This answer was written in 2009, so it is a bit outdated, also it’s rather educational for understanding the problem. Best solution I’d use today would be
let difference = arr1.filter(x => !arr2.includes(x));
(credits to other author here)
I assume you are comparing a normal array. If not, you need to change the for loop to a for .. in loop.
function arr_diff (a1, a2) { var a = [], diff = []; for (var i = 0; i < a1.length; i++) { a[a1[i]] = true; } for (var i = 0; i < a2.length; i++) { if (a[a2[i]]) { delete a[a2[i]]; } else { a[a2[i]] = true; } } for (var k in a) { diff.push(k); } return diff; } console.log(arr_diff(['a', 'b'], ['a', 'b', 'c', 'd'])); console.log(arr_diff("abcd", "abcde")); console.log(arr_diff("zxc", "zxc"));
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