Promise Gotchas

Avoiding common mistakes

Promises don’t work like other values

You cannot print their value like most values:

// This doesn't make sense w/ promises:
console.log(Promise.resolve(42));

// We must use the `.then` interface:
Promise.resolve(42).then(value => console.log(value));

Promises don’t warn you when making a mistake

Well, a likely mistake.

For a variety of reasons, it was decided by TC39 that .then and .catch may be passed null. For example, .then(null, null) is valid and the required behavior is to skip that ‘step’ in the chain.

The unfortunate consequence of this is it’s very easy to screw things up.

By Example

Let’s look at a mini challenge: which of the following option(s) will console.log 42?

// Option #1:
Promise.resolve(42).then(console.log());

// Option #2:
Promise.resolve(42).then(console.log);

// Option #3:
Promise.resolve(42).then(value => console.log(value));

// Option #4:
Promise.resolve(42)
  .then(console.log())
  .then(console.log);

The Answer

The answer is #2, #3 & #4.

Why? Let’s look at the types of what was passed to .then():

var arg1 = console.log();
var arg2 = console.log;
var arg3 = value => console.log(value);

typeof arg1 === "undefined";
typeof arg2 === "function";
typeof arg3 === "function";

Still wondering how that 4th Option works?

It’s effectively running like this:

// Option #4 - effectively
Promise.resolve(42)
  .then(undefined) // this has no affect on the value, it will be handed to following `.then(fn)`
  .then(console.log);