How to break from inside a callback in a for-loop

Question

If I bind the function find in javascript, I can do the following:

const _array = [4,10,6,5,20];

const loop = Array.prototype.find.bind(_array);
const r = loop(function (x) {
    return x === 6;
});

console.log(`Final result => ${r}`); // here prints: Final result => 6

As you can see, in the binded loop function I have a callback returned from find. Everything works and it's ok ...

But, trying to simulate something like that I ended with this:

function loop(a,callback) {
    for(i=0;i<a.length;i++)
        callback(a[i]);
};

const r = loop([4,10,6,5,20], function (x) {
    console.log("x value", x);
    return x===6; // need to break the loop function and return to 'r' the 'x' value
});

console.log(`Final result => ${r}`); // here would print the value of x, that would be 6

and I get:

x value 4
x value 10
x value 6
x value 5
x value 20
undefined

what means that the return x===6 inside the r function is not working correctly, because the for-loop continues to the end.

So, my question:

How can I break the loop function when x===6 and return the value of x ?

Uh, your `loop` function behaves like `forEach`, not like `find`? — Bergi, Sep 27 '18 at 19:01
So you're essentially asking "*[How to implement `find`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/find#Polyfill) myself?*", right? — Bergi, Sep 27 '18 at 19:10
@Bergi Not exactly, just I took the `find` function as an example, but my question is about on how to return from the callback inside the `for-loop`. That's it. — robe007, Sep 27 '18 at 19:13
The answer to that question is: you can't in general. The callback doesn't control how often it is called or what `loop` returns, it also cannot just "jump out" of control flow (except you're considering exceptions, but even those can be caught). The only way is to make the `loop` function react to the return value of the callback call. — Bergi, Sep 27 '18 at 19:20
@Bergi Yeah sure, that's why the use of an `if` in the answer below. And talking about the use of `exceptions`, I was reading that they are used in a `foreach`, because inside of it I can't return. See [here](https://stackoverflow.com/a/2641374/2954267) — robe007, Sep 27 '18 at 19:26

score 2 · Accepted Answer · answered Sep 27 '18 at 18:55

2

Check what value is returned by the callback, and then decide whether to continue or not:

function loop(a, callback) {
  for (let i = 0; i < a.length; i++) {
    const found = callback(a[i]);
    if (found) {
      return a[i];
    }
  }
}

const r = loop([4,10,6,5,20], function (x) {
  console.log("x value", x);
  return x===6;
});

console.log(`Final result => ${r}`);

answered Sep 27 '18 at 18:55

Nicholas Tower

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2

`function loop (a, callback) { for (const value of a) if (callback(value)) return value }` – Patrick Roberts Sep 27 '18 at 18:59
Yeah, sure, that's it. Thanks ! – robe007 Sep 27 '18 at 19:08
@PatrickRoberts Nice one, if you write it as an answer, I'll upvoted it ! – robe007 Sep 27 '18 at 19:11
@robe007 it's really not that different than this answer, which is why I put it in a comment instead. It's just a little more terse. – Patrick Roberts Sep 27 '18 at 19:14

Mulan · Answer 2 · 2018-09-27T19:11:17.027

1

You can also write find using recursion

const find = (f, [ x, ...xs ]) =>
  x === undefined
    ? null
  : f (x) === true
    ? x
  : find (f, xs)

console.log
  ( find
      ( x => x > 8
      , [ 5, 7, 9, 3, 1 ]
      )
      // 9
      
  , find
      ( x => x < 4
      , [ 5, 7, 9, 3, 1 ]
      )
      // 3
  )

Instead of destructuring assignment, an index parameter can be used

const find = (f, xs = [], i = 0) =>
  i >= xs.length
    ? null
  : f (xs[i]) === true
    ? xs[i]
  : find (f, xs, i + 1)

In both cases, iteration through the array stops as soon as f returns true

edited Sep 27 '18 at 19:11

answered Sep 27 '18 at 19:04

Mulan

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I think your parameters need to be swapped. The `array` in the question is the first parameter, not the second. Also recursion is going to cause a stack overflow error for large arrays, so I don't recommend this approach. – Patrick Roberts Sep 27 '18 at 19:17
PS there's [many ways](https://stackoverflow.com/a/43596323/633183) to avoid overflowing the stack – Mulan Sep 27 '18 at 20:18

How to break from inside a callback in a for-loop

2 Answers2