Validating data manually

⬅️ Execute Program Advanced TypeScript

Validating data manually

• 💡typeof null === 'object' in both JavaScript and TypeScript.

type Comment = {
  message: string
};
 
function hasMessageProperty(obj: {}): obj is {message: unknown} {
  return 'message' in obj;
}
 
function parseCommentJson(json: string): Comment | undefined {
  const parsed: unknown = JSON.parse(json);
  if (typeof parsed === 'object') {
    if (parsed !== null) {
      if (hasMessageProperty(parsed)) {
        if (typeof parsed.message === 'string') {
          return {message: parsed.message};
        }
      }
    }
  }
  return undefined;
}

Remember:

• The JSON value might not be an object at all.

• It might be null, which is an object in JavaScript but not the kind of object we expect.

• It might not have the message property that we expect.

• It might have a message, but with an unexpected type.

• 💡 We can implement checks with a third-party data validation library like io-ts or runtypes.

• 💡 When using JSON.parse, it’s best to explicitly specify the unknown type, like const parsed: unknown = JSON.parse(json).