Description

Login

Syncing account progress…

Back

Beginner · 10 min

Checking unknown input

Use runtime checks before trusting data that comes from outside typed code.

Start with unknown

unknown requires a check before specific operations. Unlike any, it keeps type checking active at the point where the value is used.

function label(value: unknown): string {
  return typeof value === "string" ? value.trim() : "Invalid";
}
console.log(label(" Ada "));

Output

Ada

Check parsed data

JSON.parse() does not validate an application shape. Assign its result to unknown, then check the required property before reading it. A type assertion alone does not validate data.

const value: unknown = JSON.parse('{"name":"Ada"}');
if (typeof value === "object" && value !== null && "name" in value && typeof value.name === "string") {
  console.log(value.name);
}

Output

Ada

Put it into practice

  1. Read the types and predict the output before running the examples.
  2. Complete the task, then check your understanding with the practice questions.

Try it yourself

Code runs on this device. When you are signed in, drafts sync to your account.

Check before reading a string

Make "label" trim a string and return "Invalid" for other inputs. Display the result for 4.

Practice