typescript-eslint · bradzacher · Apr 13, 2020 · Mar 8, 2020 · Mar 8, 2020 · Apr 13, 2020
diff --git a/packages/eslint-plugin/README.md b/packages/eslint-plugin/README.md
@@ -151,6 +151,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
 | [`@typescript-eslint/prefer-readonly-parameter-types`](./docs/rules/prefer-readonly-parameter-types.md)               | Requires that function parameters are typed as readonly to prevent accidental mutation of inputs                        |                    |          | :thought_balloon: |
 | [`@typescript-eslint/prefer-regexp-exec`](./docs/rules/prefer-regexp-exec.md)                                         | Enforce that `RegExp#exec` is used instead of `String#match` if no global flag is provided                              | :heavy_check_mark: |          | :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 | :heavy_check_mark: | :wrench: | :thought_balloon: |
+| [`@typescript-eslint/prefer-ts-expect-error`](./docs/rules/prefer-ts-expect-error.md)                                 | Recommends using `// @ts-expect-error` over `// @ts-ignore`                                                             |                    | :wrench: |                   |
 | [`@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: |
 | [`@typescript-eslint/require-array-sort-compare`](./docs/rules/require-array-sort-compare.md)                         | Requires `Array#sort` calls to always provide a `compareFunction`                                                       |                    |          | :thought_balloon: |
 | [`@typescript-eslint/restrict-plus-operands`](./docs/rules/restrict-plus-operands.md)                                 | When adding two variables, operands must both be of type number or of type string                                       |                    |          | :thought_balloon: |

diff --git a/packages/eslint-plugin/docs/rules/prefer-ts-expect-error.md b/packages/eslint-plugin/docs/rules/prefer-ts-expect-error.md
@@ -0,0 +1,47 @@
+# Recommends using `// @ts-expect-error` over `// @ts-ignore` (`prefer-ts-expect-error`)
+
+TypeScript allows you to suppress all errors on a line by placing a single-line comment starting with `@ts-ignore` immediately before the erroring line.
+While powerful, there is no way to know if a `@ts-ignore` is actually suppressing an error without manually investigating what happens when the `@ts-ignore` is removed.
+
+This means its easy for `@ts-ignore`s to be forgotten about, and remain in code even after the error they were suppressing is fixed.
+This is dangerous, as if a new error arises on that line it'll be suppressed by the forgotten about `@ts-ignore`, and so be missed.
+
+To address this, TS3.9 ships with a new single-line comment directive: `// @ts-expect-error`.
+
+This directive operates in the same manner as `@ts-ignore`, but will error if the line it's meant to be suppressing doesn't actually contain an error, making it a lot safer.
+
+## Rule Details
+
+This rule looks for usages of `@ts-ignore`, and flags them to be replaced with `@ts-expect-error`.
+
+Examples of **incorrect** code for this rule:
+
+```ts
+// @ts-ignore
+const str: string = 1;
+
+const isOptionEnabled = (key: string): boolean => {
+  // @ts-ignore: if key isn't in globalOptions it'll be undefined which is false
+  return !!globalOptions[key];
+};
+```
+
+Examples of **correct** code for this rule:
+
+```ts
+// @ts-expect-error
+const str: string = 1;
+
+const isOptionEnabled = (key: string): boolean => {
+  // @ts-expect-error: if key isn't in globalOptions it'll be undefined which is false
+  return !!globalOptions[key];
+};
+```
+
+## When Not To Use It
+
+If you are not using TypeScript 3.9 (or greater), then you will not be able to use this rule, as the directive is not supported
+
+## Further Reading
+
+- [Original Implementing PR](https://github.com/microsoft/TypeScript/pull/36014)
diff --git a/packages/eslint-plugin/src/configs/all.json b/packages/eslint-plugin/src/configs/all.json
@@ -88,6 +88,7 @@
     "@typescript-eslint/prefer-readonly-parameter-types": "error",
     "@typescript-eslint/prefer-regexp-exec": "error",
     "@typescript-eslint/prefer-string-starts-ends-with": "error",
+    "@typescript-eslint/prefer-ts-expect-error": "error",
     "@typescript-eslint/promise-function-async": "error",
     "quotes": "off",
     "@typescript-eslint/quotes": "error",

diff --git a/packages/eslint-plugin/src/rules/index.ts b/packages/eslint-plugin/src/rules/index.ts
@@ -77,6 +77,7 @@ import preferReadonly from './prefer-readonly';
 import preferReadonlyParameterTypes from './prefer-readonly-parameter-types';
 import preferRegexpExec from './prefer-regexp-exec';
 import preferStringStartsEndsWith from './prefer-string-starts-ends-with';
+import preferTsExpectError from './prefer-ts-expect-error';
 import promiseFunctionAsync from './promise-function-async';
 import quotes from './quotes';
 import requireArraySortCompare from './require-array-sort-compare';
@@ -174,6 +175,7 @@ export default {
   'prefer-readonly': preferReadonly,
   'prefer-regexp-exec': preferRegexpExec,
   'prefer-string-starts-ends-with': preferStringStartsEndsWith,
+  'prefer-ts-expect-error': preferTsExpectError,
   'promise-function-async': promiseFunctionAsync,
   quotes: quotes,
   'require-array-sort-compare': requireArraySortCompare,

