How to remove duplicate elements, of different data types, from an Array.

array_deduplicate.js

/**
 * Primitives
 */

// If an Array only contains primitive values, we can deduplicate it by only using the filter and indexOf methods.
var deduped = [ 1, 1, 'a', 'a' ].filter(function (el, i, arr) {
  return arr.indexOf(el) === i;
});
console.log(deduped); // [ 1, 'a' ]

// The ES6 (ECMAScript 2015) version
// We can write this in a more compact way using an arrow function.
var deduped = [ 1, 1, 'a', 'a' ].filter( (el, i, arr) => arr.indexOf(el) === i);
console.log(deduped); // [ 1, 'a' ]

// But with the introduction of Sets and the from method, we can achieve the same result in a more concise way.
var deduped = Array.from( new Set([ 1, 1, 'a', 'a' ]) );
console.log(deduped); // [ 1, 'a' ]

/**
 * Objects
 */

// We can't use the same approach when the elements are Objects,
// because Objects are stored by reference and primitives are stored by value.
       1 === 1        // true
     'a' === 'a'      // true
{ a: 1 } === { a: 1 } // false

// Therefore we need to change our approach and use a hash table.

function dedup(arr) {
    var hashTable = {};

    return arr.filter(function (el) {
        var key = JSON.stringify(el);
        var match = Boolean(hashTable[key]);

        return (match ? false : hashTable[key] = true);
    });
}

var deduped = dedup([
    { a: 1 },
    { a: 1 },
    [ 1, 2 ],
    [ 1, 2 ]
]);

console.log(deduped); // [ {a: 1}, [1, 2] ]

// Because a hash table in javascript is simply an Object, the keys will always be of the type String.
// This means that normally we can't distinguish between strings and numbers of the same value, i.e. 1 and '1'.

var hashTable = {};

hashTable[1] = true;
hashTable['1'] = true;

console.log(hashTable); // { '1': true }

// However, because we're using JSON.stringify, keys that are of the type String,
// will be stored as an escaped string value, giving us unique keys in our hashTable.

var hashTable = {};

hashTable[JSON.stringify(1)] = true;
hashTable[JSON.stringify('1')] = true;

console.log(hashTable); // { '1': true, '\'1\'': true }

// This means duplicate elements of the same value, but of a different type,
// will still be deduplicated using the same implementation.

var deduped = dedup([
    { a: 1 },
    { a: 1 },
    [ 1, 2 ],
    [ 1, 2 ],
    1,
    1,
    '1',
    '1'
]);

console.log(deduped); // [ {a: 1}, [1, 2], 1, '1' ]