index( undefined ) returning incorrect values

[Motty]

The index of an undefined element should return -1, but instead returns an incorrect value (​http://jsfiddle.net/Mottie/FMh69/)

// jQuery ver = value returned
// 1.6.4   = 2
// 1.7.2   = 2
// 1.8.3   = 2
// 1.9.0   = 0
// 2.0.0b1 = 0
var undef;
console.log( $('div').index( undef ) );

[dmethvin]

I feel it's important to point out that an undefined value is not one of the four things that .index() is documented to take: no arguments, a selector string, a DOM element, or a jQuery collection. So there is no "should" here, although we might decide to define new behavior. It looks like up until 1.9 it was being interpreted as the "no arguments" case. -- Correction, it *is* the no-args case but there was a bug before 1.9.

[gibson042]

The behavior change you observe stems from fixing #10977, but the invalid input undefined was treated like "no argument" (returns position in parent of the collection's first member) even before then. You probably want to do something like element ? $collection.index( element ) : -1.

[Motty]

I was just troubleshooting some code that was passing an undefined variable to the index. I'll just make sure to follow gibson042's recommendation from now on, thanks! :)