Return from a for loop but keep loop running

Question

I have a for loop that needs to return something on every iteration:

for(var i=0;i<100;++i) {
    return i;
}

but return breaks the loop. How can I return but kee the loop running?

You can't. What you can do however, is push the return value to an array for example. Can you be more specific on what you want to achieve? — stlvc, May 15 '13 at 12:59
Depending on your target system, [`yield`](https://developer.mozilla.org/en-US/docs/JavaScript/New_in_JavaScript/1.7#Generators) could be a solution ([so-question](http://stackoverflow.com/questions/2282140/whats-the-yield-keyword-in-javascript)). — Yoshi, May 15 '13 at 13:02
Return something to _where?_ By definition a function only returns one "thing", though that "thing" may be an array or object that contains multiple values. (Unless `yield` works for your situation.) — nnnnnn, May 15 '13 at 13:13

score 13 · Answer 1 · answered May 15 '13 at 12:59

13

Store it inside an array.

var array = [];
for (var i = 0; i < 100; ++i) {
    array.push(i);
}

answered May 15 '13 at 12:59

Niccolò Campolungo

11,824
4
32
39

5

@Doorknob This is not Java. – Geeky Guy May 15 '13 at 13:00
thought as much. I can't use an array in this instance. – Mike Rifgin May 15 '13 at 13:00
@MikeRifgin Then what do you want it to return? – tckmn May 15 '13 at 13:01

score 5 · Answer 2 · answered May 15 '13 at 13:00

The context here is unclear. If you are trying to execute a function for each iteration of the loop, why not something simple like calling a function from within the loop?

for(var i=0;i<100;++i) {
    processIteration(i)
}

In processIteration you could simply handle what you need to happen with that value.

score 2 · Accepted Answer · answered May 15 '13 at 12:59

2

Store the values you need to return in an array, then after the loop, return the array.

answered May 15 '13 at 12:59

Geeky Guy

9,229
4
42
62

score 0 · Answer 4 · answered Aug 15 '18 at 05:42

There are 3 ways to solve this

Promises

function loop() {
  promises = []
  for (var i = 0; i < 100; ++i) {
    // push a promise resolve to a promises array
    promises[i] = Promise.resolve(i);
  }
  return promises;
}

// Observe the promises loop
for (let p of loop()) {
  p.then(console.log)
}

Callbacks

function loop(cb) {
  for(var i = 0; i < 100; ++i) {
    // calling the callback 
    cb(i);
  }
}

// Pass a do something method
// Here it's the console log as the callback
loop(console.log)

Yield

// Generator function for loop over 100
function* loop() {
  for (var i = 0; i < 100; ++i) {
      // return value from loop 
      yield i;
  }
}

// Initialize the generator function
let it = loop();

// Call and get the resulr
// it.next().value
var result = it.next();
while (!result.done) {
  console.log(result.value); // 1 2 3 .... 99
  result = it.next();
}

Return from a for loop but keep loop running

4 Answers4

Linked