Accessing object properties and array elements in JavaScript is syntacticly assumed to be done in constant time: O(1). Performance characteristics are not guaranteed in the ECMAScript specification, but all the modern JavaScript engines retrieve object properties in constant time.
Here's a simple example showing how access times grow when the container is x1000 times bigger:
var largeObject = {};
var smallObject = {};
var x, i;
for (i = 0; i < 1000000; i++) {
largeObject['a' + i] = i;
}
for (i = 0; i < 1000; i++) {
smallObject['b' + i] = i;
}
console.time('10k Accesses from largeObject');
for (i = 0; i < 10000; i++) x = largeObject['a' + (i % 1000000)];
console.timeEnd('10k Accesses from largeObject');
console.time('10k Accesses from smallObject');
for (i = 0; i < 10000; i++) x = largeObject['a' + (i % 1000)];
console.timeEnd('10k Accesses from smallObject');
Results in Firebug, Firefox 3.6.10 (Mac OS X 10.6.4 - 2.93Ghz Intel Core 2 Duo):
10k Accesses from largeObject: 22ms
10k Accesses from smallObject: 19ms
Results in Chrome Dev Tools 6.0.472:
10k Accesses from largeObject: 15ms
10k Accesses from smallObject: 15ms
Internet Explorer 8.0.7600 with Firebug Lite on Windows 7
10k Accesses from largeObject: 250ms
10k Accesses from smallObject: 219ms