TS原生的Readonly只会限制一层写入操作，我们可以利用递归来实现深层次的Readonly。但要注意，TS对最大递归层数做了限制，最多递归5层。
```
type DeepReadony<T> = {
    readonly [P in keyof T]: DeepReadony<T[P]>
}

interface SomeObject {
  a: {
    b: {
      c: number;
    };
  };
}

const obj: Readonly<SomeObject> = { a: { b: { c: 2 } } };
obj.a.b.c = 3;    // TS不会报错

const obj2: DeepReadony<SomeObject> = { a: { b: { c: 2 } } };
obj2.a.b.c = 3;    // Cannot assign to 'c' because it is a read-only property.

```