Access Modifiers

Access modifiers describe which TypeScript code is permitted to access class members.

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

The default is public. TypeScript private and protected are primarily compile-time access checks and are normally emitted as regular JavaScript properties. For runtime-enforced privacy use ECMAScript #private fields; the separate private-fields lesson covers them.

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 Wallet {
  private balance = 0;
  deposit(amount: number): void {
    if (amount > 0) this.balance += amount;
  }
  getBalance(): number { return this.balance; }
}
const wallet = new Wallet();
wallet.deposit(25);
console.log(wallet.getBalance());

Verified output

25

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.

Wallet starts with balance 0. Calling deposit(25) passes its positive-value check and adds 25. The public getter returns 25 while direct wallet.balance access is rejected by TypeScript.

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.

TypeScript private is not a security boundary in emitted JavaScript. Do not rely on it to hide secrets from untrusted code.

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 withdraw method that refuses amounts larger than the balance, and verify both outcomes.

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.

Access Modifiers: Access modifiers describe which TypeScript code is permitted to access class members.

Wallet starts with balance 0. Calling deposit(25) passes its positive-value check and adds 25. The public getter returns 25 while direct wallet.balance access is rejected by TypeScript.

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