How To Resolve Promises When Using App With Repl

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

Solution 1:

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.

Solution 2:

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.jsconst repl=require('repl');
functionreplEvalPromise(cmd,ctx,filename,cb) {
  let result=eval(cmd);
  if (result instanceofPromise) {
    return result
      .then(response=>cb(null,response));
  }
  returncb(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> newPromise(resolve=>setTimeout(()=>resolve('Finished!'),5000));
'Finished!'
promise-aware> out = newPromise(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 = newPromise(resolve=>setTimeout(()=>resolve('Finished!'),5000));
Promise { <pending> }
> out
Promise { <pending> }
> out
Promise { <pending> }
> out
Promise { <pending> }
> out
Promise { 'Finished!' }
>

Solution 3:

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