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const vs readonly

Written byPhuoc Nguyen
20 Aug, 2020


  1. `const` is used for variables
    const message = 'Hello';

    // Does not work
    message = 'World';
    While `readonly` is used for properties. The properties can be declared as a member of class
    class Triangle {
    public readonly numberOfVertices = 3;

    const triangle = new Triangle();

    // Does not work
    triangle.numberOfVertices = 4;
    or `type`, `interface`:
    interface Person {
    firstName: string;
    lastName: string;
    readonly fullName: string;
  2. `const` declarations have to be initialized, and you can't reassign their values. The `readonly` properties can be reassigned in the constructor function.
    class Square {
    readonly numberOfVertices: number;

    constructor() {
    this.numberOfVertices = 4;
    The `readonly` properties could be changed if we don't pass their class or interface directly but passing an alias.
    Let's take a look at the `Person` interface above, and assume that we have the following function to update the person information:
    const updatePerson = (person: { firstName: string, lastName: string, fullName: string }) => {
    person.fullName = `\${firstName}, \${lastName}`;
    We can update the `fullName` property because it's an property of `person` parameter:
    let person: Person = {
    firstName: 'Foo',
    lastName: 'Bar',
    fullName: 'Foo Bar',


    person.fullName; // `Foo, Bar`
    Of course, the compiler will throw an error if we pass the original type `Person`:
    const updatePerson = (person: Person) => {
    // Error: Cannot assign to 'fullName' because it is a read only property
    person.fullName = `\${person.firstName}, \${person.lastName}`;

Good to know

  1. In a given class, if a property has only getter method and doesn't come with setter method, it will be treated as read only.
    class Square {
    side: number = 0;

    get area() {
    return this.side * this.side;

    const s = new Square();
    Setting `s.area = 100` will throw an error because `area` is a ready only property.
  2. In the React library, we don't change the props and state of a component directly. Because the props are immutable and the state could be updated via `setState()` method.
    React type definitions wrap the props and state in read only type.
    // P, S represents the props and state respectively
    class Component<P, S> {
    constructor(props: Readonly<P>);

    readonly props: Readonly<P> & Readonly<{ children?: ReactNode }>;
    state: Readonly<S>;

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Phước Nguyễn