One thing I've always longed for and ached for is some support for hashing. Specifically, let me track metadata about an object without needing to add an expando property on that object.
Java provides Object.getHashCode() which, by default, uses the underlying memory address; Python provides id(obj) to get the memory address and hash(obj) to be customizable; etc. Javascript provides nothing for either.
For example, I'm writing a Javascript library that tries to unobtrusively and gracefully enhance some objects you give me (e.g. your <li> elements, or even something unrelated to the DOM). Let's say I need to process each object exactly once. So after I've processed each object, I need a way to "mark it" as seen.
Ideally, I could make my own hashtable or set (either way, implemented as a dictionary) to keep track:
var processed = {};
function process(obj) {
var key = obj.getHashCode();
if (processed[key]) {
return; // already seen
}
// process the object...
processed[key] = true;
}
But since that's not an option, I have to resort to adding a property onto each object:
var SEEN_PROP = "__seen__";
function process(obj) {
if (obj[SEEN_PROP]) { // or simply obj.__seen__
return; // already seen
}
// process the object...
obj[SEEN_PROP] = true; // or obj.__seen__ = true
}
But these objects aren't mine, so this makes my script obtrusive. The technique is effectively a hack to work around the fact that I can't get a reliable hash key for any arbitrary object.
Another workaround is to create wrapper objects for everything, but often you need a way to go from the original object to the wrapper object, which requires an expando property on the original object anyway. Plus, that creates a circular reference which causes memory leaks in IE if the original object is a DOM element, so this isn't a safe cross-browser technique.
For developers of Javascript libraries, this is a recurring issue.