When trying to assign a typed value, the Typescript compiler often provides convoluted error messages

I am facing an ambiguous TS-Error that I find challenging to resolve. It appears to be linked to the "noImplicitAny" and "strictNullChecks" settings in the TS-Config file. Can someone offer insight into what this error signifies and what message the compiler is conveying? Please provide not just a solution but also an explanation of the issue's complexity.

interface Person {
  name: string;
  age: number;
}

interface MyState {
  someKeyA: string[];
  someKeyB: Person | null;
  isOk: boolean;
}

const getInitialState = (): MyState => {
  return {
    someKeyA: ["This is a wonderful state", "But sth is not right"],
    someKeyB: {
      name: "Francis",
      age: 33
    },
    isOk: false
  };
};

class MyWonderFullClass {
  state: MyState;

  constructor() {
    this.state = getInitialState();
  }

  setSingleStateField = (
    fieldId: keyof MyState,
    value: MyState[keyof MyState]
  ) => {
    if (!fieldId || !value) {
      return;
    }

    this.state[fieldId] = value; 
   /*The assignment fails with the following error message: Type 'true | Person | string[]' is not assignable to type 'string[] & (Person | null) & boolean'. Type 'true' is not assignable to type 'string[] & (Person | null) & boolean'.*/

  };
}

https://codesandbox.io/s/typescript-playground-export-forked-nijj9?file=/index.ts

Answer №1

The issue lies in the fact that the compiler is unaware of the type associated with value for fieldId.

To resolve this, I addressed it by isolating the key type:

  setSingleStateField = <K extends keyof MyState>(
    fieldId: K,
    value: MyState[K]
  ) => {
    if (!fieldId || !value) {
      return;
    }

    this.state[fieldId] = value;
  };

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