jQuery ID Selector (“#id”)

For id selectors, jQuery uses the JavaScript function document.getElementById(), which is extremely efficient. When another selector is attached to the id selector, such as h2#pageTitle, jQuery performs an additional check before identifying the element as a match.

Calling jQuery() (or $()) with an id selector as its argument will return a jQuery object containing a collection of either zero or one DOM element.

Each id value must be used only once within a document. If more than one element has been assigned the same ID, queries that use that ID will only select the first matched element in the DOM. This behavior should not be relied on, however; a document with more than one element using the same ID is invalid.

If the id contains characters like periods or colons you have to escape those characters with backslashes.