Function promisify

lib/internal/util.js:455–514

(original)

let validateFunction;

function promisify(original) {
  // Lazy-load to avoid a circular dependency.
  if (validateFunction === undefined)
    ({ validateFunction } = require('internal/validators'));

  validateFunction(original, 'original');

  if (original[kCustomPromisifiedSymbol]) {
    const fn = original[kCustomPromisifiedSymbol];

    validateFunction(fn, 'util.promisify.custom');

    return ObjectDefineProperty(fn, kCustomPromisifiedSymbol, {
      __proto__: null,
      value: fn, enumerable: false, writable: false, configurable: true,
    });
  }

  // Names to create an object from in case the callback receives multiple
  // arguments, e.g. ['bytesRead', 'buffer'] for fs.read.
  const argumentNames = original[kCustomPromisifyArgsSymbol];

  function fn(...args) {
    return new Promise((resolve, reject) => {
      ArrayPrototypePush(args, (err, ...values) => {
        if (err) {
          return reject(err);
        }
        if (argumentNames !== undefined && values.length > 1) {
          const obj = {};
          for (let i = 0; i < argumentNames.length; i++)
            obj[argumentNames[i]] = values[i];
          resolve(obj);
        } else {
          resolve(values[0]);
        }
      });
      if (isPromise(ReflectApply(original, this, args))) {
        process.emitWarning('Calling promisify on a function that returns a Promise is likely a mistake.',
                            'DeprecationWarning', 'DEP0174');
      }
    });
  }

  ObjectSetPrototypeOf(fn, ObjectGetPrototypeOf(original));

  ObjectDefineProperty(fn, kCustomPromisifiedSymbol, {
    __proto__: null,
    value: fn, enumerable: false, writable: false, configurable: true,
  });

  const descriptors = ObjectGetOwnPropertyDescriptors(original);
  const propertiesValues = ObjectValues(descriptors);
  for (let i = 0; i < propertiesValues.length; i++) {
    // We want to use null-prototype objects to not rely on globally mutable
    // %Object.prototype%.
    ObjectSetPrototypeOf(propertiesValues[i], null);
  }