Private Fields

The difference between TypeScript private fields and ECMAScript

TypeScript private is erased at compile time. ECMAScript #private fields are enforced by JavaScript at runtime.

Use #private when outside code must not read a field, even through casts or reflection-style access.

class SecureConfig {
  readonly name: string;
  #apiKey: string;
 
  constructor(name: string, apiKey: string) {
    this.name = name;
    this.#apiKey = apiKey;
  }
 
  makeRequest(endpoint: string): string {
    return `${endpoint}?key=${this.#apiKey}`;
  }
}
 
const config = new SecureConfig("prod", "secret-key-123");
console.log(config.name);
// console.log(config.#apiKey); // syntax error outside the class

In production

TypeScript private is fine for normal internal APIs. Use #private for runtime privacy, such as secrets that should not appear during object walking, serialization, or casual debugging.

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