Simple function to convert functions with callback APIs to have Promise APIs.

promisify.js

/**
 * Converts a function from a callback API to a Promise API.
 *
 * Example:
 *   const context = navigator.geolocation;
 *   const f = context.getCurrentPosition;
 *   const getCurrentPosition = promisify(f, 0, 1, context);
 *   const enableHighAccuracy = true;
 *   getCurrentPosition({enableHighAccuracy})
 *     .then(r => console.log(r.coords), e => console.error(e));
 *
 * @param {!Function} f a function
 * @param {number} successI parameter index of the success callback
 * @param {number=} errorI parameter index of the error callback
 * @param {?=} context context to use when calling f
 */
const promisify = (f, successI, errorI, context) => {
  return function() {
    const args = [].slice.call(arguments, 0);
    return new Promise((resolve, reject) => {
      args.splice(successI, 0, resolve);
      if (errorI != null) {
        args.splice(errorI, 0, reject);
      }
      f.apply(context || this, args);
    });
  };
};