prefer-readonly
Require private members to be marked as
readonlyif they're never modified outside of the constructor.
Some problems reported by this rule are automatically fixable by the --fix ESLint command line option.
This rule requires type information to run.
Member variables with the privacy private are never permitted to be modified outside of their declaring class.
If that class never modifies their value, they may safely be marked as readonly.
This rule reports on private members are marked as readonly if they're never modified outside of the constructor.
module.exports = {
"rules": {
"@typescript-eslint/prefer-readonly": "error"
}
};
Examples
- ❌ Incorrect
- ✅ Correct
class Container {
// These member variables could be marked as readonly
private neverModifiedMember = true;
private onlyModifiedInConstructor: number;
public constructor(
onlyModifiedInConstructor: number,
// Private parameter properties can also be marked as readonly
private neverModifiedParameter: string,
) {
this.onlyModifiedInConstructor = onlyModifiedInConstructor;
}
}
class Container {
// Public members might be modified externally
public publicMember: boolean;
// Protected members might be modified by child classes
protected protectedMember: number;
// This is modified later on by the class
private modifiedLater = 'unchanged';
public mutate() {
this.modifiedLater = 'mutated';
}
}
Options
This rule accepts the following options
type Options = [
{
onlyInlineLambdas?: boolean;
},
];
const defaultOptions: Options = [{ onlyInlineLambdas: false }];
onlyInlineLambdas
You may pass "onlyInlineLambdas": true as a rule option within an object to restrict checking only to members immediately assigned a lambda value.
{
"@typescript-eslint/prefer-readonly": ["error", { "onlyInlineLambdas": true }]
}
Example of code for the { "onlyInlineLambdas": true } options:
- ❌ Incorrect
- ✅ Correct
class Container {
private onClick = () => {
/* ... */
};
}
class Container {
private neverModifiedPrivate = 'unchanged';
}