Classes in TypeScript
Readonly Properties
Prevent mutation of class fields after initialization using the readonly modifier.
Last Updated: July 29, 2026
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10 min read
The readonly modifier ensures a field can only be assigned during declaration or inside the class constructor.
1. Introduction & Architecture
2. Deep Dive & Core Concepts
Readonly prevents reassignment of the property reference itself, but does not deeply freeze object properties inside reference types.
3. Basic Code Example
4. Advanced Production Patterns
5. Interactive Code Playground
class Config { readonly env = "production"; }
const c = new Config();
console.log(c.env);6. Common Pitfalls & Edge Cases
Warning:
readonly does not make objects deeply immutable. For deep immutability, use Object.freeze() or Readonly utility types.7. Interview Q&A & Quizzes
Q: Can a readonly class property be modified inside a constructor?
A: Yes, constructors are the only place where readonly fields can be assigned or updated.
8. Summary Comparison Table
| Context | Assignment Allowed |
|---|---|
| Declaration | Yes |
| Constructor | Yes |
| Class Methods | No |