Typescript

How the TypeScript Readonly Type Works

TypeScript has a number of utility types, which are types specifically created by TypeScript to solve a problem. In this article, let's look at the Readonly type.

TypeScript Readonly Type

As the name suggests, the Readonly type in TypeScript suggests that a particular type is read-only.

Let's look at an example. Below, we don't want anyone to update any part of myObject. We can make it a read-only object like so:

type User = {
    firstName: string,
    lastName: string
}

let firstUser:Readonly<User> = {
    firstName: "John",
    lastName: "Doe"
}

If you try to change a property of firstUser, you'll get the following error:

Cannot assign to 'firstName' because it is a read-only property.

Readonly variables don't work in TypeScript

When we define the type User above, we are creating a custom interface - i.e. something which objects have to conform to. Readonly only works with interfaces or custom types like the one we've used. As such, we can still edit Readonly variables:

let myVariable:Readonly<string> = "Hello World";
myVariable = "Goodbye World";
console.log(myVariable); // console logs "Goodbye World"

The above code is valid and will work in TypeScript. If you need read only variables, you can simply use const instead, i.e:

const myVariable:string = "Hello World";
// Throws error: Cannot assign to 'myVariable' because it is a constant.
myVariable = "Goodbye World";

More Tips and Tricks for Typescript

• Ultimate Guide to Types in Typescript
• How keyof works in Typescript
• TypeScript Optional Parameters
• How the TypeScript Exclude Type Works
• How the TypeScript Omit Type works
• How the TypeScript NonNullable Type Works
• How the TypeScript Extract Type Works
• How the TypeScript Readonly Type Works
• How the TypeScript Record Type Works
• Creating Custom Types in Typescript