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Type assertion vs casting

Learn the difference between assertion and real casting.

Assertion vs conversion

Type assertion is compile-time only; it does not change the value at runtime. For example, "5" as number does not convert the value to the number 5; at runtime it is still the string "5". For real conversion, functions like Number() and String() are used.

const s: any = "5";
console.log((s as number) + 1); // "51"
console.log(Number(s) + 1);      // 6

Real conversion functions

For real type conversion at runtime, functions like Number(), String(), and Boolean() are used. These change the runtime type of the value. Assertion only affects TypeScript's type view.

const n = Number("42");  // number 42
const t = String(42);    // string "42"

Practice Assertion vs conversion difference

The s variable is of type any with value "5". First assert it as number with as number and assign to iddia; then do real conversion with Number() and assign to gercek. Add 1 to each and print on separate lines with console.log.

Starter code

index.ts
// s degiskenini iddia yontemiyle number olarak isaretle
// Number fonksiyonuyla gercek donusum yap
// her degiskene 1 ekle ve ayri satirlarda yazdir

const s: any = "5";

Sample solution

index.ts
const s: any = "5";
const iddia = s as number;
const gercek = Number(s);
console.log(iddia + 1);
console.log(gercek + 1);

Expected output

51
6

Questions and answers

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

1 Which is used to truly convert a string to a number?

  • Assertion with <number>
  • The typeof operator
  • Assertion with as number
  • The Number() function Correct answer

Explanation: as number and <number> are compile-time only; for real conversion Number() must be called.

2 Real numeric conversion

Question 
const s: any = "42";
// gercek donusum icin
const n = ____;
console.log(n + 1);
Answer 
const s: any = "42";
// gercek donusum icin
const n = Number(s);
console.log(n + 1);

Explanation: For runtime conversion call Number(); as is compile-time only.

3 Assertion and conversion

Term Meaning
as number Compile-time only
Number(s) Runtime conversion
String(n) Conversion to string
Boolean(x) Boolean conversion

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