Docs / Language Manual / Promise

You are currently looking at the v6.0 - v8.2 docs (Reason v3.6 syntax edition). You can find the latest manual page here.

(These docs are equivalent to the old BuckleScript docs before the ReScript rebrand)

Promise

ReScript has built-in support for JavaScript promises. The 3 functions you generally need are:

Js.Promise.resolve: 'a => Js.Promise.t('a)
Js.Promise.then_: ('a => Js.Promise.t('b), Js.Promise.t('a)) => Js.Promise.t('b)
Js.Promise.catch: (Js.Promise.error => Js.Promise.t('a), Js.Promise.t('a)) => Js.Promise.t('a)

Additionally, here's the type signature for creating a promise on the ReScript side:

Reason (Old Syntax)ML (Older Syntax)

Js.Promise.make: (
  (
    ~resolve: (. 'a) => unit,
    ~reject: (. exn) => unit
  ) => unit
) => Js.Promise.t('a);

This type signature means that make takes a callback that takes 2 named arguments, resolve and reject. Both arguments are themselves uncurried callbacks (with a dot). make returns the created promise.

Usage

Using the pipe operator:

Reason (Old Syntax)ML (Older Syntax)JS Output

let myPromise = Js.Promise.make((~resolve, ~reject) => resolve(. 2));

myPromise
|> Js.Promise.then_(value => {
     Js.log(value);
     Js.Promise.resolve(value + 2);
   })
|> Js.Promise.then_(value => {
     Js.log(value);
     Js.Promise.resolve(value + 3);
   })
|> Js.Promise.catch(err => {
     Js.log2("Failure!!", err);
     Js.Promise.resolve(-2);
   });

Note: we might offer a dedicated syntax for JS promises (async/await) in the future. Stay tuned!

Lazy Values Module