Great Generator example ES6

gistfile1.txt

function *foo(x) {
    var y = 2 * (yield (x + 1));
    var z = yield (y / 3);
    return (x + y + z);
}

var it = foo( 5 );

// note: not sending anything into `next()` here
console.log( it.next() );       // { value:6, done:false }
console.log( it.next( 12 ) );   // { value:8, done:false }
console.log( it.next( 13 ) );   // { value:42, done:true }

// The yield (x + 1) is what sends out value 6. 
// The second next(12) call sends 12 to that waiting yield (x + 1) expression, so y is set to 12 * 2, value 24. 
// Then the subsequent yield (y / 3) (yield (24 / 3)) is what sends out the value 8. 
// The third next(13) call sends 13 to that waiting yield (y / 3) expression, making z set to 13.
// Finally, return (x + y + z) is return (5 + 24 + 13), or 42 being returned out as the last value.