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    <p>You should use a loop-based technique. Other answers on this page that are based on using <code>.apply</code> can fail for large arrays. </p> <p>A fairly terse loop-based implementation is:</p> <pre><code>Array.prototype.extend = function (other_array) { /* you should include a test to check whether other_array really is an array */ other_array.forEach(function(v) {this.push(v)}, this); } </code></pre> <p>You can then do the following:</p> <pre><code>var a = [1,2,3]; var b = [5,4,3]; a.extend(b); </code></pre> <p><a href="https://stackoverflow.com/a/1374131/1280629">DzinX's answer</a> (using push.apply) and other <code>.apply</code> based methods fail when the array that we are appending is large (tests show that for me large is > 150000 entries approx in Chrome, and > 500000 entries in Firefox). You can see this error occurring in <a href="http://jsperf.com/array-extending-push-vs-concat" rel="noreferrer">this jsperf</a>.</p> <p>An error occurs because the call stack size is exceeded when 'Function.prototype.apply' is called with a large array as the second argument. (The MDN has a note on the dangers of exceeding call stack size using <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply" rel="noreferrer">Function.prototype.apply</a> - see the section titled "apply and built-in functions")</p> <p>For a speed comparison with other answers on this page check out <a href="http://jsperf.com/array-extending-push-vs-concat/18" rel="noreferrer">this jsperf</a> (thanks to EaterOfCode). The loop-based implementation is similar in speed to using Array.push.apply, but tends to be a little slower than Array.slice.apply.</p> <p>Interestingly, if the array you are appending is sparse, the <code>forEach</code> based method above can take advantage of the sparsity and outperform the <code>.apply</code> based methods, check out <a href="http://jsperf.com/array-extending-push-vs-concat/5" rel="noreferrer">this jsperf</a> if you want to test this for yourself. </p> <p>By the way, do not be tempted (as I was!) to further shorten the forEach implementation to:</p> <pre><code>Array.prototype.extend = function (array) { array.forEach(this.push, this); } </code></pre> <p>because this produces garbage results! Why? Because Array.prototype.forEach provides 3 arguments to the function it calls - these are: (element_value, element_index, source_array). All of these will be pushed onto your first array for every iteration of forEach if you use "forEach(this.push, this)"!</p>
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    1. POHow to extend an existing JavaScript array with another array, without creating a new array?
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    1. COp.s. to test whether other_array really is an array, choose one of the options described here: http://stackoverflow.com/questions/767486/how-do-you-check-if-a-variable-is-an-array-in-javascript
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    2. COGood answer. I wasn't aware of this problem. I've cited your answer here: http://stackoverflow.com/a/4156156/96100
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