DEV Community

Noah
Noah

Posted on

Cautions When Using readonly in TypeScript

The basic of readonly property

In Type Script, you can make the object of the properties of an object read-only.

const person: { readonly name: string  } = { name: 'Mike' }

person.name = 21;
// → Cannot assign to 'name' because it is a read-only property.
Enter fullscreen mode Exit fullscreen mode

⚠️① readonly is only at compile-time

In the compiled JavaScript code, the readonly declaration is removed, so it will not be detected as an error at runtime.

⚠️② readonly is not recursive.

const person: {
  readonly name: string;
  readonly academicBackground: {
    primarySchool: string
  }
} = {
  name: 'Mike',
  academicBackground: {
    primarySchool: 'School A'
  }
}

person.academicBackground.primarySchool = 'School B'
// You can change `person.academicBackground.primarySchool`
Enter fullscreen mode Exit fullscreen mode

If you want to make it read-only, also you need to put readonly to primarySchool.

const person: {
  readonly name: string;
  readonly academicBackground: {
    readonly primarySchool: string
  }
} = {
  name: 'Mike',
  academicBackground: {
    primarySchool: 'School A'
  }
}

person.academicBackground.primarySchool = 'School B'
// → Cannot assign to 'primarySchool' because it is a read-only property.
Enter fullscreen mode Exit fullscreen mode

Readonly

When the number of properties increases, adding readonly to each one becomes cumbersome and increases the amount of code.
You can refactor by using Readonly.

const obj: {
  readonly a : string;
  readonly b: string;
  readonly c: string;
  readonly d: string;
} = {
  a: 'a',
  b: 'b',
  c: 'c',
  d: 'd'
}

// ↓

const obj: Readonly<{
  a : string;
  b: string;
  c: string;
  d: string;
}> = {
  a: 'a',
  b: 'b',
  c: 'c',
  d: 'd'
}
Enter fullscreen mode Exit fullscreen mode

Happy Coding☀️

Top comments (0)