typescript-eslint · JoshuaKGoldberg · Oct 21, 2024 · Oct 12, 2024 · Oct 12, 2024 · Oct 12, 2024
diff --git a/packages/eslint-plugin/docs/rules/no-base-to-string.mdx b/packages/eslint-plugin/docs/rules/no-base-to-string.mdx
@@ -1,5 +1,5 @@
 ---
-description: 'Require `.toString()` to only be called on objects which provide useful information when stringified.'
+description: 'Require `.toString()` and `.toLocaleString()` to only be called on objects which provide useful information when stringified.'
 ---
 
 import Tabs from '@theme/Tabs';
@@ -10,10 +10,10 @@ import TabItem from '@theme/TabItem';
 > See **https://typescript-eslint.io/rules/no-base-to-string** for documentation.
 
 JavaScript will call `toString()` on an object when it is converted to a string, such as when `+` adding to a string or in `${}` template literals.
-The default Object `.toString()` uses the format `"[object Object]"`, which is often not what was intended.
-This rule reports on stringified values that aren't primitives and don't define a more useful `.toString()` method.
+The default Object `.toString()` and `toLocaleString()` use the format `"[object Object]"`, which is often not what was intended.
+This rule reports on stringified values that aren't primitives and don't define a more useful `.toString()` or `toLocaleString()` method.
 
-> Note that `Function` provides its own `.toString()` that returns the function's code.
+> Note that `Function` provides its own `.toString()` and `toLocaleString()` that return the function's code.
 > Functions are not flagged by this rule.
 
 ## Examples
@@ -29,20 +29,22 @@ class MyClass {}
 const value = new MyClass();
 value + '';
 
-// Interpolation and manual .toString() calls too:
+// Interpolation and manual .toString() and `toLocaleString()` calls too:
 `Value: ${value}`;
 ({}).toString();
+({}).toLocaleString();
 ```
 
 </TabItem>
 <TabItem value="✅ Correct">
 
 ```ts
-// These types all have useful .toString()s
+// These types all have useful .toString() and `toLocaleString()` methods
 'Text' + true;
 `Value: ${123}`;
 `Arrays too: ${[1, 2, 3]}`;
 (() => {}).toString();
+(() => {}).toLocaleString();
 
 // Defining a custom .toString class is considered acceptable
 class CustomToString {
@@ -68,8 +70,8 @@ const literalWithToString = {
 
 {/* insert option description */}
 
-This is useful for types whose declarations missing `toString()`, but are known to actually have `toString()`.
-There are some types missing `toString()` in TypeScript, like `RegExp`, `URL`, `URLSearchParams` etc.
+This is useful for types missing `toString()` or `toLocaleString()` (but actually has `toString()` or `toLocaleString()`).
+There are some types missing `toString()` or `toLocaleString()` in old versions of TypeScript, like `RegExp`, `URL`, `URLSearchParams` etc.
 
 The following patterns are considered correct with the default options `{ ignoredTypeNames: ["RegExp"] }`:
 
@@ -94,4 +96,5 @@ If you don't mind a risk of `"[object Object]"` or incorrect type coercions in y
 ## Further Reading
 
 - [`Object.prototype.toString()` MDN documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/toString)
+- [`Object.prototype.toLocaleString()` MDN documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/toLocaleString)
 - [Microsoft/TypeScript Add missing toString declarations for base types that have them](https://github.com/microsoft/TypeScript/issues/38347)
diff --git a/packages/eslint-plugin/src/rules/no-base-to-string.ts b/packages/eslint-plugin/src/rules/no-base-to-string.ts
@@ -24,7 +24,7 @@ export default createRule<Options, MessageIds>({
     type: 'suggestion',
     docs: {
       description:
-        'Require `.toString()` to only be called on objects which provide useful information when stringified',
+        'Require `.toString()` and `.toLocaleString()` to only be called on objects which provide useful information when stringified',
       recommended: 'recommended',
       requiresTypeChecking: true,
     },
@@ -82,7 +82,9 @@ export default createRule<Options, MessageIds>({
     }
 
