Promisifying Callback based API

Convert callback-based APIs to promise-based API:

// A wrapper for callback-based API that returns a function which uses promises

function promisify(callbackBasedAPI) {
  return function promisfiedApi() {
    const args = [].slice.call(arguments);
    return new Promise(function (resolve, reject) {
      args.push(function (err, result) {
        if (err) {
          return reject(err);
        }
        if (arguments.length <= 2) {
          resolve(result);
        } else {
          resolve([].slice.call(arguments, 1));
        }
      });
      callbackBasedApi.apply(null, args);
    });
  };
}

// Usage:
const promiseBasedApi = promisify(callbackBasedApi);

What’s happening:

• [].slice.call(arguments) is required as arguments is an array-like object that does NOT inherit most array methods like slice
• The function pushes a special callback as the last argument. The callback simply rejects promise if an error is present. Otherwise, the callback resolves promise either with a single result or an array of results. (note that the arguments within callback refers to the parameters passed into callback, e.g. err, result)
• callbackBasedApi is called with the argument list that was built earlier