Lessons · JavaScript · classes and new
A class builds objects with new
A class describes what a kind of thing has and does. new Dog('Rex') builds one Dog and runs its constructor on it.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
A user, an order, a game piece, a connection: real programs are full of things that carry their own data and actions. A class keeps the two together instead of scattering them across plain objects and loose functions.
How to think about it
Ask: is this a KIND of thing I will make many of? Write a class for the kind; build each one with new. The class name is capitalised, the objects are ordinary variables.
Worked example
class Dog {The blueprint. constructor(name) {Runs once per new Dog.this.name = name;this is the object being built.
}
bark() { return this.name + ' barks'; }A method: a function every Dog has.}
const rex = new Dog('Rex');new builds the object and runs the constructor.console.log(rex.bark(), typeof Dog);Rex barks function
Your turn
Build one Cat from the class.
class Cat {
constructor(name) { this.name = name; }
}
const tom = Cat('Tom');Solve one with the tests running
The trap
Leaving out new. Dog('Rex') throws 'Class constructor Dog cannot be invoked without new'; a class is not a plain function call.