Oct 27, 2024 · Sep 17, 2024 · Sep 18, 2024 · Sep 18, 2024 · Sep 25, 2024 · Oct 23, 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
 >
 > 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`${}` template literals.
 JavaScript will call `toString()` on an object when it is converted to a string, such as whenconcatenated with a string (`expr + ''`), when interpolated into template literals (`${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.


 // Interpolation and manual .toString() and `toLocaleString()` calls too:
 `Value: ${value}`;
 String({});
 ({}).toString();
 ({}).toLocaleString();
 ```
 `Value: ${123}`;
 `Arrays too: ${[1, 2, 3]}`;
 (() => {}).toString();
 String(42);
 (() => {}).toLocaleString();

 // Defining a custom .toString class is considered acceptable
 </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`
 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
 /* eslint-disable @typescript-eslint/internal/prefer-ast-types-enum */
 import type { TSESTree } from '@typescript-eslint/utils';

 import { AST_NODE_TYPES } from '@typescript-eslint/utils';
    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;
      }
      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,
          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
      `,
    ),

    // String()
    ...literalList.map(i => `String(${i});`),
    `
 function someFunction() {}
 someFunction.toString();
    "'' += 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: [
    {
        },
      ],
    },
    {
      code: 'String({});',
      errors: [
        {
          data: {
            certainty: 'will',
            name: '{}',
          },
          messageId: 'baseToString',
        },
      ],
    },
    {
      code: `
 let objects = [{}, {}];
 String(...objects);
      `,
      errors: [
        {
          data: {
            certainty: 'will',
            name: '...objects',
          },
          messageId: 'baseToString',
        },
      ],
    },
    {
      code: "'' += {};",
      errors: [
Original file line number	Diff line number	Diff line change
Expand Up		@@ -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`${}` template literals.
		JavaScript will call `toString()` on an object when it is converted to a string, such as whenconcatenated with a string (`expr + ''`), when interpolated into template literals (`${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.

Expand All		@@ -31,6 +31,7 @@ value + '';

		// Interpolation and manual .toString() and `toLocaleString()` calls too:
		`Value: ${value}`;
		String({});
		({}).toString();
		({}).toLocaleString();
		```
Expand All		@@ -44,6 +45,7 @@ value + '';
		`Value: ${123}`;
		`Arrays too: ${[1, 2, 3]}`;
		(() => {}).toString();
		String(42);
		(() => {}).toLocaleString();

		// Defining a custom .toString class is considered acceptable
Expand All		@@ -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);
		```
kirkwaiblinger marked this conversation as resolved. Show resolvedHide resolved

		## Options

		### `ignoredTypeNames`
Expand All		@@ -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
Expand Down
Original file line number	Diff line number	Diff line change
		@@ -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';
Expand DownExpand Up		@@ -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;
		}
Expand DownExpand Up		@@ -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,
Expand All		@@ -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 {
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -60,6 +60,8 @@ ruleTester.run('no-base-to-string', rule, {
		`,
		),

		// String()
		...literalList.map(i => `String(${i});`),
		`
		function someFunction() {}
		someFunction.toString();
Expand DownExpand Up		@@ -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: [
		{
Expand DownExpand Up		@@ -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: [
Expand Down