Unknown Type

The any type basically turns off TypeScript's type checking:

function processData(data: any) {
  // TypeScript allows this even though it might crash
  console.log(data.toLowerCase());

  // TypeScript allows this too - it's like we're using plain JavaScript
  return data.nonExistentMethod();
}

// No errors when calling the function
processData(42); // Will crash at runtime

As we talked about earlier, when you take plain JavaScript code and run it through TypeScript tooling, almost everything is any by default.

The unknown type doesn't allow that kind of tomfoolery:

function processData(data: unknown) {
  // Error: Object is of type 'unknown'
  console.log(data.toLowerCase());

  // Error: Object is of type 'unknown'
  return data.nonExistentMethod();
}

With unknown, you can still assign any value to it (e.g. call this function with any value), but you can't use that value in a meaningful way without first checking its type:

function processData(data: unknown) {
  // We do a type assertion
  if (typeof data === "string") {
    // Now TypeScript knows data is a string
    console.log(data.toLowerCase());
    return data;
  }
  if (typeof data === "number") {
    // Now TypeScript knows data is a number
    return data * 2;
  }

  // Throw an error for other types
  // that we can't handle
  throw new Error("Expected string data");
}

Unknown is a fantastic alternative to any when it comes to dealing with values that are coming into your program from the outside world (e.g. user input, API responses, etc.). It forces you to add type checks at that I/O boundary so that you can then be confident working with the data inside your program.

Support.ai needs a robust message parser that can handle email inputs, which are received as strings, or chat inputs, which are received as arrays of strings.

Complete the parseCustomerMessage function. It receives an unknown input and returns a CustomerMessage:

1. If the input is a string:
   1. source should be "email"
   2. content should be the input string
2. If the input is an array:
   1. source should be "chat"
   2. content should be the strings in the input .join()ed together with a newline character (\n)
3. Otherwise:
   1. Source should be the string "unknown"
   2. Content should be an empty string

Use Array.isArray() to check if an input is an array.