typescript-eslint · JoshuaKGoldberg · Oct 27, 2024 · Sep 17, 2024 · Sep 18, 2024 · Sep 18, 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
@@ -9,7 +9,7 @@ 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 `${}` template literals.
+JavaScript will call `toString()` on an object when it is converted to a string, such as when concatenated with a string (`expr + ''`), when interpolated into template literals (`${expr}`), or when passed as an argument to the String constructor (`String(expr)`).
 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.
 
@@ -31,6 +31,7 @@ value + '';
 
 // Interpolation and manual .toString() and `toLocaleString()` calls too:
 `Value: ${value}`;
+String({});
 ({}).toString();
 ({}).toLocaleString();
 ```
@@ -44,6 +45,7 @@ value + '';
 `Value: ${123}`;
 `Arrays too: ${[1, 2, 3]}`;
 (() => {}).toString();
+String(42);
 (() => {}).toLocaleString();
 
 // Defining a custom .toString class is considered acceptable
@@ -64,6 +66,15 @@ const literalWithToString = {
 </TabItem>
 </Tabs>
 
+## Alternatives
+
+Consider using `JSON.stringify` when you want to convert non-primitive things to string for logging, debugging, etc.
+
+```typescript
+declare const o: object;
+const errorMessage = 'Found unexpected value: ' + JSON.stringify(o);
+```
+
 ## Options
 
 ### `ignoredTypeNames`
@@ -82,6 +93,7 @@ The following patterns are considered correct with the default options `{ ignore
 let value = /regex/;
 value.toString();
 let text = `${value}`;
+String(/regex/);
 ```
 
 ## When Not To Use It

diff --git a/packages/eslint-plugin/src/rules/no-base-to-string.ts b/packages/eslint-plugin/src/rules/no-base-to-string.ts
@@ -1,3 +1,4 @@
+/* eslint-disable @typescript-eslint/internal/prefer-ast-types-enum */
 import type { TSESTree } from '@typescript-eslint/utils';
 
 import { AST_NODE_TYPES } from '@typescript-eslint/utils';
@@ -59,7 +60,7 @@ export default createRule<Options, MessageIds>({
     const checker = services.program.getTypeChecker();
     const ignoredTypeNames = option.ignoredTypeNames ?? [];
 
-    function checkExpression(node: TSESTree.Expression, type?: ts.Type): void {
+    function checkExpression(node: TSESTree.Node, type?: ts.Type): void {
       if (node.type === AST_NODE_TYPES.Literal) {
         return;
       }
@@ -153,6 +154,19 @@ export default createRule<Options, MessageIds>({
       return Usefulness.Never;
     }
 
+    function isBuiltInStringCall(node: TSESTree.CallExpression): boolean {
+      if (
+        node.callee.type === AST_NODE_TYPES.Identifier &&
+        node.callee.name === 'String' &&
+        node.arguments[0]
+      ) {
+        const scope = context.sourceCode.getScope(node);
+        const variable = scope.set.get('String');
+        return !variable?.defs.length;
+      }
+      return false;
+    }
+
     return {
       'AssignmentExpression[operator = "+="], BinaryExpression[operator = "+"]'(
         node: TSESTree.AssignmentExpression | TSESTree.BinaryExpression,
@@ -169,6 +183,11 @@ export default createRule<Options, MessageIds>({
           checkExpression(node.left, leftType);
         }
       },
+      CallExpression(node: TSESTree.CallExpression): void {
+        if (isBuiltInStringCall(node)) {
+          checkExpression(node.arguments[0]);
+        }
+      },
       'CallExpression > MemberExpression.callee > Identifier[name = /^(toLocaleString|toString)$/].property'(
         node: TSESTree.Expression,
       ): void {

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
@@ -60,6 +60,8 @@ ruleTester.run('no-base-to-string', rule, {
       `,
     ),
 
+    // String()
+    ...literalList.map(i => `String(${i});`),
     `
 function someFunction() {}
 someFunction.toString();
@@ -132,6 +134,28 @@ tag\`\${{}}\`;
     "'' += new Error();",
     "'' += new URL();",
     "'' += new URLSearchParams();",
+    `
+let numbers = [1, 2, 3];
+String(...a);
+    `,
+    `
+Number(1);
+    `,
+    {
+      code: 'String(/regex/);',
+      options: [{ ignoredTypeNames: ['RegExp'] }],
+    },
+    `
+function String(value) {
+  return value;
+}
+declare const myValue: object;
+String(myValue);
+    `,
+    `
+import { String } from 'foo';
+String({});
+    `,
   ],
   invalid: [
     {
@@ -182,6 +206,33 @@ tag\`\${{}}\`;
         },
       ],
     },
+    {
+      code: 'String({});',
+      errors: [
+        {
+          data: {
+            certainty: 'will',
+            name: '{}',
+          },
+          messageId: 'baseToString',
+        },
+      ],
+    },
+    {
+      code: `
+let objects = [{}, {}];
+String(...objects);
+      `,
+      errors: [
+        {
+          data: {
+            certainty: 'will',
+            name: '...objects',
+          },
+          messageId: 'baseToString',
+        },
+      ],
+    },
     {
       code: "'' += {};",
       errors: [