async-repl

Say goodbye to Promise { <pending> } in REPLs, this one supports async and await!.

Example:

$ npm start
> await 123
123
> Promise.resolve('a')
'a'
> await Promise.resolve('a')
'a'
> wait = delay => new Promise(resolve => setTimeout(resolve, delay))
[Function: wait]
> log = console.log
[Function: bound log]
> log('a'); await wait(1000); log('b'); await wait(1000); log('c')
a
b
c
undefined

You can declare async functions with many await statements inside, use Promise.all and Promise.race and so on.

Warning: This is a quick and dirty experiment, not a really usable program or library.

Gotchas

Currently, variable declaration doesn’t work as expected. But global variable declations are implicit (use a = 123 instead of var a = 123 or const a = 123). See the next section to understand why.

How does it work?

When you type await 123 in the REPL, it is transformed to something like (async () => await 123)(). Then, it is evaluated and the value of the returned promise is displayed.

Since every input line is wrapped inside a function with a local scope, variables declarations are not shared with other lines. I think the best way to avoid it is to transform variable declarations into assignements to global variables, by removing the var, let or const in the root function. const variables could be mutated but it’s not a big deal.

(In fact, things are a bit more complicated with a dirty Babel hack to handle semicolons and to add a return statement at the end of the async function)