Oct 21, 2024 · Oct 12, 2024 · Oct 12, 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
 ---
 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';
 > 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()` thatreturns the function's code.
 > Note that `Function` provides its own `.toString()`and `toLocaleString()`thatreturn the function's code.
 > Functions are not flagged by this rule.

 ## Examples
 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 {

 {/* insert option description */}

 This is useful for typeswhose declarationsmissing `toString()`, but are known toactuallyhave `toString()`.
 There are some types missing `toString()`in TypeScript, like `RegExp`, `URL`, `URLSearchParams` etc.
 This is useful for types missing `toString()` or `toLocaleString()` (butactuallyhas `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"] }`:

 ## 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
    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,
    },
    }

    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;
          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
    `
 function someFunction() {}
 someFunction.toString();
 let text = \`\${someFunction}\`;
    `,
    `
 function someFunction() {}
 someFunction.toLocaleString();
 let text = \`\${someFunction}\`;
    `,
    'unknownObject.toString();',
    'unknownObject.toLocaleString();',
    'unknownObject.someOtherMethod();',
    `
 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 _ = {} * {};',
        },
      ],
    },
    {
      code: '({}).toLocaleString();',
      errors: [
        {
          data: {
            certainty: 'will',
            name: '{}',
          },
          messageId: 'baseToString',
        },
      ],
    },
    {
      code: "'' + {};",
      errors: [
        },
      ],
    },
    {
      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';
        },
      ],
    },
    {
      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 };
Original file line number	Diff line number	Diff line change
		@@ -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';
Expand All		@@ -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()` thatreturns the function's code.
		> Note that `Function` provides its own `.toString()`and `toLocaleString()`thatreturn the function's code.
		> Functions are not flagged by this rule.

		## Examples
Expand All		@@ -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 {
Expand All		@@ -68,8 +70,8 @@ const literalWithToString = {

		{/* insert option description */}

		This is useful for typeswhose declarationsmissing `toString()`, but are known toactuallyhave `toString()`.
		There are some types missing `toString()`in TypeScript, like `RegExp`, `URL`, `URLSearchParams` etc.
		This is useful for types missing `toString()` or `toLocaleString()` (butactuallyhas `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"] }`:

Expand All		@@ -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)
Original file line number	Diff line number	Diff line change
Expand Up		@@ -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,
		},
Expand DownExpand Up		@@ -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;
Expand DownExpand Up		@@ -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;
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -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 {
Expand All		@@ -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 _ = {} * {};',
Expand DownExpand Up		@@ -144,6 +158,18 @@ tag\`\${{}}\`;
		},
		],
		},
		{
		code: '({}).toLocaleString();',
		errors: [
		{
		data: {
		certainty: 'will',
		name: '{}',
		},
		messageId: 'baseToString',
		},
		],
		},
		{
		code: "'' + {};",
		errors: [
Expand DownExpand Up		@@ -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';
Expand DownExpand Up		@@ -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 };
Expand Down