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7151395
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2011-08-22T17:49:19.213
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2011-08-22T18:02:10.113
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As you have been already made aware of, your testcases were flawed. <a href="http://jsperf.com/array-push-comparision" rel="nofollow">Here is a jsperf test</a> case which uses assignment in all methods. The speed is the same for all of them in Firefox 6 (~50,000 op/s). In Chrome 13 however, setting the array length beforehand results in a huge speed improvement (~80,000 op/s vs. ~400.000 op/s). To find the reason for that, one would have to have a look at the source code of Chrome's JavaScript engine. You asked what happens on <code>.push()</code> and <code>new Array</code>. The specification describes was should happen: <blockquote> <h3><a href="http://ecma262-5.com/ELS5_HTML.htm#Section_15.4.2.2" rel="nofollow">15.4.2.2 new Array (len)</a></h3> The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object, the one that is the initial value of Array.prototype (<a href="http://ecma262-5.com/ELS5_HTML.htm#Section_15.4.3.1" rel="nofollow">15.4.3.</a>1). The [[Class]] internal property of the newly constructed object is set to "Array". The [[Extensible]] internal property of the newly constructed object is set to true. If the argument len is a Number and <code>ToUint32(len)</code> is equal to len, then the length property of the newly constructed object is set to <code>ToUint32(len)</code>. If the argument len is a Number and <code>ToUint32(len)</code> is not equal to len, a RangeError exception is thrown. If the argument len is not a Number, then the length property of the newly constructed object is set to 1 and the 0 property of the newly constructed object is set to len with attributes {[[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.. <h3><a href="http://ecma262-5.com/ELS5_HTML.htm#Section_15.4.3.1" rel="nofollow">15.4.4.7 Array.prototype.push ( [ item1 [ , item2 [ , … ] ] ] )</a></h3> The arguments are appended to the end of the array, in the order in which they appear. The new length of the array is returned as the result of the call. When the push method is called with zero or more arguments item1, item2, etc., the following steps are taken: <ol> <li>Let O be the result of calling ToObject passing the this value as the argument.</li> <li>Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".</li> <li>Let n be ToUint32(lenVal).</li> <li>Let items be an internal List whose elements are, in left to right order, the arguments that were passed to this function invocation.</li> <li>Repeat, while items is not empty <ol> <li>Remove the first element from items and let E be the value of the element.</li> <li>Call the [[Put]] internal method of O with arguments ToString(n), E, and true.</li> <li>Increase n by 1.</li> </ol></li> <li>Call the [[Put]] internal method of O with arguments "length", n, and true.</li> <li>Return n.</li> </ol> </blockquote> That said, the implementation can be different. As you saw from the difference in Firefox and Chrome, Chrome seems to optimize the array structure internally.
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1. POHow does a javascript array work whilst pushing new elements?
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1. USFelix Kling
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