Rule: prefer-readonly

Requires that private variables are marked as readonly if they’re never modified outside of the constructor.

If a private variable is only assigned to in the constructor, it should be declared as readonly.


Marking never-modified variables as readonly helps enforce the code’s intent of keeping them as never-modified. It can also help prevent accidental changes of members not meant to be changed.



If only-inline-lambdas is specified, only immediately-declared arrow functions are checked.

Config examples
"prefer-readonly": true
"prefer-readonly": [true, "only-inline-lambdas"]
  "enum": [
  "type": "string"