diff --git a/packages/eslint-plugin/src/rules/prefer-ts-expect-error.ts b/packages/eslint-plugin/src/rules/prefer-ts-expect-error.ts
@@ -0,0 +1,55 @@
+import { AST_TOKEN_TYPES } from '@typescript-eslint/experimental-utils';
+import * as util from '../util';
+
+type MessageIds = 'preferExpectErrorComment';
+
+export default util.createRule<[], MessageIds>({
+  name: 'prefer-ts-expect-error',
+  meta: {
+    type: 'problem',
+    docs: {
+      description:
+        'Recommends using `// @ts-expect-error` over `// @ts-ignore`',
+      category: 'Best Practices',
+      recommended: false,
+    },
+    fixable: 'code',
+    messages: {
+      preferExpectErrorComment:
+        'Use "// @ts-expect-error" to ensure an error is actually being suppressed.',
+    },
+    schema: [],
+  },
+  defaultOptions: [],
+  create(context) {
+    const tsIgnoreRegExp = /^\/*\s*@ts-ignore/;
+    const sourceCode = context.getSourceCode();
+
+    return {
+      Program(): void {
+        const comments = sourceCode.getAllComments();
+
+        comments.forEach(comment => {
+          if (comment.type !== AST_TOKEN_TYPES.Line) {
+            return;
+          }
+
+          if (tsIgnoreRegExp.test(comment.value)) {
+            context.report({
+              node: comment,
+              messageId: 'preferExpectErrorComment',
+              fix: fixer =>
+                fixer.replaceText(
+                  comment,
+                  `//${comment.value.replace(
+                    '@ts-ignore',
+                    '@ts-expect-error',
+                  )}`,
+                ),
+            });
+          }
+        });
+      },
+    };
+  },
+});
diff --git a/packages/eslint-plugin/tests/rules/prefer-ts-expect-error.test.ts b/packages/eslint-plugin/tests/rules/prefer-ts-expect-error.test.ts
@@ -0,0 +1,85 @@
+import rule from '../../src/rules/prefer-ts-expect-error';
+import { RuleTester } from '../RuleTester';
+
+const ruleTester = new RuleTester({
+  parser: '@typescript-eslint/parser',
+});
+
+ruleTester.run('prefer-ts-expect-error', rule, {
+  valid: [
+    '// @ts-nocheck',
+    '// @ts-check',
+    '// just a comment containing @ts-ignore somewhere',
+    '/* @ts-ignore */',
+    '/** @ts-ignore */',
+    `
+/*
+// @ts-ignore in a block
+*/
+    `,
+    '// @ts-expect-error',
+    `
+if (false) {
+  // @ts-expect-error: Unreachable code error
+  console.log('hello');
+}
+    `,
+  ],
+  invalid: [
+    {
+      code: '// @ts-ignore',
+      output: '// @ts-expect-error',
+      errors: [
+        {
+          messageId: 'preferExpectErrorComment',
+          line: 1,
+          column: 1,
+        },
+      ],
+    },
+    {
+      code: '// @ts-ignore: Suppress next line',
+      output: '// @ts-expect-error: Suppress next line',
+
+      errors: [
+        {
+          messageId: 'preferExpectErrorComment',
+          line: 1,
+          column: 1,
+        },
+      ],
+    },
+    {
+      code: '/////@ts-ignore: Suppress next line',
+      output: '/////@ts-expect-error: Suppress next line',
+      errors: [
+        {
+          messageId: 'preferExpectErrorComment',
+          line: 1,
+          column: 1,
+        },
+      ],
+    },
+    {
+      code: `
+if (false) {
+  // @ts-ignore: Unreachable code error
+  console.log('hello');
+}
+      `,
+      output: `
+if (false) {
+  // @ts-expect-error: Unreachable code error
+  console.log('hello');
+}
+      `,
+      errors: [
+        {
+          messageId: 'preferExpectErrorComment',
+          line: 3,
+          column: 3,
+        },
+      ],
+    },
+  ],
+});
diff --git a/packages/typescript-estree/src/ts-estree/ts-estree.ts b/packages/typescript-estree/src/ts-estree/ts-estree.ts
@@ -115,6 +115,7 @@ export interface LineComment extends BaseToken {
 export type Comment = BlockComment | LineComment;
 export type Token =
   | BooleanToken
+  | Comment
   | IdentifierToken
   | JSXIdentifierToken
   | JSXTextToken