When using Typescript, the keyof operator for objects may not undergo refinement and can result in error

I've been working on creating a function that validates whether a key in an object is a non-empty string.

export const validateRequiredString = <T>(
  obj: T,
  key: keyof T & (string | number)
): void => {
  if (typeof obj[key] !== "string") {
    throw new Error(`${key} is not a string`);
  }
  if (obj[key] === "") {
    throw new Error(`${key} is empty`);
  }
};

However, I'm encountering an issue and can't figure out why it's not allowing me to make this work:

file.ts:8:7 - error TS2367: This condition will always return 'false' since the types 'T[keyof T & (string | number)]' and 'string' have no overlap

Answer №1

I finally figured it out.

const validateString = <T>(
  data: T,
  property: keyof T & (string | number)
): void => {
  const value = data[property];

  if (!(typeof value === "string")) {
    throw new Error(`${property} is not a string`);
  }
  if (value === "") {
    throw new Error(`${property} is empty`);
  }
};

After storing value, the type narrowing mechanism kicks in.

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