Generate Readonly TypeScript Interfaces from JSON

Immutable Types from JSON

By default, all properties on a TypeScript interface are mutable. Adding readonly prevents reassignment after construction, catching accidental mutations at compile time.

Example JSON

{
  "id": "usr_001",
  "createdAt": "2024-01-15T08:00:00Z",
  "name": "Alice",
  "permissions": ["read", "write"]
}

Generated TypeScript (readonly)

interface User {
  readonly id: string;
  readonly createdAt: string;
  readonly name: string;
  readonly permissions: readonly string[];
}

When to Use Readonly

• IDs and timestamps — These are set by the server and should never be changed by client code.
• Configuration objects — Parsed once, used everywhere, never mutated.
• Redux/state management — Immutable state patterns benefit from readonly types.

Readonly vs Readonly<T>

You can apply readonly per-property or use the built-in utility type:

// Per-property
interface User {
  readonly id: string;
  name: string; // mutable
}

// All properties
type ImmutableUser = Readonly<User>;

Readonly<T> is shallow — nested objects remain mutable unless you apply it recursively:

type DeepReadonly<T> = {
  readonly [K in keyof T]: T[K] extends object
    ? DeepReadonly<T[K]>
    : T[K];
};

Readonly Arrays

Use readonly string[] or ReadonlyArray<string> to prevent .push(), .pop(), and direct index assignment. This is especially important for arrays in state management where mutations cause bugs.

Practical Guideline

Make everything readonly by default and only remove it when you explicitly need mutation. This aligns with the principle of least privilege and catches a large class of bugs.