Calling functions with setTimeout()

Question

Simply put...

why does

setTimeout('playNote('+currentaudio.id+', '+noteTime+')', delay);

work perfectly, calling the function after the the specified delay, but

setTimeout(playNote(currentaudio.id,noteTime), delay);

calls the function playNote all at the same time?

(these setTimeout()s are in a for loop)

or, if my explanation is too hard to read, what is the difference between the two functions?

Answer 1

Try this.

setTimeout(function() { playNote(currentaudio.id,noteTime) }, delay);

Answer 2

Because the second one you're telling it to call the playNote function first and then pass the return value from it to setTimeout.

Answer 3

The first form that you list works, since it will evaluate a string at the end of delay. Using eval() is generally not a good idea, so you should avoid this.

The second method doesn't work, since you immediately execute a function object with the function call operator (). What ends up happening is that playNote is executed immediately if you use the form playNote(...), so nothing will happen at the end of the delay.

Instead, you have to pass an anonymous function to setTimeout, so the correct form is:

setTimeout(function() { playNote(currentaudio.id,noteTime) }, delay);

Note that you are passing setTimeout an entire function declaration, so it will hold on to the declaration and only execute it at the end of the delay.

You can also pass setTimeout a reference, since a reference isn't executed immediately, but then you can't pass arguments:

setTimeout(playNote, delay);

---

Note:

For repeated events you can use setInterval() and you can set setInterval() to a variable and use the variable to stop the interval with clearInterval().

You say you use setTimeout() in a for loop. In many situations, it is better to use setTimeout() in a recursive function. This is because in a for loop, the variables used in the setTimeout() will not be the variables as they were when setTimeout() began, but the variables as they are after the delay when the function is fired.

Just use a recursive function to sidestep this entire problem.

Using recursion to deal with variable delay times:

  // Set original delay
var delay = 500;

  // Call the function for the first time, to begin the recursion.
playNote(xxx, yyy);

  // The recursive function
function playNote(theId, theTime)
{
    // Do whatever has to be done
    // ...

    // Have the function call itself again after a delay, if necessary
    //   you can modify the arguments that you use here. As an
    //   example I add 20 to theTime each time. You can also modify
    //   the delay. I add 1/2 a second to the delay each time as an example.
    //   You can use a condition to continue or stop the recursion

    delay += 500;

    if (condition)
    { setTimeout(function() { playNote(theID, theTime + 20) }, delay); }
}

Answer 4

Don't use string-timeouts. It's effective an eval, which is a Bad Thing. It works because it's converting currentaudio.id and noteTime to the string representations of themselves and hiding it in the code. This only works as long as those values have toString()s that generate JavaScript literal syntax that will recreate the value, which is true for Number but not for much else.

setTimeout(playNote(currentaudio.id, noteTime), delay);

that's a function call. playNote is called immediately and the returned result of the function (probably undefined) is passed to setTimeout(), not what you want.

As other answers mention, you can use an inline function expression with a closure to reference currentaudio and noteTime:

setTimeout(function() {
    playNote(currentaudio.id, noteTime);
}, delay);

However, if you're in a loop and currentaudio or noteTime is different each time around the loop, you've got the Closure Loop Problem: the same variable will be referenced in every timeout, so when they're called you'll get the same value each time, the value that was left in the variable when the loop finished earlier.

You can work around this with another closure, taking a copy of the variable's value for each iteration of the loop:

setTimeout(function() {
    return function(currentaudio, noteTime) {
        playNote(currentaudio.id, noteTime);
    };
}(currentaudio, noteTime), delay);

but this is getting a bit ugly now. Better is Function#bind, which will partially-apply a function for you:

setTimeout(playNote.bind(window, currentaudio.id, noteTime), delay);

(window is for setting the value of this inside the function, which is a feature of bind() you don't need here.)

However this is an ECMAScript Fifth Edition feature which not all browsers support yet. So if you want to use it you have to first hack in support, eg.:

// Make ECMA262-5 Function#bind work on older browsers
//
if (!('bind' in Function.prototype)) {
    Function.prototype.bind= function(owner) {
        var that= this;
        if (arguments.length<=1) {
            return function() {
                return that.apply(owner, arguments);
            };
        } else {
            var args= Array.prototype.slice.call(arguments, 1);
            return function() {
                return that.apply(owner, arguments.length===0? args : args.concat(Array.prototype.slice.call(arguments)));
            };
        }
    };
}