[javascript] Compare 2 arrays which returns difference

What's the fastest/best way to compare two arrays and return the difference? Much like array_diff in PHP. Is there an easy function or am I going to have to create one via each()? or a foreach loop?

Array operations like this is not jQuery's strongest point. You should consider a library such as Underscorejs, specifically the difference function.

if you also want to compare the order of the answer you can extend the answer to something like this:

Array.prototype.compareTo = function (array2){
    var array1 = this;
    var difference = [];
    $.grep(array2, function(el) {
        if ($.inArray(el, array1) == -1) difference.push(el);
    });
    if( difference.length === 0 ){
        var $i = 0;
        while($i < array1.length){
            if(array1[$i] !== array2[$i]){
                return false;
            }
            $i++;
        }
        return true;
    }
    return false;
}

var arrayDiff = function (firstArr, secondArr) {
    var i, o = [], fLen = firstArr.length, sLen = secondArr.length, len;


    if (fLen > sLen) {
        len = sLen;
    } else if (fLen < sLen) {
        len = fLen;
    } else {
        len = sLen;
    }
    for (i=0; i < len; i++) {
        if (firstArr[i] !== secondArr[i]) {
            o.push({idx: i, elem1: firstArr[i], elem2: secondArr[i]});  //idx: array index
        }
    }

    if (fLen > sLen) {  // first > second
        for (i=sLen; i< fLen; i++) {
            o.push({idx: i, 0: firstArr[i], 1: undefined});
        }
    } else if (fLen < sLen) {
        for (i=fLen; i< sLen; i++) {
            o.push({idx: i, 0: undefined, 1: secondArr[i]});
        }
    }    

    return o;
};

I know this is an old question, but I thought I would share this little trick.

var diff = $(old_array).not(new_array).get();

diff now contains what was in old_array that is not in new_array

The short version can be like this:

const diff = (a, b) => b.filter((i) => a.indexOf(i) === -1);

result:

diff(['a', 'b'], ['a', 'b', 'c', 'd']);

["c", "d"]

This should work with unsorted arrays, double values and different orders and length, while giving you the filtered values form array1, array2, or both.

function arrayDiff(arr1, arr2) {
    var diff = {};

    diff.arr1 = arr1.filter(function(value) {
        if (arr2.indexOf(value) === -1) {
            return value;
        }
    });

    diff.arr2 = arr2.filter(function(value) {
        if (arr1.indexOf(value) === -1) {
            return value;
        }
    });

    diff.concat = diff.arr1.concat(diff.arr2);

    return diff;
};

var firstArray = [1,2,3,4];
var secondArray = [4,6,1,4];

console.log( arrayDiff(firstArray, secondArray) );
console.log( arrayDiff(firstArray, secondArray).arr1 );
// => [ 2, 3 ]
console.log( arrayDiff(firstArray, secondArray).concat );
// => [ 2, 3, 6 ]

use underscore as :

_.difference(array1,array2)

In this way you don't need to worry about if the first array is smaller than the second one.

var arr1 = [1, 2, 3, 4, 5, 6,10],
    arr2 = [1, 2, 3, 4, 5, 6, 7, 8, 9];

function array_diff(array1, array2){
    var difference = $.grep(array1, function(el) { return $.inArray(el,array2) < 0});
    return difference.concat($.grep(array2, function(el) { return $.inArray(el,array1) < 0}));;
}

console.log(array_diff(arr1, arr2));

/** SUBTRACT ARRAYS **/
function subtractarrays(array1, array2){
    var difference = [];
    for( var i = 0; i < array1.length; i++ ) {
        if( $.inArray( array1[i], array2 ) == -1 ) {
                    difference.push(array1[i]);
        }
    }

    return difference;
}   

You can then call the function anywhere in your code.

var I_like    = ["love", "sex", "food"];
var she_likes = ["love", "food"];

alert( "what I like and she does't like is: " + subtractarrays( I_like, she_likes ) ); //returns "Naughty"!

This works in all cases and avoids the problems in the methods above. Hope that helps!