Mysterious 'function' being attached to array

I have the following code:

// button sets

    var sets = ['.diamond-colour','.diamond-cut','.diamond-clarity','.diamond-certificate'];

    // for each set

    for (set in sets){

        console.log('Set: '+set);

        console.log(sets[set]);

    }

The console log shows:

Set: 0

.diamond-colour

Set: 1

.diamond-cut

Set: 2

.diamond-clarity

Set: 3

.diamond-certificate

Set: findIndex

function (value){

var ctr = "";

for (var i=0; i < this.length; i++) {

if (this[i] == value) {

return i;

}

}

return ctr;

}

It appears to be a new array element with the key of findIndex and a value being that function.

Would anyone have any idea what this is and why it's appearing?