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    <p>I used your jsperf as a reference, for some of the finer points of script. I really, really liked your 'hash' code, so I totally stole it. Mine uses a different method to generate the string used to make the hash, which seems to be a little faster, which increases the performance, according to the 'browserscope' charts. I include in my test a 'too much recursion' proof of concept to show that it has recursion protection, like JSON.stringify and .toSource(). </p> <p><a href="http://jsfiddle.net/dTC4g/" rel="noreferrer">My jsfiddle</a> shows that the code returns the format you need. <a href="http://jsperf.com/group-by-complex-fast" rel="noreferrer">My jsperf</a> seems to indicate that it outperforms the posted solution. I also included the linq.js solution, but it performs pretty bad in FireFox for me. It works comparably in Safari, Chrome, an IE, but not faster than mine, except in IE. I even tried it on my phone, and still I have the same performance difference. I have personally tested it in the latest versions of all browsers in a side-by-side to the posted solution, and mine out performs by around 40% across each of them. What are everyone's thoughts?</p> <p>Here is my code:</p> <pre><code>var arr = [ { N: 10, Name: "Foo" }, { N: 10, Name: "Bar" }, { N: 20, Name: "Foo" }, { N: 20, Name: "Bar" } ]; var poc = { name:'blah', obj:{} }; poc.obj = poc; var arrComplex = [ { N: { Value: 10, TooMuchRecursionProofPOC:poc }, Name: "Foo" }, { N: { Value: 10, TooMuchRecursionProofPOC:poc }, Name: "Bar" }, { N: { Value: 20, TooMuchRecursionProofPOC:poc }, Name: "Foo" }, { N: { Value: 20, TooMuchRecursionProofPOC:poc }, Name: "Bar" } ]; var eArr = Enumerable.From(arr); var eArrComplex = Enumerable.From(arrComplex); function setup_hashers() { // recursion protection idea var rp = '_rp'+(Math.random()*10000000); function tstr() { var out = '', i = ''; if (this[rp]) { this[rp] = undefined; return out; } for (i in this) if (i != rp &amp;&amp; this.hasOwnProperty(i)) out += this[i] instanceof Object ? ((this[rp] = true) &amp;&amp; this[i] != this &amp;&amp; !this[i][rp] ? tstr.call(this[i]) : '') : (this[i].toString || tstr).call(this[i]); return out; }; Number.prototype.GetHashCode = function() { return this.valueOf(); }; Object.prototype.GetHashCode = function() { var s = (this instanceof Object ? tstr : this.toString || tstr).call(this), h = 0; if (s.length) for (var i = 0; i &lt; s.length; i++) h = ((h &lt;&lt; 5) - h) + s.charCodeAt(i); return h; }; } function group_by(a, keyFunc, valFunc, comp, as_array) { if (!a.length) return as_array ? [] : {}; var keyFunc = keyFunc || function (e) { return e; }, valFunc = valFunc || function (e) { return e; }; var comp = comp || { Equals: function (a, b) { return a == b; }, Hash: function (e) { return e.GetHashCode(); } }; var hashs = {}, key = '', hash = ''; for (var i = 0; i &lt; a.length; i++) { key = keyFunc(a[i]); hash = comp.Hash(key); if (typeof hashs[hash] != 'undefined') hash = comp.Equals(key, hashs[hash].Key) ? hash : hash + '-' + i; hashs[hash] = hashs[hash] || { Key: key, Elements: [] }; hashs[hash].Elements.push(valFunc(a[i])); } if (as_array) { var out = [], j = '', keys = Object.keys(hashs); for (var j = 0; j &lt; keys.length; j++) out.push(hashs[keys[j]]); return out; } return hashs; }; function group_by_control(a, keyFunc, valFunc) { if (!a.length) return as_array ? [] : {}; var keyFunc = keyFunc || function (e) { return e; }, valFunc = valFunc || function (e) { return e; }; var hashs = {}, key = '', hash = ''; for (var i = 0; i &lt; a.length; i++) { key = keyFunc(a[i]); hashs[key] = hashs[key] || { Key: key, Elements: [] }; hashs[key].Elements.push(valFunc(a[i])); } var out = [], j = '', keys = Object.keys(hashs); for (var j = 0; j &lt; keys.length; j++) out.push(hashs[keys[j]]); return out; }; setup_hashers(); console.log(group_by_control( arr, function(e) { return e.N }, function(e) { return e.Name } )); console.log(group_by( arrComplex, function(e) { return e.N; }, function(e) { return e.Name; }, { Equals: function(a, b) { return a.Value == b.Value }, Hash: function(e) { return e.GetHashCode(); } } )); console.log(group_by( arrComplex, function(e) { return e.N; }, function(e) { return e.Name; }, { Equals: function(a, b) { return a.Value == b.Value }, Hash: function(e) { return e.GetHashCode(); } }, true )); </code></pre>
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    1. POWhat is the most efficient way to implement GroupBy in Javascript?
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    1. COWhat a nice present you are going to get huh. Thanks for liking to code. Thanks for understanding my problem. Thanks for providing useful information. Happy holidays.
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    2. COno problem :) it was a fun task to occupy my sunday :)
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