feat(eslint-plugin): added new rule prefer-readonly (#555)

* feat(eslint-plugin): added new rule prefer-readonly

Adds the equivalent of TSLint's `prefer-readonly` rule.

* Added docs, auto-fixing

* Updated docs; love the new build time checks!

* Fixed linting errors (ha) and corrected internal source

* PR feedback: non recommended; :exit; some test coverage

* I guess tslintRuleName isn't allowed now?

* Added back recommended as false

* Removed :exit; fixed README.md table

packages/eslint-plugin-tslint/src/custom-linter.ts

@@ -4,7 +4,7 @@ import { Program } from 'typescript';
 const TSLintLinter = Linter as any;
 
 export class CustomLinter extends TSLintLinter {
-  constructor(options: ILinterOptions, private program: Program) {
+  constructor(options: ILinterOptions, private readonly program: Program) {
     super(options, program);
   }
 

packages/eslint-plugin/README.md

@@ -171,6 +171,7 @@ Then you should add `airbnb` (or `airbnb-base`) to your `extends` section of `.e
 | [`@typescript-eslint/prefer-function-type`](./docs/rules/prefer-function-type.md)                         | Use function types instead of interfaces with call signatures                                                                                       |                    | :wrench: |                   |
 | [`@typescript-eslint/prefer-includes`](./docs/rules/prefer-includes.md)                                   | Enforce `includes` method over `indexOf` method                                                                                                     |                    | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/prefer-namespace-keyword`](./docs/rules/prefer-namespace-keyword.md)                 | Require the use of the `namespace` keyword instead of the `module` keyword to declare custom TypeScript modules                                     | :heavy_check_mark: | :wrench: |                   |
+| [`@typescript-eslint/prefer-readonly`](./docs/rules/prefer-readonly.md)                                   | Requires that private members are marked as `readonly` if they're never modified outside of the constructor                                         |                    | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/prefer-regexp-exec`](./docs/rules/prefer-regexp-exec.md)                             | Prefer RegExp#exec() over String#match() if no global flag is provided                                                                              |                    |          | :thought_balloon: |
 | [`@typescript-eslint/prefer-string-starts-ends-with`](./docs/rules/prefer-string-starts-ends-with.md)     | Enforce the use of `String#startsWith` and `String#endsWith` instead of other equivalent methods of checking substrings                             |                    | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/promise-function-async`](./docs/rules/promise-function-async.md)                     | Requires any function or method that returns a Promise to be marked async                                                                           |                    |          | :thought_balloon: |

packages/eslint-plugin/ROADMAP.md

@@ -122,7 +122,7 @@
 | [`no-require-imports`]       | ✅  | [`@typescript-eslint/no-require-imports`]          |
 | [`object-literal-sort-keys`] | 🌓  | [`sort-keys`][sort-keys] <sup>[2]</sup>            |
 | [`prefer-const`]             | 🌟  | [`prefer-const`][prefer-const]                     |
-| [`prefer-readonly`]          | 🛑  | N/A                                                |
+| [`prefer-readonly`]          | ✅  | [`@typescript-eslint/prefer-readonly`]             |
 | [`trailing-comma`]           | 🌓  | [`comma-dangle`][comma-dangle] or [Prettier]       |
 
 <sup>[1]</sup> Only warns when importing deprecated symbols<br>
@@ -611,6 +611,7 @@ Relevant plugins: [`chai-expect-keywords`](https://github.com/gavinaiken/eslint-
 [`@typescript-eslint/prefer-interface`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/prefer-interface.md
 [`@typescript-eslint/no-array-constructor`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-array-constructor.md
 [`@typescript-eslint/prefer-function-type`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/prefer-function-type.md
+[`@typescript-eslint/prefer-readonly`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/prefer-readonly.md
 [`@typescript-eslint/no-for-in-array`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-for-in-array.md
 [`@typescript-eslint/no-unnecessary-qualifier`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-unnecessary-qualifier.md
 [`@typescript-eslint/semi`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/semi.md

