如何保证在函数返回之前保存异步返回值？

Question

As I understand it the 100 asyncFunctions in the code below will not be executed until after func has returned true, and at this point the referens to i will not be valid. Nothing in this code would work as expected I guess.

Psedo example code:

function func(){
  var needToKnow = []
  for i = 1 to 100 {
    needToKnow[i] = asyncFunction( i )
  }
  //Do some work on needToKnow[i]
  return true
}

What would be the Javascript way to do something like this?

Answer 1

Use callbacks :

function func(callback) {
    var needToKnow = [],
        max = 100;

    for (var i = 0; i < max; i++) {
        asyncFunction(i, function (result) {
            needToKnow.push(result);

            if (needToKnow.length == max) { // or something that let you know that its finished
                callback(needToKnow);
            }
        });
    }
}

function asyncFunction(i, callback) {
    setTimeout(function () {
        callback({ index: i });
    }, 1000); // Im an async func!
}

And use it this way:

func(function (result) {
    console.log(result);
});

Be careful, don't get in callback hell

Answer 2

Here is an example using the Q Promise library:

function functionThatCouldThrowError(i){
  //It doesn't, but just to give an idea of error propagation.
  return { index: i };
}

function asyncFunction(i) {
  var deferred = Q.defer();
    setTimeout(function () {
      try{
        var data = functionThatCouldThrowError(i);
        deferred.resolve(data);
      } catch (error) {
        deferred.reject({ index: i, error: error });
      }
    }, 1000);
    return deferred.promise;
}

function doAll() {
  var needToKnow = []
  for (var i = 0; i < 100; i++) {
    needToKnow[i] = asyncFunction( i );
  }
  return Q.all(needToKnow);
}

doAll().then(function(arg) {
  //arg contains all 100 elements
  alert("All done");
})

Update: expanded the example to demonstrate how to handle errors. Plunker: http://plnkr.co/edit/djWpTKxgvzK2HmkVwvTy?p=preview