code: function promisify(f) {                              // given an underlying function,
    return function _promisify(...args) {              // return a function which
      return new Promise(                              // returns a promise
        resolve => Promise.all([this, ...args])        // which, when all args are resolved,
          .then(
            parms => resolve(f.call(...parms))         // resolves to the function result
          )
      );
    };
  }
docs: "Promisify" a function, meaning to create a function which returns a promise for the value of the function once `this` and all arguments have been fulfilled.