packages/eslint-plugin/docs/rules/prefer-readonly.md

@@ -0,0 +1,81 @@
+# require never-modified private members be marked as `readonly`
+
+This rule enforces that private members are marked as `readonly` if they're never modified outside of the constructor.
+
+## Rule Details
+
+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`.
+
+Examples of **incorrect** code for this rule:
+
+```ts
+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 reaodnly
+    private neverModifiedParameter: string,
+  ) {
+    this.onlyModifiedInConstructor = onlyModifiedInConstructor;
+  }
+}
+```
+
+Examples of **correct** code for this rule:
+
+```ts
+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, in its default state, does not require any argument.
+
+### onlyInlineLambdas
+
+You may pass `"onlyInlineLambdas": true` as a rule option within an object to restrict checking only to members immediately assigned a lambda value.
+
+```cjson
+{
+    "@typescript-eslint/prefer-readonly": ["error", { "onlyInlineLambdas": true }]
+}
+```
+
+Example of **correct** code for the `{ "onlyInlineLambdas": true }` options:
+
+```ts
+class Container {
+  private neverModifiedPrivate = 'unchanged';
+}
+```
+
+Example of **incorrect** code for the `{ "onlyInlineLambdas": true }` options:
+
+```ts
+class Container {
+  private onClick = () => {
+    /* ... */
+  };
+}
+```
+
+## Related to
+
+- TSLint: ['prefer-readonly'](https://palantir.github.io/tslint/rules/prefer-readonly)

packages/eslint-plugin/src/configs/all.json

@@ -57,6 +57,7 @@
     "@typescript-eslint/prefer-function-type": "error",
     "@typescript-eslint/prefer-includes": "error",
     "@typescript-eslint/prefer-namespace-keyword": "error",
+    "@typescript-eslint/prefer-readonly": "error",
     "@typescript-eslint/prefer-regexp-exec": "error",
     "@typescript-eslint/prefer-string-starts-ends-with": "error",
     "@typescript-eslint/promise-function-async": "error",

packages/eslint-plugin/src/rules/indent-new-do-not-use/OffsetStorage.ts

@@ -9,13 +9,13 @@ import { TokenInfo } from './TokenInfo';
  * A class to store information on desired offsets of tokens from each other
  */
 export class OffsetStorage {
-  private tokenInfo: TokenInfo;
-  private indentSize: number;
-  private indentType: string;
-  private tree: BinarySearchTree;
-  private lockedFirstTokens: WeakMap<TokenOrComment, TokenOrComment>;
-  private desiredIndentCache: WeakMap<TokenOrComment, string>;
-  private ignoredTokens: WeakSet<TokenOrComment>;
+  private readonly tokenInfo: TokenInfo;
+  private readonly indentSize: number;
+  private readonly indentType: string;
+  private readonly tree: BinarySearchTree;
+  private readonly lockedFirstTokens: WeakMap<TokenOrComment, TokenOrComment>;
+  private readonly desiredIndentCache: WeakMap<TokenOrComment, string>;
+  private readonly ignoredTokens: WeakSet<TokenOrComment>;
   /**
    * @param tokenInfo a TokenInfo instance
    * @param indentSize The desired size of each indentation level

packages/eslint-plugin/src/rules/indent-new-do-not-use/TokenInfo.ts

@@ -8,7 +8,7 @@ import { TokenOrComment } from './BinarySearchTree';
  * A helper class to get token-based info related to indentation
  */
 export class TokenInfo {
-  private sourceCode: TSESLint.SourceCode;
+  private readonly sourceCode: TSESLint.SourceCode;
   public firstTokensByLineNumber: Map<number, TSESTree.Token>;
 
   constructor(sourceCode: TSESLint.SourceCode) {

packages/eslint-plugin/src/rules/index.ts

@@ -46,6 +46,7 @@ import preferFunctionType from './prefer-function-type';
 import preferIncludes from './prefer-includes';
 import preferInterface from './prefer-interface';
 import preferNamespaceKeyword from './prefer-namespace-keyword';
+import preferReadonly from './prefer-readonly';
 import preferRegexpExec from './prefer-regexp-exec';
 import preferStringStartsEndsWith from './prefer-string-starts-ends-with';
 import promiseFunctionAsync from './promise-function-async';
@@ -105,6 +106,7 @@ export default {
   'prefer-includes': preferIncludes,
   'prefer-interface': preferInterface,
   'prefer-namespace-keyword': preferNamespaceKeyword,
+  'prefer-readonly': preferReadonly,
   'prefer-regexp-exec': preferRegexpExec,
   'prefer-string-starts-ends-with': preferStringStartsEndsWith,
   'promise-function-async': promiseFunctionAsync,