setTimeout Never Executes Function 2

Question

I tried to write asynchronous function calls in javascript using setTimeout

function async(fn, callback) {
setTimeout(function () {
 fn();
 callback();
}, 1000);
}

This is how the function is being called. I want function foo() to run first, and then my next function. Foo() runs successfully but the next function never comes out. Any idea?

async(foo(), function () {
 var check = checkField();
 alert('Check: ' + check); //somehow never comes out
});

Answer 1

You're calling foo in a synchronous way, because instead of passing a reference to foo to the async function, you're calling foo and passing to async the result of foo, which is probably undefined (although just guessing here, because I don't know how foo looks like), so when the timeout hits and async tries to call fn it throws error that undefined is not a function and thus never reaches the line that calls the second callback. You should call async this way:

async(foo, function () {
 var check = checkField();
 alert('Check: ' + check);
});

• ichachan: Thanks but what if there is a parameter for foo and I need to pass that parameter before calling async?
• Rafael: @ichachan In this case use .bind method. async(foo.bind(null, param1, param2), function () {/*here the logic of the second callback */});, where param1 and param2 are the values you want to pass to foo. Documentation on bind
• ichachan: Thanks but for some reason now foo is never called, even though the function runs successfully with the defined parameter. I make it call a dummy function instead and execute foo beforehand like this: foo(parameter); async(dummy), function () { var check = checkField(); alert('Check: ' + check); //now this is working })
• Rafael: @ichachan Without seeing your code it's difficult to say what is the issue. Please see this JSFiddle example which defines two callbacks with pre-defined params. .bind basically defines a dummy function that calls foo.

Answer 2

You should pass foo instead of calling it while calling async:

async(foo, function () {
var check = checkField();
alert('Check: ' + check); //somehow never comes out
});