javascript - setTimeout return

Question

How can I use setTimeout if I want to return a value

$.each(pCodes, function(index, pCode) {
    setTimeout(func(parm1), 2000);      
});


function func(in)         


        

Answer 1

First of all, make sure you pass to setTimeout a function, in your example you passed undefined to it, as you func(param1) executes func directly. What you want is something like this:

setTimeout(function() { func(parm1); }, 2000);

And for 'returning' the value: Use some kind of callback function that is executed with the value when timeout expired. Like so:

function callback(value) {
  //  doSomethingWithNewValue
}

$.each(pCodes, function(index, pCode) {
    setTimeout(function() { func(parm1, callback); }, 2000);      
});


function func(in, callback)
{
  var value = 999;
  callback(value);
}

This is the general pattern used in such scenario (see event driven programming).

Answer 2

First of all, your call to setTimeout is wrong. You are calling the function func and then using the result in the setTimeout method. Your code is equivalent to:

$.each(pCodes, function(index, pCode) {
  var temp = func(parm1);
  setTimeout(temp, 2000);      
});

As func returns 999, you will be doing setTimeout(999, 2000), which of course doesn't make sense. To call a function that takes a parameter from setTimeout you need a function that makes that function call:

$.each(pCodes, function(index, pCode) {
  setTimeout(function() { func(parm1); }, 2000);
});

To handle the return value from func is a bit more complicated. As it's called later on, you have to handle the return value later on. Usually that is done with a callback method that is called when the return value is available:

var callback = function(value) {
  // Here you can use the value.
};
$.each(pCodes, function(index, pCode) {
  setTimeout(function() { func(parm1, callback); }, 2000);
});

function func(in, callback) {
  var value = 999;
  callback(value);
}

Answer 3

It's pretty ugly, but you can use output parameters, since js objects are pass by reference:

function a() {
    var param1 = 42;
    var result = {};
    b(param1, result);
}

function b(val, output) {
    something();
    output.returned = 4;
}

Or, you can use a callback (the better option):

function a() {
    var param1 = 42;
    b(param1, function (newVal) {
        something();
    });
}

function b(val, callback) {
    //something
    callback(4);
}

By the way, your call to setTimeout is wrong. setTimeout receives a function as a first parameter, and a delay as a second - the first argument is still seen as regular javascript code, so it evaluates it, so your setTimeout call turns out to be like this:

setTimeout(999, 2000);

Since you're returning 999 from the function.

However, setTimeout can also receive a list of arguments after the second parameter, so it can be turned into this:

setTimeout(func, 2000, param1);

Answer 4

change it to :

var defValue;

$.each(pCodes, function(index, pCode) {
    setTimeout(function(){defValue=func(parm1)}, 2000);      
});

this way you can use the defValue in your function to access the returned value