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    <p>Accessing object properties and array elements in JavaScript is syntacticly assumed to be done in <a href="http://en.wikipedia.org/wiki/Time_complexity#Constant_time" rel="noreferrer">constant time</a>: O(1). Performance characteristics are not guaranteed in the ECMAScript specification, but all the modern JavaScript engines retrieve object properties in constant time.</p> <p>Here's a simple example showing how access times grow when the container is x1000 times bigger:</p> <pre><code>var largeObject = {}; var smallObject = {}; var x, i; for (i = 0; i &lt; 1000000; i++) { largeObject['a' + i] = i; } for (i = 0; i &lt; 1000; i++) { smallObject['b' + i] = i; } console.time('10k Accesses from largeObject'); for (i = 0; i &lt; 10000; i++) x = largeObject['a' + (i % 1000000)]; console.timeEnd('10k Accesses from largeObject'); console.time('10k Accesses from smallObject'); for (i = 0; i &lt; 10000; i++) x = largeObject['a' + (i % 1000)]; console.timeEnd('10k Accesses from smallObject'); </code></pre> <p>Results in Firebug, Firefox 3.6.10 (Mac OS X 10.6.4 - 2.93Ghz Intel Core 2 Duo):</p> <pre><code>10k Accesses from largeObject: 22ms 10k Accesses from smallObject: 19ms </code></pre> <p>Results in Chrome Dev Tools 6.0.472:</p> <pre><code>10k Accesses from largeObject: 15ms 10k Accesses from smallObject: 15ms </code></pre> <p>Internet Explorer 8.0.7600 with Firebug Lite on Windows 7</p> <pre><code>10k Accesses from largeObject: 250ms 10k Accesses from smallObject: 219ms </code></pre>
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    1. POWhat's the big O for JavaScript's array when used as a hash?
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    1. USDaniel Vassallo
    1. USDaniel Vassallo
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    1. COCan you point to the paragraph in the ECMAScript or JavaScript specifications which says that accessing array elements or object properties is guaranteed to be O(1)? AFAIK, JScript has a perfectly standards-compliant O(n) implementation of objects as simple linked lists.
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      1. USJörg W Mittag
    2. CO@Jörg: It's not mentioned in the spec, so in practice it is implementation dependent... That's why I chose to say that it is "syntactically assumed". Nevertheless, I rephrased my answer to be more accurate.
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      1. USDaniel Vassallo
    3. COI don't know how the IE team can live with such embarrassing results...
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      1. USChaosPandion
 

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