Exploring how a gen-runner works... using a recursive promise chain

1.js

// commented `run(..)`:

function run(it) {
  // return a promise for the generator completing
  return Promise.resolve()
    .then( function handleNext(value){
      // run to the next yielded value
      var next = it.next( value );

      return (function handleResult(next){
        // generator has completed running?
        if (next.done) {
          return next.value;
        }
        // otherwise keep going
        else {
          return Promise.resolve( next.value )
            .then(
              // resume the async loop on
              // success, sending the resolved
              // value back into the generator
              handleNext,

              // if `value` is a rejected
              // promise, propagate error back
              // into the generator for its own
              // error handling
              function handleErr(err) {
                return Promise.resolve(
                  it.throw( err )
                )
                .then( handleResult );
              }
            );
        }
      })(next);
    } );
}

2.js

// un-commented `run(..)`:

function run(it) {
  return Promise.resolve()
    .then( function handleNext(value){
      var next = it.next( value );

      return (function handleResult(next){
        if (next.done) {
          return next.value;
        }
        else {
          return Promise.resolve( next.value )
            .then(
              handleNext,
              function handleErr(err) {
                return Promise.resolve(
                  it.throw( err )
                )
                .then( handleResult );
              }
            );
        }
      })(next);
    } );
}

3.js

// ... with some ES6 sugar

function run(it) {
  return Promise.resolve()
    .then( function handleNext(value){
      var next = it.next( value );

      return (function handleResult(next){
        return next.done ? next.value :
          Promise.resolve( next.value )
            .then(
              handleNext,
              err =>
                Promise.resolve(
                  it.throw( err )
                )
                .then( handleResult )
            );
        }
      })( next );
    } );
}