ES6 classes are often called syntactic sugar. What does that mean, and are they only sugar?

"Syntactic sugar" means a nicer way to write something that already exists. A class is mostly that: it still uses constructor functions and prototypes underneath. But it also adds real behaviour, so it is not only sugar.

What is the same as prototypes:

• class Dog extends Animal {} creates a constructor function. typeof Dog is 'function'.
• Methods go on Dog.prototype.
• extends links Dog.prototype to Animal.prototype. It also links Dog itself to Animal, so static methods are inherited.
• There is no separate class-based object model.

What is different from constructor functions:

• Calling a class without new throws an error.
• Class bodies are always in strict mode.
• Methods are non-enumerable, so they do not show up in for...in loops.
• A class declaration is in the temporal dead zone (TDZ) until its line runs, so you cannot use it earlier.
• In a derived class, you must call super() before you use this.

Classes also offer features that are hard to build by hand. Examples are truly private #fields and static initialization blocks.