Revisions to How to merge two arrays in JavaScript and de-duplicate items

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edited Feb 24, 2023 at 23:11

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https://web.archive.org/web/20200803234803/httpJSPerf://jsperf.com/merge-two-arrays-keeping-only-unique-values "Merge two arrays keeping only unique values" (archived)

A comment pointed out that one of my setup variables was causing my loop to pull ahead of the rest, because it didn't have to initialize an empty array to write to. I agree with that, so I've rewritten the test to even the playing field, and included an even faster option.

https://web.archive.org/web/20200803235454/httpJSPerf://jsperf.com/merge-two-arrays-keeping-only-unique-values/52 "Merge two arrays keeping only unique values" (archived)

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edit approved Feb 24, 2023 at 23:11

tagurit

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http://jsperf.com/merge-two-arrays-keeping-only-unique-values https://web.archive.org/web/20200803234803/http://jsperf.com/merge-two-arrays-keeping-only-unique-values

http://jsperf.com/merge-two-arrays-keeping-only-unique-values/52 https://web.archive.org/web/20200803235454/http://jsperf.com/merge-two-arrays-keeping-only-unique-values/52

This change corrects the "uniqueifying" behavior by populating initial assoc hash with the keys from the first array. It doesn't _appear_ to hurt the speed too much. I've also updated it to not use a falsy check for a speed boost, and not use booleans for a little additional speed bump.

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edit approved Jan 17, 2019 at 23:35

Brak

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http://jsperf.com/merge-two-arrays-keeping-only-unique-values/21 http://jsperf.com/merge-two-arrays-keeping-only-unique-values/52

varlet whileLoopAlt = function (array1, array2) {
    varconst array3 = [];array1.slice(0);
    varlet arrlen1 = array1.concat(array2);length;
    varlet lenlen2 = arrarray2.length;
    varconst assoc = {};

    while (lenlen1--) {
        varassoc[array1[len1]] = null;
    }

    while (len2--) {
        let itm = arr[len];array2[len2];

        if (!assoc[itm] === undefined) { // Eliminate the indexOf call
            array3.unshiftpush(itm);
            assoc[itm] = true;null;
        }
    }

    return array3;
};

http://jsperf.com/merge-two-arrays-keeping-only-unique-values/21

var whileLoopAlt = function(array1, array2) {
    var array3 = [];
    var arr = array1.concat(array2);
    var len = arr.length;
    var assoc = {};

    while(len--) {
        var itm = arr[len];

        if(!assoc[itm]) { // Eliminate the indexOf call
            array3.unshift(itm);
            assoc[itm] = true;
        }
    }

    return array3;
};

http://jsperf.com/merge-two-arrays-keeping-only-unique-values/52

let whileLoopAlt = function (array1, array2) {
    const array3 = array1.slice(0);
    let len1 = array1.length;
    let len2 = array2.length;
    const assoc = {};

    while (len1--) {
        assoc[array1[len1]] = null;
    }

    while (len2--) {
        let itm = array2[len2];

        if (assoc[itm] === undefined) { // Eliminate the indexOf call
            array3.push(itm);
            assoc[itm] = null;
        }
    }

    return array3;
};

Active reading. Dressed the naked links.

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edited Oct 18, 2017 at 6:36

Peter Mortensen

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replaced http://stackoverflow.com/ with https://stackoverflow.com/

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edited May 23, 2017 at 12:18

URL Rewriter Bot

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Removing last edit because code does not work properly. It does not satisfy the result of deduping the two arrays and combining them.

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edited Apr 17, 2015 at 13:12

slickplaid

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added one last test case

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edited Apr 16, 2015 at 18:46

slickplaid

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Added some qualifications to other answers. There are better answers in this list.

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edited Apr 16, 2015 at 17:35

slickplaid

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fixed issue with my original answer as one of the comments had pointed out. Added a second, faster result with the use of an associative array rather than indexOf on an basic array.

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edited Apr 16, 2015 at 16:39

slickplaid

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Missed defining some variables when copying it from my jsperf tests

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edited Sep 4, 2014 at 13:01

slickplaid

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added results

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edited Aug 4, 2014 at 14:19

slickplaid

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created Aug 4, 2014 at 14:14

slickplaid

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