Conditional types

⬅️ Execute Program Advanced TypeScript

Conditional types

• with the ternary operator

type WrapStringInArray<T> = T extends string ? Array<string> : T;
const s: WrapStringInArray<string> = ['hello'];

• with mapped types

type WrapStringsInArrays<T> = {
  [K in keyof T]: T[K] extends string ? Array<string> : T[K]
};
 
type User = {
  name: string
  email: string
  age: number
};
 
const amir: WrapStringsInArrays<User> = {
  name: ['Amir'],
  email: ['amir@example.com'],
  age: 36,
};
amir.name;
 
// another example, replacing nums with null
type ReplaceNumberPropertiesWithNull<T> = {
   [K in keyof T]: T[K] extends number ? null : T[K]
};

• 💡 Regular application code rarely needs conditional types.
• However, conditional and mapped types are critical in code that needs to be highly generic, like library and framework code.