Question

On MDN’s page for setTimer, there is a little shim / compatibility layer for setTimer that will let Internet Explorer accept additional arguments in the setTimer method that will be passed to the callback.

I pretty much understand all of the code below:

if (document.all && !window.setTimeout.isPolyfill) {
  var __nativeST__ = window.setTimeout;
  window.setTimeout = function (
                          vCallback, 
                          nDelay /*, 
                          argumentToPass1, 
                          argumentToPass2, etc. */
                          ) {
    var aArgs = Array.prototype.slice.call(arguments, 2);
    return __nativeST__(vCallback instanceof Function ? function () {
      vCallback.apply(null, aArgs);
    } : vCallback, nDelay);
  };
  window.setTimeout.isPolyfill = true;
}

Except for one line:

var aArgs = Array.prototype.slice.call(arguments, 2);

It references arguments, but I cannot see that name being referenced anywhere before this line. It is not on the Reserved words list either, so it does not seem to be magic in any way. In order for me to make any sense out of it, it must somehow refer to the arguments of the overridden setTimeout function, and then use slice() to get every argument after the first two.

Answer 1

The object arguments holds all the arguments that were passed to a function including those, that were not named in the function declaration.

Remember, that assuming the following function

function doStuf( param1 ) { /* do something */ }

it is also valid to make such a call

doStuff( 'stuff', 'morestuff', 2, 'evenmorestuff' );

In that case you could reference all parameters using the arguments object.

---

So in your particular code, the following row

var aArgs = Array.prototype.slice.call(arguments, 2);

copies all arguments passed to the shim-function, but the first two. The first two, however, were explicitly named and are referenced as such (vCallback and nDelay).

• MDN documentation