Recursive readonly object type in TypeScript

We already solved several challenges with readonly:

• Readonly under the hood
• Partial readonly

Today we discuss Deep Readonly

We apply readonly modifier to all object types inside our object type recursively if we need to prohibit mutations of the whole object, but not only the first layer.

Recursive conditional types

If we want to make an object immutable for the first layer only, we use normal Readonly:

To make other layers immutable, we need to apply it recursively.

Solving Unwrapping the Promises, we already used Recursive conditional types:

We need to check if a current value is of an object type. If so, apply DeepReadonly recursively:

PropertyKey means everything that can be a key in an object (it's one of built-ins in Typescript):

Check out the solution on Playground

Have a good week ☀️