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typescript

TS2540: Cannot assign to 'X' because it is a read-only property

You're trying to modify a property that's been marked as readonly in its type definition. Readonly properties can only be set during initialization — any reassignment afterward is a type error.

Common causes

Reassigning a property declared as readonly in an interface or class
Mutating an object created with as const, which makes all properties deeply readonly
Modifying a property on an object typed as Readonly<T> or using Object.freeze()
Trying to reassign a class constructor parameter marked as readonly

How to fix it

Remove the readonly modifier if the property should be mutable
Create a new object instead of mutating: const updated = { ...original, property: newValue }
If using as const, only use it when you intend the object to be immutable
Use a mutable copy: const mutable = { ...readonlyObj } to get a writable version

Example

interface Config { readonly baseUrl: string; }
const config: Config = { baseUrl: 'https://api.example.com' };
config.baseUrl = 'https://other.com';
// error TS2540: Cannot assign to 'baseUrl' because it is a read-only property.

Reassigning a readonly interface property after the object has been initialized

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