keyof

interface P {
  name: string;
  age: number;
  gender: string;
}
class T {
  constructor(private info: P) {}
  getInfo(key: string) {
    return this.info[key]
  }
}
const t = new T({
  name: 'pw',
  age: 18,
  gender: 'male'
})
const test = t.getInfo('name'); // 没有 if判断，test是 any
console.log(test);

interface P {
  name: string;
  age: number;
  gender: string;
}
class T {
  constructor(private info: P) {}
  getInfo<T extends keyof P>(key: T): P[T] { 
    // T extends name === type T = name
    // T extends age === type T = age
    // T extends gender === type T = gender
    return this.info[key]
  }
}
const t = new T({
  name: 'pw',
  age: 18,
  gender: 'male'
})
const test = t.getInfo('name'); // test 的类型判断是正确的了
console.log(test);