Summary - Type assertion

Write a program that safely parses an API response with assertion.

Summary - Type assertion

In this group you learned type assertion methods. The as keyword and <type> angle bracket assert types at compile time; neither changes the value at runtime. unknown values are first narrowed with typeof. For real conversion, functions like Number() are used.

const response: unknown = "5";
if (typeof response === "string") {
  const n = Number(response);
  console.log(n + 1);
}

Exercise Parse API response

The yanit variable is of type unknown with value "42". First check if it is string with typeof. If so, do real conversion with Number(), add 1, and print the result with console.log.

Show example solution

const yanit: unknown = "42";
if (typeof yanit === "string") {
  const n = Number(yanit);
  console.log(n + 1);
}

Test yourself

What is the best order to safely process an API response received as unknown?
- Receive as unknown, force cast with as
- Receive as unknown, call methods directly
- Receive as any, do no checks
- Receive as unknown, narrow with typeof, then use
Show answer

Receive as unknown, narrow with typeof, then use — The safest way: receive as unknown, narrow with typeof, then use. Direct use or forced casting is error-prone.

Narrow unknown with string

const yanit: unknown = "basarili";
if (typeof yanit === "____") {
  console.log(yanit.length);
}

Show answer

const yanit: unknown = "basarili";
if (typeof yanit === "string") {
  console.log(yanit.length);
}

Type assertion summary

as

Universal assertion
<>

Angle bracket syntax
unknown

Safe any alternative
typeof

Narrowing operator

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