28 — Classes in TypeScript

One line: Almost everything TypeScript adds to a class is checked by tsc and then erased, so each modifier is a compile-time promise, not a run-time wall.

JavaScript classes already have fields, # private names, static members and inheritance; this chapter covers what TypeScript layers on top. It starts with private against #private, the clearest case of a check that exists only at compile time, then parameter properties, the class feature Node's type stripping refuses. abstract and implements come next, then the readonly and override modifiers, and finally the two uses of this in types: the this parameter and the polymorphic this return type.

The pages

Page	Level	In one line	Status
private versus #private	201	private is a compile-time check that Node erases: acct["pin"] type-checks and prints 1234, and JSON.stringify and Object.keys both show the field. A #balance field is enforced by the JavaScript engine and shows up in neither.	stub
Parameter properties	201	constructor(public x: number) declares and assigns a field in one go, which means TypeScript has to generate code — so Node's type stripping refuses it with ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX, and erasableSyntaxOnly makes tsc refuse it first.	stub
abstract and implements	201	implements checks a class against an interface but types nothing inside it — check(s) in a class implementing check(name: string) is still an implicit-any error — and abstract makes new Shape() and a subclass missing area() compile errors.	stub
readonly and override	201	readonly blocks this.port = 0 inside the class but not a write through a { port: number } alias of the same object; override catches a misspelt method, but leaving it off is an error only under noImplicitOverride, which strict leaves out.	stub
this in types	301	A method whose return type is this returns the subclass, so new HtmlBuilder().add("x").bold() type-checks where a : Builder return type loses bold; a this: parameter makes tsc reject a bare greet() call.	stub

Boundaries

Classes as JavaScript defines them — prototypes, # fields, super and static — are in Prototypes and classes.