What is supposed to happen is that ‘named elements’ are added as apparent properties of the
document object. This is a really bad idea, as it allows element names to clash with real properties of
IE made the situation worse by also adding named elements as properties of the
window object. This is doubly bad in that now you have to avoid naming your elements after any member of either the
document or the
window object you (or any other library code in your project) might want to use.
It also means that these elements are visible as global-like variables. Luckily in this case any real global
function declarations in your code shadow them, so you don’t need to worry so much about naming here, but if you try to do an assignment to a global variable with a clashing name and you forget to declare it
var, you’ll get an error in IE as it tries to assign the value to the element itself.
It’s generally considered bad practice to omit
var, as well as to rely on named elements being visible on
window or as globals. Stick to
document.getElementById, which is more widely-supported and less ambiguous. You can write a trivial wrapper function with a shorter name if you don’t like the typing. Either way, there’s no point in using an id-to-element lookup cache, because browsers typically optimise the
getElementById call to use a quick lookup anyway; all you get is problems when elements change
id or are added/removed from the document.
Opera copied IE, then WebKit joined in, and now both the previously-unstandardised practice of putting named elements on
document properties, and the previously-IE-only practice of putting them on
window are being standardised by HTML5, whose approach is to document and standardise every terrible practice inflicted on us by browser authors, making them part of the web forever. So Firefox 4 will also support this.
What are ‘named elements’? Anything with an
id, and anything with a
name being used for ‘identifying’ purposes: that is, forms, images, anchors and a few others, but not other unrelated instances of a
name attribute, like control-names in form input fields, parameter names in
<param> or metadata type in
names are the ones that should be avoided in favour of