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    <p>Here is a solution using ECMA5 methods</p> <p>Javascript</p> <pre><code>var makeRows1 = (function () { "use strict"; function reduceParticipants(previous, participant) { previous[participant.id] = [participant.name]; return previous; } function reduceResult(previous, subResult) { previous[subResult.participantId].push(subResult.answer); return previous; } function filterParticipants(participant) { return participant; } return function (participants, results) { var row = participants.reduce(reduceParticipants, []); results.forEach(function (result) { result.reduce(reduceResult, row); }); return row.filter(filterParticipants); }; }()); </code></pre> <p>This will not be as fast as using raw <code>for</code> loops, like @Esailija answer, but it's not as slow as you may think. It's certainly faster than using <code>Underscore</code>, like your example or the answer given by @Maroshii</p> <p>Anyway, here is a <a href="http://jsfiddle.net/fMBAe/" rel="nofollow">jsFiddle</a> of all three answers that demonstrates that they all give the same result. It uses quite a large data set, I don't know it compares to the size you are using. The data is generated with the following:</p> <p>Javascript</p> <pre><code>function makeName() { var text = "", possible = "abcdefghijklmnopqrstuvwxy", i; for (i = 0; i &lt; 5; i += 1) { text += possible.charAt(Math.floor(Math.random() * possible.length)); } return text; } var count, count2, index, index2, participants = [], results = []; for (index = 0, count = 1000; index &lt; count; index += 4) { participants.push({ id: index, name: makeName() }); } for (index = 0, count = 1000; index &lt; count; index += 1) { results[index] = []; for (index2 = 0, count2 = participants.length; index2 &lt; count2; index2 += 1) { results[index].push({ question: index, participantId: participants[index2].id, answer: "test" + index }); } } </code></pre> <p>Finally, we have a <a href="http://jsperf.com/make-rows-comparison" rel="nofollow">jsperf</a> that compares these three methods, run on the generated data set.</p>
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    1. COI updated your jsperf example to show that the original function doesn't work, and to reflect Esailija's update.
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    2. COLooks like your answer is faster on IE 10 and Esailija's is faster on Firefox 24.
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    3. CO@Kastor wow, that's quite impressive! (I'm not an MS fan).
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