Correct way to get a count of matching elements in Nightwatch?

I found the following very elegant solution within a VueJS template. It shows how to add a custom assertion into Nightwatch that counts the number of elements returned by a selector. See http://nightwatchjs.org/guide#writing-custom-assertions for details of how to write custom assertions within Nightwatch.

Once installed, usage is as simple as:

browser.assert.elementCount('#todo-list li', 2)

The plugin:

// A custom Nightwatch assertion.
// the name of the method is the filename.
// can be used in tests like this:
//
//   browser.assert.elementCount(selector, count)
//
// for how to write custom assertions see
// http://nightwatchjs.org/guide#writing-custom-assertions
exports.assertion = function (selector, count) {
  this.message="Testing if element <" + selector + '> has count: ' + count;
  this.expected = count;
  this.pass = function (val) {
    return val === this.expected;
  }
  this.value = function (res) {
    return res.value;
  }
  this.command = function (cb) {
    var self = this;
    return this.api.execute(function (selector) {
      return document.querySelectorAll(selector).length;
    }, [selector], function (res) {
      cb.call(self, res);
    });
  }
}

This code was added to vuejs-templates by yyx990803 in 2016. So full credit goes to yyx990803.