Why `Readonly` in TypeScript Should Be Your Default

In TypeScript, the Readonly utility type can quietly save you from hard-to-diagnose bugs. Let’s understand how:

For demonstration purposes, let’s suppose we have a shared User object across multiple modules:

type User = {
  name: string
  age: number
}
 
const user: User = {
  name: 'Carlos',
  age: 18
}

At some point, a developer unknowingly modified the user name when developing a new screen and didn’t notice it:

user.name = 'Roberto' // we get no error

This change led to unexpected behavior in parts of the app relying on the original value. And things like this can be avoided using Readonly utility type:

type User = Readonly<{
  name: string
  age: number
}>
 
const user: User = {
  name: 'Carlos',
  age: 18
}
 
user.name = 'Roberto' // Error: Cannot assign to 'name' because it is a read-only property.

With Readonly, the compiler stopped us from making accidental mutations. This simple change makes the User type safe by design. When data is meant to stay constant, let the compiler protect it.