How to interrupt/break/stop forEach in ES6 Map?

Question

Is there a nice way (except using JS exceptions) to stop forEach loop in ES6 Map object (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/forEach )

From example provided on MDN - is there a way to stop enumeration on 'bar' (skip bar):

function logMapElements(value, key, map) {
    console.log(`m[${key}] = ${value}`);
}
new Map([['foo', 3], ['bar', {}], ['baz', undefined]]).forEach(logMapElements);

For the people who suggest to close this question: yes, it is similar to the questions about Array.prototype.forEach.
But at the same time is different: most of the suggested answers won't work with ES6 set and map. Only throwing the exception will work, but I ask for some other ways

It is similar, but at the same time is different. Most of suggested answers won't work with ES6 set and map. Only throwing the exception will work, but I ask for some other ways. — vmg, Mar 10 '17 at 22:56

score 10 · Answer 1 · edited May 29 '18 at 19:43

10

There is no good reason to use forEach any more in ES6. You should use iterators and for … of loops from which you can ordinarily break:

const m = new Map([['foo', 3], ['bar', {}], ['baz', undefined]]);
for (let [key, value] of m) {
    console.log(`m[${key}] = ${value}`);
}

edited May 29 '18 at 19:43

John Jiang

11,069
12
51
60

answered Mar 10 '17 at 23:29

Bergi

630,263
148
957
1,375

IE11 does not support iterators – vmg Mar 11 '17 at 20:03
@vmg Either switch to Edge or use a transpiler then :-) – Bergi Mar 11 '17 at 23:00
1

@JDB `forEach` does *not* support chaining, it returns `undefined`. It is only suitable for side effects in the callback, and for side effects `for … of` is the better solution. For functional programming, you'll be looking for `map`, `filter` and the others that build on `reduce`. – Bergi Mar 13 '17 at 16:03
@vmg It also doesn't support forEach, or does it? – Mathijs Segers Apr 12 '18 at 12:13

dhilt · Answer 2 · 2021-01-15T00:52:46.507

For those who don't want for-of, the following approach can be applied:

const length = myMap.size;
const iterator = myMap.entries();
for (let i = 0; i < length; i++) {
  const item = iterator.next().value;
  console.log(`myMap[${itemkey}] = ${value}`);
}

A benefit that I have personally encountered is that this approach produces less code during es5 transpilation (using Typescript transpiler with all its __values and __read definitions)...

How to interrupt/break/stop forEach in ES6 Map?

2 Answers2

Linked