Readonly vs const

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;
}

Exercise 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.

Show example solution

class App {
  readonly version: string = "1.0";
}

const a = new App();
console.log(a.version);

Test yourself

Which is used to make a class field immutable?
- readonly
- static
- const
- let
Show answer

readonly — 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.

Complete the right modifier for a class field

class App {
  ____ version: string = "1.0";
}

Show answer

class App {
  readonly version: string = "1.0";
}

readonly vs const parts

const

Constant for a variable
readonly

Constant for a class field
const MAX = 100

Standalone constant
readonly limit: number

Class constant field

Next lesson: Summary - Readonly

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