#Promise.try S4

#Promise.try

ECMAScript Proposal, specs, and reference implementation for Promise.try

Spec drafted by @ljharb.

This proposal is currently stage 4 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's Promise.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