Why is Typescript showing an error?

Question

typescript

I started to study the TS, and I can not understand why it shows an error here?

const data: {
  hello: {
    wtf: string
  }
} = {
  hello: {
    wtf: 'world'
  }
}

for (const [key] of Object.entries(data)) {
  console.log(data[key]); // ошибка тут data[key]
}

How to access object property without error?

Answer 1

Typescript shows an error because it types any object keys passed to Object.keys and Object.entries as strings (not string constants). This is inherent behavior. The reason the developers say is: "an object can have more properties at runtime, so the type is string[]" ( thread1 , thread2 ).
Access to the property of the object without an error in this case can be obtained only if you set the typing yourself.
The simplest solution:

const data: {
    hello: {
        wtf: string;
    };
} = {
    hello: {
        wtf: 'world',
    },
};

for (const [key] of Object.entries(data) as [keyof typeof data, any][]) {
    console.log(data[key]); 
}

The readability of such code, of course, suffers greatly. More suitable option:

interface Data {
    hello: {
        wtf: string;
    };
}

type DataKey = keyof Data;

const data: Data = {
    hello: {
        wtf: 'world',
    },
};

for (const [key] of Object.entries(data) as [DataKey, any][]) {
    console.log(data[key]);
}