Summary - Readonly

Write an immutable configuration with readonly fields.

Summary - Readonly

A Settings class creates an immutable configuration model with readonly fields. Each field is assigned in the constructor and frozen afterwards; this keeps the configuration consistent.

class Settings {
  readonly env: string;
  readonly version: number;
  constructor(env: string, version: number) {
    this.env = env;
    this.version = version;
  }
}
const s = new Settings("prod", 2);
console.log(s.env);
console.log(s.version);

Exercise Settings class with readonly fields

Define a Settings class with readonly env: string and readonly version: number fields. Add a constructor that takes env and version parameters and assigns them with this. Create a new Settings("prod", 2) and print env and version.

Show example solution

class Settings {
  readonly env: string;
  readonly version: number;
  constructor(env: string, version: number) {
    this.env = env;
    this.version = version;
  }
}

const s = new Settings("prod", 2);
console.log(s.env);
console.log(s.version);

Test yourself

In which cases can a readonly field be assigned?
- From outside, always
- Inside a method
- At declaration or in the constructor
- Only in a static method
Show answer

At declaration or in the constructor — A readonly field can only be assigned at its declaration (initial value) or inside the constructor. Methods, external code, or static methods cannot assign it.

Complete the readonly env field

class Settings {
  ____ env: string;
  readonly version: number;
  constructor(env: string, version: number) {
    this.env = env;
    this.version = version;
  }
}

Show answer

class Settings {
  readonly env: string;
  readonly version: number;
  constructor(env: string, version: number) {
    this.env = env;
    this.version = version;
  }
}

Settings parts

readonly env

Immutable environment
readonly version

Immutable version
new Settings("prod", 2)

Instance creation
s.env

Read configuration

Next lesson: What is a generic?

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