Understanding 'Yield' keyword in javascript?

Question

I came across yield keyword in javascript today and I am aware that currently it's not supported in browsers that are not ECMA 6 upgraded. Meanwhile in firefox, how can i rewrite the following code without yield

if (currentNode) {
   yield currentNode;
   currentNode = null;
}

Answer 1

There is no direct equivalent. However, one can fake it by returning a "generator" object. Basically, the continuation code is moved into the next() of the generator.

Consider this fib-generator example on MDN:

function fib() {
  var i = 0, j = 1;
  while (true) {
    yield i;
    var t = i;
    i = j;
    j += t;
  }
}

var g = fib();
for (var i = 0; i < 10; i++) {
  console.log(g.next());
}

And re-written using a fake generator:

function fib() {
  var i = 0, j = 1;
  return {
      'next': function () {
          var yieldRet = i;
          // These haven't occurred before the `yield` in the above generator,
          // but it makes it easier to do it in the same order here.
          // Just make sure there are no OBSERVABLE side-effects.
          var t = i; 
          i = j;
          j += t;
          return yieldRet;
      }
  };
}

var g = fib();
for (var i = 0; i < 10; i++) {
  console.log(g.next());
}

Now, this does become a bit trickier with the addition of observable mutable state; the given example could still be expressed as a state machine. Note that each next can "advance" the state.

var currentNode;
function yield1 () {
    var y = { next: st0 };
    return y;
    function st0 () {
        if (currentNode) {
            y.next = st1;
            return currentNode;
        } else {
            y.next = stZ;
        }
    }
    function st1 () {
        y.next = stZ;
        currentNode = null; // observable side-effect!
    }
    function stZ () {
    }
}

var g = yield1();
currentNode = "x";
console.log(g.next());    // "x"
console.log(currentNode); // still "x"
g.next();
console.log(currentNode); // null