Readonly vs Const - TypeScript

Introduction

In TypeScript, readonly and const are used to ensure that values are not modified. Both keywords appear to be similar, but they have a few key differences.

Const

const is used to declare a variable whose value is not allowed to change. The value must be assigned when the variable is declared, and cannot be reassigned thereafter.

For example:

const PI = 3.14159;
PI = 3.14; // Compile error!

Readonly

readonly is used to declare a property or a class member whose value cannot be modified once it is set. Unlike const, readonly can be declared without an initial value. The value can be set in the constructor of the class.

For example:

class Person {
    readonly name: string;
    readonly age: number;

    constructor(name: string, age: number) {
        this.name = name;
        this.age = age;
    }
}

let p1 = new Person("John Doe", 23);
p1.name = "Jane Doe"; // Compile error!

Differences

1. const is used for variables, while readonly is used for properties and class members.

2. const values must be assigned when declared, whereas readonly values can be set in the constructor.

3. const values cannot be reassigned, while readonly values cannot be modified once they are set.

Conclusion

In conclusion, const and readonly serve different purposes in TypeScript. const is used to declare variables whose values cannot be changed, while readonly is used to declare class members or properties that should not be modified once they are set.