Index error persists despite using the 'in' operator for verification

Is it possible to use a string as an object index? Although using keyof is the recommended approach due to its specificity, I am curious why using a string is not allowed even after confirming it is a valid index with the in operator.

For example:

if (fieldName in someObject) {
  const fieldValue = someObject[fieldName];
}

Typescript raises an error when attempting this. While I understand that determining the type of fieldValue may be challenging, assigning it an any type could suffice in such cases.

typescript

Answer №1

Typescript raises errors during compilation due to the use of the in keyword to verify if a string is a valid object key. This check occurs at run-time, highlighting the distinction between compile time and runtime in programming. If JavaScript were used instead of TypeScript, this issue would not arise since JS is an interpreted language. An alternative solution is to employ 

fieldName as keyof typeof someObject
to inform TypeScript during compilation that the specified value is a valid object key. However, this method may come with its own limitations.

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