Javascript Array indexFor( item )

Array.prototype.indexFor = function( item ) {
    var output=-1;
    for (var i = 0 ; i < this.length ; i ++) {
        if (this[i]===item) {
            output= i; /*from w  ww .j ava2  s  .  c  om*/
            break;
        }
    }
    return output; 
};
[2,3,5,5,6,8,9,10,22,49].indexFor("a");