What is the difference between readonly and const?

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.

Understanding `const`

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.

Example of `const`

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.

Understanding `readonly`

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.

Example of `readonly`

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.

Key Differences

Best Practices

• Use `const` for variables that should not be reassigned, promoting immutability where possible.
• Utilize `readonly` for properties in classes to enforce immutability and maintain integrity of data.
• Be cautious with objects declared as `const`; remember that their properties can still be modified unless marked as `readonly`.

Common Mistakes

• Assuming `const` makes the object itself immutable; it only prevents reassignment of the variable.
• Forgetting to use `readonly` in class properties, which can lead to unintended modifications.
• Using `const` for variables that should be mutable, which can lead to confusion and errors in the code.

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.