Awesome
Promise.try
ECMAScript Proposal, specs, and reference implementation for Promise.try
Spec drafted by @ljharb.
This proposal is currently stage 3 of the process.
Rationale
A common use case that I, and many others, have, is that I have a function, f
. This function may be async, and return a Promise, or it may not - I don’t wish to have to know. However, I'd like to wrap it in a Promise so that if it is async, or if it throws, I can lean on Promise semantics and .catch
to handle it.
The typical “easy to remember” way this is achieved in JS Promises is with Promise.resolve().then(f)
. This works great! It catches any exceptions thrown, and it Promise-wraps any thenable or value returned from the function. However, f
is needlessly run asynchronously, on a future tick.
If I want f
to be run on the same tick - since, after all, it might be synchronously returning a value - or if I want parallel semantics with an async function
up to the first await
- then I need to use new Promise(resolve => resolve(f()))
. This achieves my goal, but is not ergonomic to write nor easy to remember.
Using Promise.try(f)
, I can get the same semantics as the new Promise
mess above, nicely mirroring async function
, and allowing optimistically synchronous, but safe, execution of a function, and being able to work with a Promise afterwards. Yay!
Userland implementations
p-try
package (44M weekly downloads, 8.9B total downloads; alternate link)- Bluebird:
Promise.try
/Promise.attempt
- takes one function, calls it with no args. - Q:
Q.try
/Promise.prototype.fcall
-Q.try
takes one function, calls it with no args.Promise#fcall
is deprecated, but takes a list of arguments, and invokes the given function with that list of arguments. - when:
when.try
/when.attempt
- takes one function, and an optional list of arguments, and invokes the given function with that list of arguments. - ES6:
es6-promise-try
- takes one function, calls it with no args. Functionally equivalent to Bluebird'sPromise.try
, but a stand-alone implementation using ES6 Promises. - dojo/when
Further reading
- http://cryto.net/~joepie91/blog/2016/05/11/what-is-promise-try-and-why-does-it-matter/
- https://twitter.com/RReverser/status/695678489937186816
Naming
The most common name is try
, which has a clean parallel with a syntactic try
block. A common alternative name is attempt
, but this has been primarily for ES3 compatibility, and is not necessary here.
Spec
You can view the spec rendered as HTML.
Polyfills
- core-js v3.37+ (docs)
- es-shims: promise.try