     function collectToStringCertainty(type: ts.Type): Usefulness {
-      const toString = checker.getPropertyOfType(type, 'toString');
+      const toString =
+        checker.getPropertyOfType(type, 'toString') ??
+        checker.getPropertyOfType(type, 'toLocaleString');
       const declarations = toString?.getDeclarations();
       if (!toString || !declarations || declarations.length === 0) {
         return Usefulness.Always;
@@ -167,7 +169,7 @@ export default createRule<Options, MessageIds>({
           checkExpression(node.left, leftType);
         }
       },
-      'CallExpression > MemberExpression.callee > Identifier[name = "toString"].property'(
+      'CallExpression > MemberExpression.callee > Identifier[name = /^(toLocaleString|toString)$/].property'(
         node: TSESTree.Expression,
       ): void {
         const memberExpr = node.parent as TSESTree.MemberExpression;

diff --git a/packages/eslint-plugin/tests/docs-eslint-output-snapshots/no-base-to-string.shot b/packages/eslint-plugin/tests/docs-eslint-output-snapshots/no-base-to-string.shot
diff --git a/packages/eslint-plugin/tests/rules/no-base-to-string.test.ts b/packages/eslint-plugin/tests/rules/no-base-to-string.test.ts
@@ -63,9 +63,15 @@ ruleTester.run('no-base-to-string', rule, {
     `
 function someFunction() {}
 someFunction.toString();
+let text = \`\${someFunction}\`;
+    `,
+    `
+function someFunction() {}
+someFunction.toLocaleString();
 let text = \`\${someFunction}\`;
     `,
     'unknownObject.toString();',
+    'unknownObject.toLocaleString();',
     'unknownObject.someOtherMethod();',
     `
 class CustomToString {
@@ -79,14 +85,22 @@ class CustomToString {
 const literalWithToString = {
   toString: () => 'Hello, world!',
 };
-'' + literalToString;
+'' + literalWithToString;
     `,
     `
 const printer = (inVar: string | number | boolean) => {
   inVar.toString();
 };
 printer('');
 printer(1);
+printer(true);
+    `,
+    `
+const printer = (inVar: string | number | boolean) => {
+  inVar.toLocaleString();
+};
+printer('');
+printer(1);
 printer(true);
     `,
     'let _ = {} * {};',
@@ -144,6 +158,18 @@ tag\`\${{}}\`;
         },
       ],
     },
+    {
+      code: '({}).toLocaleString();',
+      errors: [
+        {
+          data: {
+            certainty: 'will',
+            name: '{}',
+          },
+          messageId: 'baseToString',
+        },
+      ],
+    },
     {
       code: "'' + {};",
       errors: [
@@ -183,6 +209,21 @@ tag\`\${{}}\`;
         },
       ],
     },
+    {
+      code: `
+        let someObjectOrString = Math.random() ? { a: true } : 'text';
+        someObjectOrString.toLocaleString();
+      `,
+      errors: [
+        {
+          data: {
+            certainty: 'may',
+            name: 'someObjectOrString',
+          },
+          messageId: 'baseToString',
+        },
+      ],
+    },
     {
       code: `
         let someObjectOrString = Math.random() ? { a: true } : 'text';
@@ -213,6 +254,21 @@ tag\`\${{}}\`;
         },
       ],
     },
+    {
+      code: `
+        let someObjectOrObject = Math.random() ? { a: true, b: true } : { a: true };
+        someObjectOrObject.toLocaleString();
+      `,
+      errors: [
+        {
+          data: {
+            certainty: 'will',
+            name: 'someObjectOrObject',
+          },
+          messageId: 'baseToString',
+        },
+      ],
+    },
     {
       code: `
         let someObjectOrObject = Math.random() ? { a: true, b: true } : { a: true };