You can delete properties of an object while you're running a for loop over
that object. No special tricks are required.
If you delete a property that's already been iterated (including the current
iteration), nothing special happens - the iteration continues on its merry
way. If you delete a property that hasn't yet been iterated, it won't be.
Just for fun, here's a primitive little cache implementation that lets you
add/remove/get items and also prune items from the beginning of the list:
function Cache() {
var itemsByKey = {};
var keysByIndex = [];
this.add = function( key, value ) {
var item = itemsByKey[key];
if( item ) {
item.value = value;
}
else {
itemsByKey[key] = { value: value, index: keysByIndex.length
};
keysByIndex.push( key );
}
};
this.remove = function( key ) {
var item = itemsByKey[key];
if( item ) {
keysByIndex.splice( item.index, 1 );
delete itemsByKey[key];
}
};
this.get = function( key ) {
var item = itemsByKey[key];
return item && item.value;
};
this.prune = function( n ) {
n = Math.min( n, keysByIndex.length );
for( var i = 0; i < n; ++i ) {
delete itemsByKey[ keysByIndex[i] ];
}
keysByIndex.splice( 0, n );
};
// for testing
this.dump = function() {
console.debug( keysByIndex, itemsByKey );
};
}
var c = new Cache;
c.add('a','A'); c.add('b','B'); c.add('c','C'); c.add('d','D');
c.dump();
console.debug( c.get('b') );
c.add('b','BB'); // duplicate key
console.debug( c.get('b') );
c.dump();
c.remove('b');
console.debug( c.get('b') );
c.dump();
c.prune(2);
c.dump();
Pasted into the Firebug multiline console, this should dump:
["a", "b", "c", "d"] Object a=Object b=Object c=Object d=Object
B
BB
["a", "b", "c", "d"] Object a=Object b=Object c=Object d=Object
undefined
["a", "c", "d"] Object a=Object c=Object d=Object
["d"] Object d=Object
-Mike
> From: Pops
>
> What I wanted to use this for was my cache and to "truncate"
> old data. I see that using a real array will allow me to use
> the inherited .length property to set a new size and it will
> do truncation. But since I am using an associated array, the
> length property is no longer usable.
>
> So if I use the for loop with delete, will I be pulling the
> "rug from under its feet?"
>
> Typically, in code designs like this, you would do a reverse
> traversal to delete the last entries first so you can keep
> with the internal loop counters and/or references.
>
> In other words, is this "safe?"
>
> function pruneCache(amt) {
> var n = 0;
> for (var i in cache) {
> if (amt > 0) {
> amt--;
> delete cache[i];
> return;
> }
> n++;
> }
> cache.length = n;
> }
>
> If not, then in lieu of a reverse loop syntax, I would probably need
> to copy the cache first? I have not checked but if returning false
> stops the traveral that would be more efficient.
>
> I could recode all this into a pure indexed array and then
> use the .length, but then I would need a fast lookup method
> or 2nd matrix to map the associated name with the array index.
