Proper way to dynamically add functions to ES6 classes

Your solution is fine, though it’ll be better to create all those methods once on a prototype level:

['GET', 'PUT', 'POST', 'DEL'].forEach((method) => {
  Executor.prototype[method] = function (body) {
    return this.request(method, body)
  }
})

prototype approach is slightly faster, because this code is executed only once, while constructor code is executed every time new instance is created.

Another advantage of prototype over constructor is that its compatible with classes inheritance. So, if you’ll extend your class later nothing will break even if you’ll redefine any of those methods.

By the way, you can use require('http').METHODS or methods package instead of hard-coded array of HTTP verbs here.