What is the use case of Promise.resolve(promise)?

Question

We can call Promise.resolve(value) as well as Promise.resolve(promise). I can understand where and how I should use the first call but am not sure about the second one.

Why not simply use the promise as argument in the first place? I mean calling:

promise1.then()

instead of:

Promise.resolve(promise1).then()

?

Answer 1

Promise.resolve is basically "I don't care what this is, give me a promise". Generally, whenever you get an argument that might be a promise Promise.resolve is the correct way to work with it (vs. detecting it's a promise clunkily).

It's useful for:

• Working with foreign promises from different libraries.
• Working with values that might be promises and might be plain values.

It's also implicitly called a lot with promises.

• When you pass an array of mixed promises, foreign "thenables" and values to Promise.all or Promise.race - it implicitly Promise.resolves all values.
• Similarly, when you return a foreign "thenable" or a value from Promise#then(...) it will call Promise.resolve on it.
• When you await a value Promise.resolve is implicitly called on it.

And so on.

Answer 2

If you have a value that might be a Promise, Promise.resolve is great.

In the program that I am working on now, I have a lot of places were I create a UI component with callbacks to handle various inputs. Some times, those callbacks need to return promises, sometimes they don't. Rather than put the onus on the callback, I always wrap the return value in Promise.resolve and boom, problem solved.