ECMAScript Private Fields

ECMAScript #private fields are private at JavaScript runtime, not merely restricted by TypeScript’s access checker.

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

A #balance field cannot be read using normal bracket or dot access from outside the class. This differs from TypeScript’s private keyword, which is commonly a compile-time-only restriction. Private fields also have distinct inheritance and branding behavior.

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 Vault {
  #balance = 0;
  deposit(amount: number): void {
    if (amount > 0) this.#balance += amount;
  }
  balance(): number { return this.#balance; }
}
const vault = new Vault();
vault.deposit(40);
console.log(vault.balance());
console.log("#balance" in vault);

Verified output

40
false

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.

Vault starts with a hidden runtime #balance slot. deposit adds 40 through class code, and balance returns it. The public property-name check with the string "#balance" is false because a private field is not an ordinary string-keyed property.

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 attempt vault["#balance"] to access the private field; private names are not equivalent to regular property keys.

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.

Write a #pin field and expose a verifyPin(input) method. Explain what TypeScript private would change.

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.

ECMAScript Private Fields: ECMAScript #private fields are private at JavaScript runtime, not merely restricted by TypeScript’s access checker.

Vault starts with a hidden runtime #balance slot. deposit adds 40 through class code, and balance returns it. The public property-name check with the string "#balance" is false because a private field is not an ordinary string-keyed property.

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