JS - compare two arrays and find the mismatched elements

compareTwoArrays.js

var current = [1, 2, 3, 4, 7, 8],
    prev = [1, 2, 4],
    isMatch = false,
    missing = null;

var i = 0, y = 0,
    lenC = current.length,
    lenP = prev.length;

for ( ; i < lenC; i++ ) {
    isMatch = false;
    for ( y = 0; y < lenP; y++ ) {
        if (current[i] == prev[y]) isMatch = true;
    }
    if ( !isMatch ){
	    missing = current[i];
	    alert(missing);
    }  // Current[i] isn't in prev
}