public, private, and protected

public members are accessible wherever the object is visible; private members are limited to their declaring class; protected members are accessible within the class and its subclasses.

What you will learn: the purpose of public, private, and protected, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

These are TypeScript access-control rules, distinct from JavaScript #private runtime semantics. A subclass can use a protected base member, but outside code cannot access that member through the instance in checked TypeScript.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

class Person {
  constructor(public name: string, protected prefix: string) {}
  label(): string { return this.prefix + this.name; }
}
class Teacher extends Person {
  describe(): string { return `Teacher: ${this.prefix}${this.name}`; }
}
const teacher = new Teacher("Mira", "Dr. ");
console.log(teacher.label());
console.log(teacher.describe());

Verified output

Dr. Mira
Teacher: Dr. Mira

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

Person initializes name and protected prefix. Teacher inherits label and can use prefix in its own describe method. Both lines include Dr. Mira, but each has different surrounding text.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Do not read teacher.prefix directly outside the class hierarchy; protected allows subclass access, not arbitrary client access.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Add a private numeric id to Person and try to use it from Teacher; observe the access error.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

public, private, and protected: public members are accessible wherever the object is visible; private members are limited to their declaring class; protected members are accessible within the.

Person initializes name and protected prefix. Teacher inherits label and can use prefix in its own describe method. Both lines include Dr. Mira, but each has different surrounding text.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.