Readonly vs const
- TypeScript course
- Intermediate
- Lesson 13/89
- 4 min
Learn the difference between readonly and const.
readonly vs const
const is for standalone variables, readonly is for class fields. const prevents reassigning the variable; readonly makes the field immutable after construction.
const MAX = 100;
class Config {
readonly limit: number = MAX;
}Which one in which case?
Use const for a standalone constant and readonly for a class field. const is not valid in a class body; readonly only works on class members.
// Standalone constant:
const VERSION = "1.0";
// Class field:
class App {
readonly version: string = VERSION;
}Practice App class with a readonly field
Define an App class with a readonly version: string = "1.0" field (with an initial value). Create a new App and print the version field.
Starter code
// App sınıfı tanımla (değiştirilemez sürüm alanı, başlangıç değeri 1.0)
// yeni App oluştur, version'ı yazdırSample solution
class App {
readonly version: string = "1.0";
}
const a = new App();
console.log(a.version);Expected output
1.0
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which is used to make a class field immutable?
- readonly Correct answer
- static
- const
- let
Explanation: readonly is used for class fields. const is not valid in a class body; it is only for standalone variables. let is mutable, and static is a class-level modifier.
2 Complete the right modifier for a class field
class App {
____ version: string = "1.0";
}class App {
readonly version: string = "1.0";
}Explanation: To make a class field immutable you use readonly. const is invalid in a class body, static moves to the class level (mutable), and public opens access (mutable).
3 readonly vs const parts
| Term | Meaning |
|---|---|
const |
Constant for a variable |
readonly |
Constant for a class field |
const MAX = 100 |
Standalone constant |
readonly limit: number |
Class constant field |