This question already has an answer here:
Going around Modernizr source code I have found strange thing:
window.Modernizr = (function(window,document,undefined) {
After that this function will be called with two arguments (this, this.document).
Why undefined is used here? Is it meaningful or it's just strange code-style?
Creates a local undefined in case someone redefines undefined somewhere in the scope chain.
Also provides a faster lookup since undefined is a global property, so the scope chain traversal is reduced.
来源：https://stackoverflow.com/questions/4650442/javascript-function-with-undefined-parameter