How to resolve promises when using app with REPL

问题

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)

回答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.

回答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.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!' }
>

回答3:

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

来源：https://stackoverflow.com/questions/44169542/how-to-resolve-promises-when-using-app-with-repl

标签

javascript

node.js

promise

read-eval-print-loop

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