REPL with standalone await + babel transform

await-babel-repl.js

const repl = require('repl');
const babel = require('babel-core');

function preprocess(input) {
  const awaitMatcher = /^(?:\s*(?:(?:let|var|const)\s)?\s*([^=]+)=\s*|^\s*)(await\s[\s\S]*)/;
  const asyncWrapper = (code, binder) => {
    let assign = binder ? `global.${binder} = ` : '';
    return `(function(){ async function _wrap() { return ${assign}${code} } return _wrap();})()`;
  };

  // match & transform
  const match = input.match(awaitMatcher);
  if (match) {
    input = `${asyncWrapper(match[2], match[1])}`;
  }
  return input;
}

function myEval(cmd, context, filename, callback) {
  const code = babel.transform(preprocess(cmd), {
    presets: ['es2015', 'stage-3'],
    plugins: [
      ["transform-runtime", {
        "regenerator": true
      }]
    ]
  }).code;
  _eval(code, context, filename, callback);
}


const replInstance = repl.start({ prompt: '> ' });
const _eval = replInstance.eval;
replInstance.eval = myEval;

By running the latest stable node version with the --harmony flag, you can remove the need for babel altogether. Just replace function myEval by:

function myEval(cmd, context, filename, callback) {
  const code = preprocess(cmd);
  _eval(code, context, filename, callback);
}

and run node --harmony await-babel-repl.js

a = await new Promise(res => res(1)) // => 1

@christophemarois I installed the latest node (it has native async function support), but it's REPL still does not understand await. Top level await support was never supported by node... I tried with your code and it gives back the promise, it does not wait it out. I tried with this:

const timeout = function (delay) {  
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve()
    }, delay)
  })
}
await Promise.resolve(timeout(3000));

Currently there is a discussion about REPL await support here: nodejs/node#13209 If there is a way to make the REPL hang, then it is possible to implement this. Probably it is possible with a while(1) + break, I'll try it out later.

Can't hang with while(1) because it hangs the event loop and so it does not let the promise to be fulfilled.

function syncAwait(condition, timeout){
	let expirationTime = Date.now() + timeout;
	while(1){
		if (condition())
			return "awaited";
		if (Date.now() > expirationTime)
			return "expired";
	}
}

var i = false;
setTimeout(function (){
	i = true;
}, 2000);

syncAwait(function (){
	return i;
}, 50000);

This returns only expired because the setTimeout can't run until the while loop is finished.