How do I make an array with unique elements (i.e. remove duplicates)?

Or for those looking for a one-liner (simple and functional):

var a = ["1", "1", "2", "3", "3", "1"];
var unique = a.filter(function(item, i, ar){ return ar.indexOf(item) === i; });

Using a plain array and returning the keys of associative array (containing only the "unique" values from given array) is more efficient:

function ArrNoDupe(a) {
    var temp = {};
    for (var i = 0; i < a.length; i++)
        temp[a[i]] = true;
    var r = [];
    for (var k in temp)
        r.push(k);
    return r;
}

$(document).ready(function() {
    var arr = [10, 7, 8, 3, 4, 3, 7, 6];
    var noDupes = ArrNoDupe(arr);
    $("#before").html("Before: " + arr.join(", "));
    $("#after").html("After: " + noDupes.join(", "));
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="before"></div>
<div id="after"></div>

Note: The function does not preserve the order of the items, so if this is important use different logic.

As of IE9 and on all other modern browsers (e.g. Chrome, Firefox) this can become even more efficient by using the Object.keys() method:

function ArrNoDupe(a) {
    var temp = {};
    for (var i = 0; i < a.length; i++)
        temp[a[i]] = true;
    return Object.keys(temp);
}

$(document).ready(function() {
    var arr = [10, 7, 8, 3, 4, 3, 7, 6];
    var noDupes = ArrNoDupe(arr);
    $("#before").html("Before: " + arr.join(", "));
    $("#after").html("After: " + noDupes.join(", "));
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="before"></div>
<div id="after"></div>

_{Thanks wateriswet for bringing this to my attention. :)}

You could use the new native new Set(list) object in ES6/ES2015. (i.e. Babel, Typescript, or those lucky enough that all target browsers support ES2015).

// I never use this, because it's an iterator, not an array
let s = new Set(list);

or, if you want to chain to array helpers use the new ... spread operator in ES6/ES2015 to spread it into an array:

const unique = (list) => {
  return [...new Set(list)];
}

You need an array to chain methods like sort():

const convertText = (textToConvert) => {
  let list = unique(textToConvert.split(/\r?\n/g))
    .sort() // this will error if using uniqueAsIterator() version...
    .filter(x => x != "NULL");
  return list;
}

How do I make an array with unique elements (i.e. remove duplicates)?

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