如何在 TypeScript 中描述这样的类型？

Question

As below, personParam is always number | { name: string; } number | { name: string; } number | { name: string; } in func , but I hope it's type can be determined by key

type Params = {
  jack: number;
  tom: {name: string};
};
const func = <K extends keyof Params>(key: K, personParam: Params[K]) => {
  switch (key) {
    case 'jack':
      console.log(personParam);
    case 'tom':
      // I hope personParams to be {name: string} type, but unfortunately it's number | { name: string; }
      // TS2339: Property 'name' does not exist on type 'number | { name: string; }'.   Property 'name' does not exist on type 'number'.
      console.log(personParam.name);
  }
};

Answer 1

One solution is using a type predicates

type Params = {
  jack: number;
  tom: {name: string};
};
const func = <K extends keyof Params>(key: K, personParam: Params[K]) => {
  if(isTom(key, personParam)) {
     console.log(personParam.name);
  } else {
    console.log(personParam);
  }
};

function isTom(k: keyof Params, personParam: Params[keyof Params]): personParam is {name: string} {
  return k === 'tom';
}

TypeScript Playground

Answer 2

I don't know if this way can be useful for you. But check this out.

type Params = {
    jack: number;
    tom: {name: string};
  };
  const func = <K extends keyof Params>(personParam: Params[K]) => {
    if(typeof(personParam) ===  'object') {
       personParam.name;
    } else {
      console.log(personParam);
    }
  };