Promises and then function return values

The return type of Promise function will dictate how future chained then functions behave. If you return a Promise then the next chained then function will execute when the Promise that you returned is resolved.

Promise.resolve('foo').
  then(() => {return Promise.resolve('bar')}).
  then((v) => console.log(v))

// prints bar

If you return something different from the then function, then the argument of the next chained then function will be the previous returned value.

Promise.resolve('foo').
  then(() => {return 'baz'}).
  then((v) => console.log(v))

// prints baz

Check out the MDN docs for Promise.prototype.then

H/T Brian Dunn

H/T Josh Branchaud