How Can I Remove Duplicate Values from a JavaScript Array?

Remove Duplicate Values from JS Array

Duplicating elements in JavaScript arrays is a common issue. To address this, various methods can be employed.

Set and Spread Syntax:

The most concise solution utilizes the Set constructor and the spread syntax:

uniq = [...new Set(array)];

Naïve Approach:

For simplicity, a straightforward approach involves filtering elements based on their index:

uniqueArray = a.filter(function(item, pos) {
    return a.indexOf(item) == pos;
});

Using Hashtables:

A more efficient method leverages hashtables, using object properties as keys:

function uniq(a) {
    var seen = {};
    return a.filter(function(item) {
        return seen.hasOwnProperty(item) ? false : (seen[item] = true);
    });
}

Combining Approaches:

To handle diverse arrays, a hybrid solution combines hashtables for primitives and linear search for objects:

function uniq(a) {
    var prims = {"boolean":{}, "number":{}, "string":{}}, objs = [];

    return a.filter(function(item) {
        var type = typeof item;
        if(type in prims)
            return prims[type].hasOwnProperty(item) ? false : (prims[type][item] = true);
        else
            return objs.indexOf(item) >= 0 ? false : objs.push(item);
    });
}

Sorting and Filtering:

Sorting before filtering provides another option, removing duplicates based on adjacent elements:

function uniq(a) {
    return a.sort().filter(function(item, pos, ary) {
        return !pos || item != ary[pos - 1];
    });
}

Custom Comparison:

For unique comparisons, a callback can be passed, with equal "keys" being filtered:

function uniqBy(a, key) {
    var seen = {};
    return a.filter(function(item) {
        var k = key(item);
        return seen.hasOwnProperty(k) ? false : (seen[k] = true);
    })
}

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