103 lines
2.4 KiB
Plaintext
103 lines
2.4 KiB
Plaintext
---
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description: "Require private members to be marked as `readonly` if they're never modified outside of the constructor."
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---
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> 🛑 This file is source code, not the primary documentation location! 🛑
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>
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> See **https://typescript-eslint.io/rules/prefer-readonly** for documentation.
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Private member variables (whether using the `private` modifier or private `#` fields) are never permitted to be modified outside of their declaring class.
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If that class never modifies their value, they may safely be marked as `readonly`.
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This rule reports on private members are marked as `readonly` if they're never modified outside of the constructor.
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## Examples
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<!--tabs-->
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### ❌ Incorrect
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```ts
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class Container {
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// These member variables could be marked as readonly
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private neverModifiedMember = true;
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private onlyModifiedInConstructor: number;
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#neverModifiedPrivateField = 3;
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public constructor(
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onlyModifiedInConstructor: number,
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// Private parameter properties can also be marked as readonly
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private neverModifiedParameter: string,
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) {
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this.onlyModifiedInConstructor = onlyModifiedInConstructor;
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}
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}
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```
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### ✅ Correct
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```ts
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class Container {
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// Public members might be modified externally
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public publicMember: boolean;
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// Protected members might be modified by child classes
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protected protectedMember: number;
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// This is modified later on by the class
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private modifiedLater = 'unchanged';
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public mutate() {
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this.modifiedLater = 'mutated';
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}
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// This is modified later on by the class
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#modifiedLaterPrivateField = 'unchanged';
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public mutatePrivateField() {
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this.#modifiedLaterPrivateField = 'mutated';
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}
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}
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```
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## Options
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### `onlyInlineLambdas`
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You may pass `"onlyInlineLambdas": true` as a rule option within an object to restrict checking only to members immediately assigned a lambda value.
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```jsonc
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{
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"@typescript-eslint/prefer-readonly": [
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"error",
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{ "onlyInlineLambdas": true },
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],
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}
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```
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Example of code for the `{ "onlyInlineLambdas": true }` options:
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<!--tabs-->
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#### ❌ Incorrect
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```ts option='{ "onlyInlineLambdas": true }'
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class Container {
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private onClick = () => {
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/* ... */
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};
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}
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```
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#### ✅ Correct
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```ts option='{ "onlyInlineLambdas": true }'
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class Container {
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private neverModifiedPrivate = 'unchanged';
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}
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```
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## When Not To Use It
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If you aren't trying to enforce strong immutability guarantees, this rule may be too restrictive for your project.
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