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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);
}

Practice 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.

Starter code

index.ts
// yanit degiskeninin string olup olmadigini typeof ile kontrol et
// eger stringse Number ile donustur, 1 ekle ve yazdir

const yanit: unknown = "42";

Sample solution

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

Expected output

43

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 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 Correct answer

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

2 Narrow unknown with string

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

Explanation: To narrow an unknown string value, compare with "string" using typeof.

3 Type assertion summary

Term Meaning
as Universal assertion
<> Angle bracket syntax
unknown Safe any alternative
typeof Narrowing operator

All lessons in Intermediate

1 Classes

  1. Class basics
  2. Constructor and fields
  3. Methods
  4. Static members
  5. Summary - Classes

2 Access modifiers

  1. public
  2. private
  3. protected
  4. Parameter properties
  5. Summary - Access modifiers

3 Readonly properties

  1. readonly fields
  2. Readonly parameter
  3. Readonly vs const
  4. Summary - Readonly

4 Generics

  1. What is a generic?
  2. Generic functions
  3. Generic classes
  4. Multiple type parameters
  5. Summary - Generics

5 Generic constraints

  1. extends constraint
  2. keyof constraint
  3. Default type parameter
  4. Summary - Generic constraints

6 Utility types

  1. Partial
  2. Required and Pick
  3. Omit and Record
  4. Summary - Utility types

7 keyof and typeof

  1. keyof operator
  2. typeof operator
  3. Indexing with keyof
  4. Summary - keyof and typeof

8 Mapped types

  1. What is a mapped type?
  2. Readonly mapped type
  3. Optional mapped type
  4. Summary - Mapped types

9 Conditional types

  1. What is a conditional type?
  2. Distributive conditional
  3. Logic with conditional
  4. Summary - Conditional types

10 infer

  1. infer keyword
  2. Extracting from Promise
  3. Array element type
  4. Summary - infer

11 Decorators

  1. What is a decorator?
  2. Method decorator
  3. Property decorator
  4. Decorator factories
  5. Summary - Decorators

12 Modules

  1. export and import
  2. Default export
  3. Re-export
  4. Type-only import
  5. Summary - Modules

13 Namespaces

  1. What is a namespace?
  2. Nested namespace
  3. Namespace vs module
  4. Summary - Namespaces

14 Declaration files

  1. What is .d.ts?
  2. Ambient module
  3. Triple-slash directive
  4. Summary - Declaration files

15 Advanced classes

  1. abstract class
  2. implements
  3. Get and set
  4. Summary - Advanced classes

16 Type assertion

  1. as keyword
  2. angle bracket syntax
  3. Type assertion vs casting
  4. Casting from unknown
  5. Summary - Type assertion

17 Project: Library package

  1. Package design
  2. Exporting types
  3. Generic helpers
  4. Declaration file
  5. Integration

18 Project: Quiz app

  1. Question model
  2. Question pool
  3. Answer check
  4. Score calculation
  5. Integration

19 Project: Weather app

  1. Weather model
  2. API response type
  3. Data transform
  4. Display
  5. Integration

20 Intermediate final

  1. Challenge 1: Classes
  2. Challenge 2: Generics
  3. Challenge 3: Type systems
  4. Challenge 4: Mini app