The unknown type

What a session covers

The tutor works through these with you, in conversation and code. A session ends when you have shown them, not when a video does.

1. Explain the difference between unknown and any
2. Narrow an unknown value before using it
3. Type a caught error as unknown and extract its message safely

In practice

A few lines that show The unknown type doing its job.

function message(error: unknown): string {
  if (error instanceof Error) return error.message;
  if (typeof error === "string") return error;
  return "Unknown error";
}

try {
  JSON.parse("{bad json");
} catch (error) {              // error: unknown under useUnknownInCatchVariables
  console.error(message(error));
}

The common mistake

What to know first

Your roadmap puts these before The unknown type, unless you already know them.

• typeof and in guards

How it stays learned

Once a session shows you understand The unknown type, it comes back for review. Each review takes a minute and tells the memory model how well you still know it.

1. Day 3First recall, while it is fresh
2. Day 7Second, a week on
3. Day 18Last, when it has settled

Where it leads

Topics that build on The unknown type.

• Runtime validation