degenerator

Compiles sync functions into async functions

Sometimes you need to write sync looking code that's really async under the hood. This module takes a String to one or more synchronous JavaScript functions, and returns a new String that with those JS functions transpiled into async functions.

So this:

function foo() {
  return a('bar') || b();
}

Gets compiled into:

async function foo() {
    return await a('bar') || await b();
}

With the compiled output code, you can evaluate the code using the vm module in Node.js, or save the code to a file and require it, or whatever.

Example

You must explicitly specify the names of the functions that should be "asyncified". So say we wanted to expose a get(url) function that did and HTTP request and returned the response body.

The user has provided us with this implementation:

function myFn() {
  const one = get('https://google.com');
  const two = get('http://nodejs.org');
  const three = JSON.parse(get('http://jsonip.org'));
  return [one, two, three];
}

Now we can compile this into an asynchronous function, implement the async get() function, and finally evaluate it into a real JavaScript function instance with the vm module:

import vm from 'vm';
import { degenerator } from 'degenerator';

// The `get()` function is Promise-based (error handling omitted for brevity)
function get(endpoint: string) {
  return new Promise((resolve, reject) => {
    var mod = 0 == endpoint.indexOf('https:') ? require('https') : require('http');
    var req = mod.get(endpoint);
    req.on('response', function (res) {
      var data = '';
      res.setEncoding('utf8');
      res.on('data', function (b) { data += b; });
      res.on('end', function () {
        resolve(data);
      });
    });
  });
}

// Convert the JavaScript string provided from the user (assumed to be `str` var)
str = degenerator(str, [ 'get' ]);

// Turn the JS String into a real async function instance
const asyncFn = vm.runInNewContext(`(${str})`, { get });

// Now we can invoke the function asynchronously
asyncFn().then((res) => {
  // Do something with `res`...
});

API

degenerator(code: string, names: Array<string|RegExp>): String

Returns a "degeneratorified" JavaScript string, with async/await transplanted.