The distinction between `readonly` and `const` is crucial for developers working with TypeScript and JavaScript, particularly when dealing with variable declarations and object properties. Understanding these two keywords can significantly impact how you manage data immutability and variable scope in your applications.
Both `readonly` and `const` serve to prevent reassignment, but they do so in different contexts and with different implications.
The `const` keyword is used to declare variables that cannot be reassigned after their initial assignment. This means that once a variable is declared with `const`, it cannot be assigned a new value. However, it is important to note that `const` does not make the value itself immutable. If the value is an object or an array, the contents can still be modified.
const myArray = [1, 2, 3];
myArray.push(4); // This is allowed
console.log(myArray); // Output: [1, 2, 3, 4]
myArray = [5, 6, 7]; // This will throw an error: Assignment to constant variable.
The `readonly` modifier is primarily used in TypeScript to denote that a property of an object cannot be reassigned after its initial value has been set. This is particularly useful in class definitions or interfaces, where you want to ensure that certain properties remain unchanged throughout the lifecycle of an object.
class User {
readonly id: number;
name: string;
constructor(id: number, name: string) {
this.id = id;
this.name = name;
}
}
const user = new User(1, 'Alice');
user.name = 'Bob'; // This is allowed
user.id = 2; // This will throw an error: Cannot assign to 'id' because it is a read-only property.
| Feature | const | readonly |
|---|---|---|
| Scope | Variable scope | Property of an object or class |
| Reassignment | No reassignment allowed | No reassignment allowed for properties |
| Mutability of value | Mutable if object/array | Immutable reference to the property |
In summary, while both `const` and `readonly` serve to prevent reassignment, they operate in different contexts and have unique implications for variable and property management. Understanding these differences is essential for writing robust and maintainable code.