TypeScriptconditional-utils.ts
Copy
1
// Utility: is T an array?
2
type IsArray<T> = T extends any[] ? true : false;
3
type T1 = IsArray<string[]>;  // true
4
type T2 = IsArray<string>;    // false
5
 
6
// Flatten one level of array
7
type FlattenArray<T> = T extends Array<infer U> ? U : T;
8
type T3 = FlattenArray<string[][]>;  // string[]
9
type T4 = FlattenArray<string[]>;    // string
10
type T5 = FlattenArray<string>;      // string (not an array)
11
 
12
// Deep readonly
13
type DeepReadonly<T> = T extends (infer U)[]
14
  ? ReadonlyArray<DeepReadonly<U>>
15
  : T extends object
16
  ? { readonly [K in keyof T]: DeepReadonly<T[K]> }
17
  : T;
18
 
19
interface Config {
20
  server: { host: string; port: number };
21
  db: { url: string; pool: { min: number; max: number } };
22
}
23
 
24
type FrozenConfig = DeepReadonly<Config>;
25
// All nested fields are readonly — even server.host and db.pool.min
26
 
27
// Filter union members
28
type NonNullableUnion<T> = T extends null | undefined ? never : T;
29
type T6 = NonNullableUnion<string | null | undefined | number>;
30
// string | number
31
 
32
// Test runner
33
type Assert<T extends true> = T;
34
type Test1 = Assert<IsArray<string[]>>;
35
type Test2 = Assert<IsArray<number> extends false ? true : false>;
36
type Test3 = Assert<FlattenArray<string[]> extends string ? true : false>;
37
 
38
console.log('All type tests pass (TypeScript validates at compile time)');