NodeJS：从交互式控制台处理承诺

Question

I'm learning NodeJS. One thing I'm finding really annoying is when working with promises from an interactive console while debugging and/or just poking around at the interfaces of various libraries. The work flow is normally; call some library to do some work, wait for result, poke around at the result to see how it actually behaves and looks like.

For example (and just for example) I want to interactively inspect the image data structure returned from Jimp . I'm finding myself doing this from the console:

const Jimp = require('jimp');
const fs = require('fs');
let data = fs.readFileSync('path/to/my/test/images')
let image = null
Jimp.read(data).then((result, error) => { image = result; });
// Work with image here ...

This boiler plate gets annoying to write .. I think I missed a trick. Is there a more convenient way around this? A setting can set, or a library I can load or something?

I'd rather do this:

...
let image = await Jimp.read(data);
// Work with image here ...

But this gives "SyntaxError: await is only valid in async function". Or if not that, then maybe:

...
let image = resolveMe(Jimp.read(data))
// Work with image here ...

---

PS "async / await" is not an answer to this question. In order for an async function to be evaluated you have to give control back to the event loop, and you can't do that at an interactive prompt currently AFAIK.

Answer 1

I stumbled on a SO answer for my Q. For node v12+ you can set the --experimental-repl-await option. Was right under my nose all along ... Summary:

Without:

$ node
Welcome to Node.js v12.18.1.
Type ".help" for more information.
> await Promise.resolve(7)
await Promise.resolve(7)
^^^^^
Uncaught SyntaxError: await is only valid in async function

With:

$ node --experimental-repl-await
Welcome to Node.js v12.18.1.
Type ".help" for more information.
> await Promise.resolve(7)
7

Answer 2

That's what async - await is for. It lets you write async code in a way that looks like synchronous code, even though under the hood it is exactly the same as using Promises. Promises end up being very awkward with all the nesting of functions.

You can just write:

let image = await Jimp.read(data); 
// do something with image

And the only requirement is that the function you do it in has to be declared async. That means you need at the very least to have one function, you can't use await at the outermost level.

Answer 3

Node.js doesn't really have a way to wait until an event, AFAICT. But in interactive mode, assuming you waited, all you care about is doing something with the result, so I tried to find convenient ways to get it.

The closest I can come up with is some helper like this:

Promise.prototype.a = function() {
    this
        .then(res => {
            Promise.res = res;
            console.log("async", res);
        })
        .catch(console.error)
}

Example usage:

Welcome to Node.js v12.2.0.
Type ".help" for more information.
> Promise.prototype.a = function() {
...     this
...         .then(res => {
.....           Promise.res = res;
.....           console.log("async", res);
.....       })
...         .catch(console.error)
... }
[Function]
> 
> function sleep(ms) {
...     return new Promise((resolve) => {
.....       setTimeout(resolve, ms);
.....   });
... }  
undefined
> 
> async function afterSleep(sec) {
...     await sleep(sec * 1000);
...     return 100;
... }
undefined
> afterSleep(2).a()
undefined
> async 100

> Promise.res
100

You still cannot wait for it to finish, but at least the result is available.