feat(eslint-plugin): [prefer-string-starts-ends-with] add allowSingleElementEquality option (typescript-eslint#8374)

JoshuaKGoldberg · auvred · bradzacher · web-flow · commit 32d8a8f68af8 · 2024-03-05T17:40:51.000-05:00
* feat(eslint-plugin): [prefer-string-starts-ends-with] add allowSingleElementEquality option

* Update test snapshots

* Update packages/eslint-plugin/docs/rules/prefer-string-starts-ends-with.md

Co-authored-by: auvred &lt;61150013+auvred@users.noreply.github.com&gt;

---------

Co-authored-by: auvred &lt;61150013+auvred@users.noreply.github.com&gt;
Co-authored-by: Brad Zacher &lt;brad.zacher@gmail.com&gt;
diff --git a/.eslintrc.js b/.eslintrc.js
@@ -53,7 +53,6 @@ module.exports = {
 
     // TODO(#7130): Investigate changing these in or removing these from presets
     '@typescript-eslint/no-confusing-void-expression': 'off',
-    '@typescript-eslint/prefer-string-starts-ends-with': 'off',
 
     //
     // our plugin :D
@@ -91,6 +90,12 @@ module.exports = {
         allowBitwiseExpressions: true,
       },
     ],
+    '@typescript-eslint/prefer-string-starts-ends-with': [
+      'error',
+      {
+        allowSingleElementEquality: 'always',
+      },
+    ],
     '@typescript-eslint/unbound-method': 'off',
     '@typescript-eslint/restrict-template-expressions': [
       'error',
diff --git a/packages/eslint-plugin/docs/rules/prefer-string-starts-ends-with.md b/packages/eslint-plugin/docs/rules/prefer-string-starts-ends-with.md
@@ -52,6 +52,26 @@ foo.startsWith('bar');
 foo.endsWith('bar');
 ```
 
+<!--/tabs-->
+
+## Options
+
+### `allowSingleElementEquality`
+
+If switched to `'always'`, the rule will allow equality checks against the first or last character in a string.
+This can be preferable in projects that don't deal with special character encodings and prefer a more succinct style.
+
+The following code is considered incorrect by default, but is allowed with `allowSingleElementEquality: 'always'`:
+
+```ts option='{ "allowSingleElementEquality": "always" }' showPlaygroundButton
+declare const text: string;
+
+text[0] === 'a';
+text[0] === text[0].toUpperCase();
+text[0] === text[1];
+text[text.length - 1] === 'b';
+```
+
 ## When Not To Use It
 
 If you don't mind which style of string checking is used, you can turn this rule off safely.
diff --git a/packages/eslint-plugin/src/rules/prefer-string-starts-ends-with.ts b/packages/eslint-plugin/src/rules/prefer-string-starts-ends-with.ts
@@ -17,9 +17,19 @@ import {
 const EQ_OPERATORS = /^[=!]=/;
 const regexpp = new RegExpParser();
 
-export default createRule({
+type AllowedSingleElementEquality = 'always' | 'never';
+
+export type Options = [
+  {
+    allowSingleElementEquality?: AllowedSingleElementEquality;
+  },
+];
+
+type MessageIds = 'preferEndsWith' | 'preferStartsWith';
+
+export default createRule<Options, MessageIds>({
   name: 'prefer-string-starts-ends-with',
-  defaultOptions: [],
+  defaultOptions: [{ allowSingleElementEquality: 'never' }],
 
   meta: {
     type: 'suggestion',
@@ -33,11 +43,24 @@ export default createRule({
       preferStartsWith: "Use 'String#startsWith' method instead.",
       preferEndsWith: "Use the 'String#endsWith' method instead.",
     },
-    schema: [],
+    schema: [
+      {
+        additionalProperties: false,
+        properties: {
+          allowSingleElementEquality: {
+            description:
+              'Whether to allow equality checks against the first or last element of a string.',
+            enum: ['always', 'never'],
+            type: 'string',
+          },
+        },
+        type: 'object',
+      },
+    ],
     fixable: 'code',
   },
 
-  create(context) {
+  create(context, [{ allowSingleElementEquality }]) {
     const globalScope = context.sourceCode.getScope(context.sourceCode.ast);
 
     const services = getParserServices(context);
@@ -401,8 +424,15 @@ export default createRule({
         }
 
         const isEndsWith = isLastIndexExpression(indexNode, node.object);
+        if (allowSingleElementEquality === 'always' && isEndsWith) {
+          return;
+        }
+
         const isStartsWith = !isEndsWith && isNumber(indexNode, 0);
-        if (!isStartsWith && !isEndsWith) {
+        if (
+          (allowSingleElementEquality === 'always' && isStartsWith) ||
+          (!isStartsWith && !isEndsWith)
+        ) {
           return;
         }
 
diff --git a/packages/eslint-plugin/src/util/collectUnusedVariables.ts b/packages/eslint-plugin/src/util/collectUnusedVariables.ts
@@ -437,7 +437,7 @@ function isExported(variable: TSESLint.Scope.Variable): boolean {
     }
 
     // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
-    return node.parent!.type.indexOf('Export') === 0;
+    return node.parent!.type.startsWith('Export');
   });
 }
 
diff --git a/packages/eslint-plugin/tests/rules/prefer-string-starts-ends-with.test.ts b/packages/eslint-plugin/tests/rules/prefer-string-starts-ends-with.test.ts
diff --git a/packages/eslint-plugin/tests/schema-snapshots/prefer-string-starts-ends-with.shot b/packages/eslint-plugin/tests/schema-snapshots/prefer-string-starts-ends-with.shot

Original file line number	Diff line number	Diff line change
`@@ -437,7 +437,7 @@ function isExported(variable: TSESLint.Scope.Variable): boolean {`
`437`	`437`	`}`
`438`	`438`
`439`	`439`	`// eslint-disable-next-line @typescript-eslint/no-non-null-assertion`
`440`		`- return node.parent!.type.indexOf('Export') === 0;`
	`440`	`+ return node.parent!.type.startsWith('Export');`
`441`	`441`	`});`
`442`	`442`	`}`
`443`	`443`