JavaScript classes are syntactic sugar over what?

Classes in JavaScript are syntactic sugar over prototypal inheritance. Under the hood, classes are implemented using constructor functions and prototypes.

For example, a class:

class Animal {
  constructor(name) {
    this.name = name;
  }
  speak() {
    console.log(`${this.name} makes a sound.`);
  }
}

is equivalent to:

function Animal(name) {
  this.name = name;
}
Animal.prototype.speak = function() {
  console.log(this.name + ' makes a sound.');
};

Thus, classes simplify and make object creation and inheritance more readable, but do not change the fundamental prototype model in JavaScript.