How is array.filter checking for Array differences in my sample code?

Question

I am trying to understand how array.filter is returning array differences by checking if indexOf(x) is equal to -1? Why -1?

function array_diff(a, b) {
    return a.filter(function(x) { return b.indexOf(x) == -1; });
}

Answer 1

indexOf(x) return the index of your property. So 0 is the first property, -1 means there is no property found.

Answer 2

b.indexOf(x) returns the index of parameter x in array b, or -1 if x is not found in array b.

So, if x doesn't exist in both the a and b array, indexOf() returns -1, allowing x to appear in the result.

Answer 3

function array_diff(a, b) {
    return a.filter(function(x) { return b.indexOf(x) == -1; });
}

For each item of array a, see if it exists in array b. If it does not (indexOf() returns -1), so return true, and keep it in the array. If it is in both arrays, return false.