2-way generator - My explanation of the example at https://davidwalsh.name/es6-generators

2-way-generator.js

/*************************

First call it.next(): has the effect of starting the function, there is no yield waiting for input at this point, 
                      therefore yield (line 13) passes back the value 6, and is now waiting for input
Second call it.next(12): yield (line 13) is paused and waiting for input, input is 12, therefore y is assigned the value 24 (2 * 12).
                         yield (line 14) passes back 8 (24(y) / 3), and is now waiting for input
Final call it.next(13): yield (line 14) is paused and waiting for input, input is 13, therefore z is assigned the value 13.
                        function returns x + y + z (5 + 24 + 13) = 42

*************************/

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 }