Question

Why doesn’t the following work in Firefox (v16)?

var t = setTimeout(foo(), 1000);

The error I get in the console is: “useless setTimeout call (missing quotes around argument?)”. Wrapping it in quotes doesn’t seem to do much, except making it render as a string (unsurprisingly).

It does however work fine when I wrap it in an anonymous function like so:

var t =
    setTimeout(function(){
       foo();
     }, 1000);

But why is it necessary? Why doesn’t it explode in Webkit or Opera? Stroke of luck?

Answer 1

When you pass a method to setTimeout(), it will be executed in global scope. this will point to window at execution time. Read more HERE.

If foo is not global, it won’t be found, ergo ReferenceError.

var __nativeST__ = window.setTimeout, __nativeSI__ = window.setInterval;
// just backed up the defaults. Now basically creating timeout and setInterval
//functions that take scope as a parameter,
//so you can use them in whichever invocation context you want.

window.setTimeout = function (vCallback, nDelay /*, argumentToPass1, argumentToPass2, etc. */) {
  var oThis = this, aArgs = Array.prototype.slice.call(arguments, 2);
  return __nativeST__(vCallback instanceof Function ? function () {
    vCallback.apply(oThis, aArgs);
  } : vCallback, nDelay);
};

window.setInterval = function (vCallback, nDelay /*, argumentToPass1, argumentToPass2, etc. */) {
  var oThis = this, aArgs = Array.prototype.slice.call(arguments, 2);
  return __nativeSI__(vCallback instanceof Function ? function () {
    vCallback.apply(oThis, aArgs);
  } : vCallback, nDelay);
};