c", "d");
m.set("e", "f");
m.set("g", "f");

// Then I want to do something like this:
m.find_value((v) => v === "f");

Ideally it would return the key and value as a tuple (2-element array), i.e. ["e", "f"].

Obviously this is trivial to code manually with a for loop, but is there any neater built in way?

Use the `.entries()` method and then `.find()` (the Array method). — Pointy, May 15 '21 at 12:44
And before you suggest it, no I do not want to make my application unnecessarily slow by turning the map into an array just to search through it. — Timmmm, May 15 '21 at 12:44
No it doesn't. It uses `for of` to iterate through the map. It never turns the map into an array. And iterators do not work as arrays as far as I can tell. — Timmmm, May 15 '21 at 12:46
[It most definitely doesn't!](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/entries) — Timmmm, May 15 '21 at 12:46
Ah. Well, I stand corrected. That's actually pretty cool and good to know. — Pointy, May 15 '21 at 12:47

score 1 · Accepted Answer · answered May 15 '21 at 12:44

There does not seem to be a better way than writing it myself, like a commoner.

function find_map_value<K, V>(m: Map<K, V>, predicate: (v: V) => boolean): [K, V] | undefined {
  for (const [k, v] of m.entries()) {
    if (predicate(v)) {
        return [k, v];
    }
  }
  return undefined;
}

score -2 · Answer 2 · answered May 15 '21 at 12:51

const m = new Map();
m.set("a", "b");
m.set("c", "d");
m.set("e", "f");
m.set("g", "f");

Map.prototype.find_value = function (predicate) { 
  return Array.from(this.entries())
    .find(([, v]) => predicate(v)) }

// Then I want to do something like this:
const result = m.find_value((v) => v === "f");

console.log(result)

Find a value in a Javascript Map

2 Answers2