javascript and promise: how to refactor/flatten the following nested promises?

Question

I am new to promise, and I am reading a piece of codes which is pretty difficult for me to understand:

  return promise
    .then(function helper0(instances) {
      return helper1(instances, options)
        .then(function helper2() {
            return bluebird.delay(3000)
              .then(function helper3() {
                return helper4(localParams, options);
              });
          }
        });
    });

How to re-factor it to promise.then().then()...? Thanks

Answer 1

Nesting promises is a known anti-pattern, you should chain them instead:

// the structure is vertical instead of a nested pyramid
return promise
  .then(function helper0(instances) {
    return helper1(instances, options)
  })
  .then(function helper2() {
    return bluebird.delay(3000);
  })
  .then(function helper3() {
    return helper4(localParams, options);
  });

Returning a promise from the callback passed to then adds that promise to the chain.

Using arrow functions would clean this up further:

return promise
  .then(instances => helper1(instances, options))
  .then(() =>  bluebird.delay(3000))
  .then(() =>  helper4(localParams, options);

But note that using named functions would be a better approach for debugging purposes because of a more readable stack trace.