Typing

Strong typing

• arr :Array = [1,2,3]
• symbol from ES6+

• never : type with no possible values. scenarios:

  function throwError(message: string): never {  throw new Error(message);}
  function infiniteLoop(): never {  while (true) {}}
  const emptyArray: never[] = []; // You can't push anything into it:
  type Impossible = string & number;
  

• tuple : heterogeneous array - fixed or vary size

• Type aliases
• type mytype = "abc" | "xyx"
• type mytype = String | number // OR, UNION
• type mytype = Partial
• type mytype = Partial
• type mytype = keyof Class1 // "key1" | "key2" | "key3" | ...

// &, INTERSECTION

interface A { a: number }
interface B { b: string }
type AB = A & B; // { a: number, b: string } 

• Utility Types

  • type mytype = Partial<T> / Required<T> : all properties of T to optional /required
  • type mytype = Readonly<T> : all properties of Type set to readonly.
  • type mytype = Pick<T> / omit<T>
  • type mytype = Record<K,T>
  • type mytype = ReturnType<() => string>
  • type mytype = NonNullable<T>
  • ...
  • so basically, source type : T,K || target type : mytype
  • more: https://chat.deepseek.com/a/chat/s/3c19b702-9118-49ed-9599-35975a1fbef5

• Index signature

  interface MyObject {
    [propName: string]: any; // Index signature
  }
  
  const obj: MyObject = {
    name: "Alice", // OK (string)
    age: 30,       // OK (number)
    isAdmin: true, // OK (boolean)
    someMethod() { return 42; } // OK (function)
  };
  
  // ----
  interface MyObject {
    [propName: Class1]: any; // INVALID : can only have keys of type string, number, or symbol
  }