Why Does jQuery Use `(function(window, undefined){...})(window);`?

Exploring the Syntax of JavaScript Encapsulation in jQuery

When examining the jQuery 1.4 source code, one may encounter the following encapsulating syntax:

(function( window, undefined ) {

  //All the JQuery code here 
  ...

})(window);

This syntax raises questions about the purpose of the "undefined" variable and the repeated appearance of "window."

The Meaning of "undefined"

The "undefined" variable within the parentheses is not a reserved keyword. Rather, it is a normal variable that can be assigned a new value. In this case, jQuery sets "undefined" equal to "undefined" to ensure that the global "undefined" variable remains truly undefined.

The Significance of "window"

The "window" variable refers to the global JavaScript object. By passing "window" as an argument to the anonymous function, jQuery creates a local reference to the global object. This local reference enhances performance.

When JavaScript searches for a variable, it first checks the local scope. If the variable is not found locally, the search continues to the next scope, and so on, until the global scope is reached. By creating a local reference to the global object, JavaScript can quickly locate "window" without having to search through all the scopes.

This optimization, as described by Nicholas C. Zakas in his article "Speed Up Your JavaScript," significantly improves the execution speed of the jQuery library.

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