Readonly Members
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Readonly Properties
A readonly property can be assigned when it is declared or initialized in a constructor, but any assignment attempted after that point is rejected at compile time, even from within the class itself.
Example: Readonly Properties
class Config {
readonly version: string = "1.0";
}
const config = new Config();
// config.version = "2.0"; // rejected: readonly
console.log(config.version);Readonly with Constructors
Readonly properties are often initialized through constructor parameters, which is useful for values like an ID or creation timestamp that should be fixed for the entire lifetime of the object.
Example: Readonly with Constructors
class User {
readonly id: number;
constructor(id: number) {
this.id = id; // allowed: assigned inside the constructor
}
}
console.log(new User(101).id);Readonly Arrays
TypeScript also provides readonly array types; a readonly array cannot be modified through mutating methods like push or splice, though a new array can still be produced and assigned elsewhere.
Example: Readonly Arrays
const tags: readonly string[] = ["ts", "types"];
// tags.push("more"); // rejected: readonly arrays block mutation
console.log(tags);Readonly Method Access
Readonly affects assignment to a property, not reading it — code can freely read and pass around a readonly value, TypeScript only blocks the specific act of writing a new value into it.
Example: Readonly Method Access
class Point {
constructor(public readonly x: number) {}
}
const p = new Point(5);
console.log(p.x * 2); // reading is fineWhen to Use Readonly
Readonly is appropriate for identifiers, creation dates, configuration values, and other data that should never legitimately change after an object is built, since it turns an accidental mutation into a compile-time error instead of a subtle runtime bug.
Example: When to Use Readonly
class Event {
readonly createdAt: Date = new Date();
}
console.log(new Event().createdAt instanceof Date);Chapter Quiz — Complete all 9 topics to unlock
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