What is the difference between prototype-based class syntax and class syntax in JavaScript?

Question

Are these interchangeable syntax\'s to create a JS class? I\'m used to the class syntax but don\'t understand the differences between them exactly.

Answer 1

From MDN:

JavaScript classes, introduced in ECMAScript 2015, are primarily syntactical sugar over JavaScript's existing prototype-based inheritance. The class syntax does not introduce a new object-oriented inheritance model to JavaScript.

You can use a tool like the Babel REPL to see how the ES6, class-based code you have above is then transpiled into ES5, prototype-based code:

ES6 Class

class User {

 constructor(name) {
   this.name = name;
 }

 sayHi() {
   alert(this.name);
 }

}

let user = new User("John");
user.sayHi();

Transpiled to ES5

"use strict";

var _createClass = function () { function defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } } return function (Constructor, protoProps, staticProps) { if (protoProps) defineProperties(Constructor.prototype, protoProps); if (staticProps) defineProperties(Constructor, staticProps); return Constructor; }; }();

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

var User = function () {
  function User(name) {
    _classCallCheck(this, User);

    this.name = name;
  }

  _createClass(User, [{
    key: "sayHi",
    value: function sayHi() {
      alert(this.name);
    }
  }]);

  return User;
}();

var user = new User("John");
user.sayHi();

All ES6+ functionality will be able to be transpiled into ES5-- it is all, effectively, syntactic sugar. All "new features" can still be achieved in ES5. This is called, I believe, the "One JavaScript" doctrine.