How to catch the situation when the backend sends data of the wrong type that the application expects?

To catch the situation when the backend sends data of an unexpected type for the frontend, you can use several approaches:

1. Client-side data validation — after receiving data from the API, check the types and structure. For example, if a number is expected, verify using typeof or Array.isArray.

2. Using TypeScript or PropTypes (in React) — static typing helps identify type mismatches during development.

3. JSON Schema validation — describe the expected data structure in JSON Schema and validate the received JSON using libraries (e.g., ajv).

4. Handling parsing and validation errors — if data does not meet expectations, throw an error or display a message to the user.

A simple example of validation in JavaScript:

fetch('/api/data')
  .then(res => res.json())
  .then(data => {
    if (typeof data.count !== 'number') {
      throw new Error('Invalid data type: count should be a number');
    }
    // proceed with data processing
  })
  .catch(err => {
    console.error('Data error:', err.message);
  });