var arr = [4,5,6,7,8,9]; for (var v of arr) { console.log( v ); } // 4 5 6 7 8 9 // define iterator that only produces values // from odd indexes arr[Symbol.iterator] = function*() { var idx = 1; do { yield this[idx]; } while ((idx += 2) < this.length); }; for (var v of arr) { console.log( v ); } // 5 7 9