Ts keyPoint

unknown any void never different

(num as unknown as string).split('')
foo(p as unknown){
  if(Array.isArray(p)){
    return p.length
  }
  return p.length // error: unknown can't own property
}

 foo:()=>void //I don't care the foo return type

// confirm return error or never return
function foo():never {throw new Error('error message')}
function foo():never { while(true){}}

Cumpute

const ref = useRef();
ref.current!.name // confirm have the current property
a?.b?.c  // while a isn't null, then a.b
a ?? 10  // while a isn't null or undefined ,then 10 , false '' 0  NaN is true 
 

Operator

interface Person {name: string}
 const getValue = (obj: Person , k: keyof Person)=>{
    return obj[k]
  }

 const getValue = <T, K extends keyof T>(obj: T, k: K)=>{
    return obj[k]
  }

type typeToNumber<T>{
  [k in keyof T]: number // 遍歷 traverse
}
type Person {name: string}
typeToNumber(Person) //{name: number}

范型

 type Person2 <T> = {
    name: string,
    age: T
  }

  const p: Person2<number> = {
    name: 'guolei',
    age: 18
  }

extends

// 用來約束范性的類型 T:number是不對的因為T是一個類不是變量
 function sum<T extends number>(value: T[]): number {
    // let count = 0;
    // value.forEach(v => count += v);
    // return count;
    
    return value.reduce((a,b)=>a+b, 0)
  }

infer 變量指代 類似let i = 0

  type foo<T> = T extends { t: infer K } ? K : boolean;

  const num: foo<{t: number} > = 100;

common unit

  interface Person {
    name: string
  }
  const getValue = (obj: Person, k: keyof Person)=>{
    return obj[k]
  }

  type Person2 <T> = {
    name: string,
    age: T
  }

  const p: Person2<number> = {
    name: 'guolei',
    age: 18
  }

  function sum<T extends number>(value: T[]): number {
    // let count = 0;
    // value.forEach(v => count += v);
    // return count;
    
    return value.reduce((a,b)=>a+b, 0)
  }

  type foo<T> = T extends { t: infer K } ? K : boolean;

  const num: foo<{ t: number }> = 100;
  

  type Animal = {
    name: string,
    age: number,
    sex?: string
  }

  type Partial2<T> = {
    [p in keyof T]?: T[p]
  }

  const particalAnimal: Partial2<Animal> = { name: 'xingming' }
  
  type Record2<T extends keyof any, K> = {
    [p in T]: K
  }

  const recordAnimal: Record2<string, string> = { 'k1': 'k661' }

  type Pick2<T, K extends keyof T> = {
    [p in K]: T[p] 
  }
  
  const pickAnimal: Pick2<Animal, 'name' | 'age'> = { name: 'kkkk', age: 100 }


  
  type Exclude2<T, K> = T extends K ? never: T
  
  const excludeAnimal: Exclude2<keyof Animal, 'name' | 'sex'> = 'age';

  type Omit2<T, K> = Pick2<T, Exclude2<keyof T, K>>;
  const omitAnimal: Omit2<Animal, 'name' | 'age'> = { sex: 'nan' }

  type Require2<T> = {
    [p in keyof T]-?: T[p]
  }

  const requireAnimal: Require2<Animal> = { name: 'k', age: 1232, sex: 'man'}


  type ReturnType2<T> = T extends () => infer K ? K : any;


    type foo2 = () => number;
  
  const num2: ReturnType2<foo2> = 100