Understanding undefined as argument

Question

I see javascript libraries set up like this:

   (function (global, undefined) {
      //..
    })(this)

What is the meaning of undefined in this context?

Answer 1

In older browsers, undefined wasn't immutable or non-writable. Setting undefined as a name for an unused function parameter meant that it was undefined, even if it got set in a higher scope.

// Old browser
undefined = 1;
undefined; // 1

// Modern browser
undefined = 1;
undefined; // undefined

It should be noted that the non-writability is only applicable in the global scope, in both cases the following is true;

(function () {         // creating a new scope means..
    var undefined = 1; // this is now different to the `undefined` global
    return undefined;  // so can be written to, e.g. set as 1
}()); // 1

Answer 2

Setting undefined as a parameter of the function allow minifiers to replace it by a shorter name.

For example this:

(function (global, undefined) {
    alert(undefined)
})(this)

May be minified to this:

(function(a,b){alert(b)})(this)

So all occurrences of the undefined variable inside the function will be renamed to some short name like b. Irrespective of the variable name, it'll have the same value, and so will serve the same purpose.