无法编辑属性，错误类型“字符串”不可分配给类型“从不”与 TypeScript

Question

I want to edit an attribute from an object in TypeScript and React but I get the following error: Type 'string' is not assignable to type 'never' despite the fact that I check the type of this attribute before:

const familySetter = (newValue: string, attributeToSet: keyof Family) => {
    setNewFamily((currNewFam) => {
      if (!currNewFam) return;
      const newNewFam: Family = { ...currNewFam };
      if (typeof newNewFam[attributeToSet] === 'string') {

        newNewFam[attributeToSet] = newValue; //error at this line

      }
      return newNewFam;
    });
  };

Answer 1

It will occured when your class properties have several types (for sample string and number)

class Family {
    test: string;
    test2: number;
};

in that case the compilator will not know if newNewFam[attributeToSet] is of one type or another

one solution is to declare your method by extending the keyof Family and say value is of type Family[K]

const familySetter = <K extends keyof Family>(newValue: Family[K], attributeToSet: K)

the if if (typeof newNewFam[attributeToSet] === 'string') is not enought because it's evaluate at runtime dependings on function parameter

complete sample is

class Family {
    test: string;
    test2: number;
};

const familySetter = <K extends keyof Family>(newValue: Family[K], attributeToSet: K) => {
    setNewFamily((currNewFam) => {
      if (!currNewFam) return;
      const newNewFam: Family = { ...currNewFam };
      if (typeof newNewFam[attributeToSet] === 'string') {
        newNewFam[attributeToSet] = newValue; 
      }
      return newNewFam;
    });
  };