Question

All this is one expression. What exactly gets put inside div in the end? What’s it called when you have a bunch of comma delimited expressions?

var div = document.createElement( "div" ),
    documentElement = document.documentElement,
    all,
    a,
    select,
    opt,
    input,
    tds,
    events,
    eventName,
    i,
    isSupported;

Answer 1

It is just the declaration of a bunch of variables inside a function scope. The first two variables are also initialized with their declaration. You can declare/initialize multiple variables at once by separating each with a comma like is done here.

In other uses, the comma character is an operator in javascript. From MDN: The comma operator evaluates both of its operands (from left to right) and returns the value of the second operand.

The variable div is initialized here to an empty div tag. There is nothing inside it yet until later code uses it. The code following this in jQuery, puts some content into that div and then runs a series of test operations on it to do feature tests and see what features are supported in the current browser.