How to resolve promises when using app with REPL

Question

I've got a basic Node webserver (Koa.js + a ORM). I like to start it with a REPL meaning I can use my app like a CLI-tool.

All my queries return Promises but I don't know how I can resolve them in the REPL. How can I resolve them?

For example the following code (fetch() queries the database and returns a promise) gives only this output Promise {_bitField: 4325376, _fulfillmentHandler0: undefined, _rejectionHandler0: undefined …}

Transaction.where('reference', '1').fetch().then((res) => return res)

This looks very much like a Bluebird promise. In that case, you can do [`_.value()`](http://bluebirdjs.com/docs/api/value.html) in the following line — Bergi, May 24 '17 at 23:25

Benjamin Gruenbaum · Accepted Answer · 2017-12-20T21:32:09.237

14

Update: Node.js now does this by default and resolves promises

Old answer:

You can't resolve them proper - but you can extract their references to the global scope:

> Transaction.where('reference', '1').fetch().then((res) => out = res)
[Object Promise]
> out
  /* your data outputted here since the global was assigned to*/

We might allow await in the REPL in the future in Node which would solve the issue more cleanly.

edited Dec 20 '17 at 21:32

answered May 24 '17 at 22:37

Benjamin Gruenbaum

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Woah, this is AMAZING and will save me a ton of time. Is there an ongoing discussion somewhere about the inclusion of `await` in the REPL? Having it land in 8.0 would be the bomb :) – Hedge May 24 '17 at 22:45
@Hedge you have `util.promisify` in 8.0 but await in the REPL likely won't make it (the release is in 5 days), we want to see how people are using async/await in Node. – Benjamin Gruenbaum May 24 '17 at 22:47
1

Opened an issue for tracking https://github.com/nodejs/node/issues/13209 – Benjamin Gruenbaum May 24 '17 at 22:52
1

What version of Node.js does this work on? I can't seem to get it to work. Thanks. – mfulton26 Mar 16 '18 at 15:13
1

@mfulton26 9.x latest and 10.x nightly – Benjamin Gruenbaum Mar 16 '18 at 20:55
5

With 10.0.0, you need to start it `node --experimental-repl-await` and then `await Transaction.where(...)` – MnZrK May 06 '18 at 13:56

score 6 · Answer 2 · answered May 26 '17 at 06:02

Just setting a global return value may (and often will) show wrong results if not waiting for the promise fulfillment.

To ensure the user have a fulfilled promise you may provide your own evaluator to the repl server:

// sample-repl.js
const repl=require('repl');
function replEvalPromise(cmd,ctx,filename,cb) {
  let result=eval(cmd);
  if (result instanceof Promise) {
    return result
      .then(response=>cb(null,response));
  }
  return cb(null, result);
}
repl.start({ prompt: 'promise-aware> ', eval: replEvalPromise });

Such REPL only returns control to the user after the promise got resolved:

$ node sample-repl.js
promise-aware> new Promise(resolve=>setTimeout(()=>resolve('Finished!'),5000));
'Finished!'
promise-aware> out = new Promise(resolve=>setTimeout(()=>resolve('Finished!'),5000));
'Finished!'
promise-aware> out
'Finished!'
promise-aware>

Note as it sets the proper global return variable with the resolved value.

The standard node REPL works like that instead:

> out = new Promise(resolve=>setTimeout(()=>resolve('Finished!'),5000));
Promise { <pending> }
> out
Promise { <pending> }
> out
Promise { <pending> }
> out
Promise { <pending> }
> out
Promise { 'Finished!' }
>

score 1 · Answer 3 · edited Jan 09 '20 at 12:37

1

There are userland packages that implement this, for example https://github.com/skyrising/await-repl and https://github.com/StreetStrider/repl.js

edited Jan 09 '20 at 12:37

Hedge

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answered May 24 '17 at 23:08

Refael Ackermann

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How to resolve promises when using app with REPL

3 Answers3

Update: Node.js now does this by default and resolves promises

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