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.

---

⚠️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`

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.

---

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'
}

Happy Coding☀️

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