Dec 14, 2025 · Oct 25, 2025 · Oct 25, 2025 · Oct 25, 2025 · Oct 25, 2025 · Oct 25, 2025
diff --git a/packages/eslint-plugin/docs/rules/no-useless-default-assignment.mdx b/packages/eslint-plugin/docs/rules/no-useless-default-assignment.mdx
 ---
 description: 'Disallow default values that will never be used.'
 ---

 import Tabs from '@theme/Tabs';
 import TabItem from '@theme/TabItem';

 > 🛑 This file is source code, not the primary documentation location! 🛑
 >
 > See **https://typescript-eslint.io/rules/no-useless-default-assignment** for documentation.

 [Default parameters](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters) and [default values](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#default_value) are only used if the parameter or property is `undefined`.
 That can happen when a value is missing, or when one is provided and set to `undefined`.
 If a non-`undefined` value is guaranteed to be provided, then there is no need to define a default.

 ## Examples

 <Tabs>
 <TabItem value="❌ Incorrect">

 ```ts
 function Bar({ foo = '' }: { foo: string }) {
  return foo;
 }

 const { foo = '' } = { foo: 'bar' };

 const [foo = ''] = ['bar'];

 [1, 2, 3].map((a = 42) => a + 1);
 ```

 </TabItem>
 <TabItem value="✅ Correct">

 ```ts
 function Bar({ foo = '' }: { foo?: string }) {
  return foo;
 }

 const { foo = '' } = { foo: undefined };

 const [foo = ''] = [undefined];

 [1, 2, 3, undefined].map((a = 42) => a + 1);
 ```

 </TabItem>
 </Tabs>

 ## When Not To Use It

 If you use default values defensively against runtime values that bypass type checking, or for documentation purposes, you may want to disable this rule.
diff --git a/packages/eslint-plugin/src/configs/eslintrc/all.ts b/packages/eslint-plugin/src/configs/eslintrc/all.ts
 '@typescript-eslint/no-use-before-define':'error',
 'no-useless-constructor':'off',
 '@typescript-eslint/no-useless-constructor':'error',
 '@typescript-eslint/no-useless-default-assignment':'error',
 '@typescript-eslint/no-useless-empty-export':'error',
 '@typescript-eslint/no-wrapper-object-types':'error',
 '@typescript-eslint/non-nullable-type-assertion-style':'error',
diff --git a/packages/eslint-plugin/src/configs/eslintrc/disable-type-checked.ts b/packages/eslint-plugin/src/configs/eslintrc/disable-type-checked.ts
 '@typescript-eslint/no-unsafe-return':'off',
 '@typescript-eslint/no-unsafe-type-assertion':'off',
 '@typescript-eslint/no-unsafe-unary-minus':'off',
 '@typescript-eslint/no-useless-default-assignment':'off',
 '@typescript-eslint/non-nullable-type-assertion-style':'off',
 '@typescript-eslint/only-throw-error':'off',
 '@typescript-eslint/prefer-destructuring':'off',
diff --git a/packages/eslint-plugin/src/configs/eslintrc/strict-type-checked-only.ts b/packages/eslint-plugin/src/configs/eslintrc/strict-type-checked-only.ts
 '@typescript-eslint/no-unsafe-member-access':'error',
 '@typescript-eslint/no-unsafe-return':'error',
 '@typescript-eslint/no-unsafe-unary-minus':'error',
 '@typescript-eslint/no-useless-default-assignment':'error',
 'no-throw-literal':'off',
 '@typescript-eslint/only-throw-error':'error',
 'prefer-promise-reject-errors':'off',
diff --git a/packages/eslint-plugin/src/configs/eslintrc/strict-type-checked.ts b/packages/eslint-plugin/src/configs/eslintrc/strict-type-checked.ts
 '@typescript-eslint/no-unused-vars':'error',
 'no-useless-constructor':'off',
 '@typescript-eslint/no-useless-constructor':'error',
 '@typescript-eslint/no-useless-default-assignment':'error',
 '@typescript-eslint/no-wrapper-object-types':'error',
 'no-throw-literal':'off',
 '@typescript-eslint/only-throw-error':'error',
diff --git a/packages/eslint-plugin/src/configs/flat/all.ts b/packages/eslint-plugin/src/configs/flat/all.ts
 '@typescript-eslint/no-use-before-define':'error',
 'no-useless-constructor':'off',
 '@typescript-eslint/no-useless-constructor':'error',
 '@typescript-eslint/no-useless-default-assignment':'error',
 '@typescript-eslint/no-useless-empty-export':'error',
 '@typescript-eslint/no-wrapper-object-types':'error',
 '@typescript-eslint/non-nullable-type-assertion-style':'error',
diff --git a/packages/eslint-plugin/src/configs/flat/disable-type-checked.ts b/packages/eslint-plugin/src/configs/flat/disable-type-checked.ts
 '@typescript-eslint/no-unsafe-return':'off',
 '@typescript-eslint/no-unsafe-type-assertion':'off',
 '@typescript-eslint/no-unsafe-unary-minus':'off',
 '@typescript-eslint/no-useless-default-assignment':'off',
 '@typescript-eslint/non-nullable-type-assertion-style':'off',
 '@typescript-eslint/only-throw-error':'off',
 '@typescript-eslint/prefer-destructuring':'off',
diff --git a/packages/eslint-plugin/src/configs/flat/strict-type-checked-only.ts b/packages/eslint-plugin/src/configs/flat/strict-type-checked-only.ts
 '@typescript-eslint/no-unsafe-member-access':'error',
 '@typescript-eslint/no-unsafe-return':'error',
 '@typescript-eslint/no-unsafe-unary-minus':'error',
 '@typescript-eslint/no-useless-default-assignment':'error',
 'no-throw-literal':'off',
 '@typescript-eslint/only-throw-error':'error',
 'prefer-promise-reject-errors':'off',
diff --git a/packages/eslint-plugin/src/configs/flat/strict-type-checked.ts b/packages/eslint-plugin/src/configs/flat/strict-type-checked.ts
 '@typescript-eslint/no-unused-vars':'error',
 'no-useless-constructor':'off',
 '@typescript-eslint/no-useless-constructor':'error',
 '@typescript-eslint/no-useless-default-assignment':'error',
 '@typescript-eslint/no-wrapper-object-types':'error',
 'no-throw-literal':'off',
 '@typescript-eslint/only-throw-error':'error',
diff --git a/packages/eslint-plugin/src/rules/index.ts b/packages/eslint-plugin/src/rules/index.ts
 import noUnusedVars from './no-unused-vars';
 import noUseBeforeDefine from './no-use-before-define';
 import noUselessConstructor from './no-useless-constructor';
 import noUselessDefaultAssignment from './no-useless-default-assignment';
 import noUselessEmptyExport from './no-useless-empty-export';
 import noVarRequires from './no-var-requires';
 import noWrapperObjectTypes from './no-wrapper-object-types';
  'no-unused-vars': noUnusedVars,
  'no-use-before-define': noUseBeforeDefine,
  'no-useless-constructor': noUselessConstructor,
  'no-useless-default-assignment': noUselessDefaultAssignment,
  'no-useless-empty-export': noUselessEmptyExport,
  'no-var-requires': noVarRequires,
  'no-wrapper-object-types': noWrapperObjectTypes,
diff --git a/packages/eslint-plugin/src/rules/no-useless-default-assignment.ts b/packages/eslint-plugin/src/rules/no-useless-default-assignment.ts
 import type { TSESTree } from '@typescript-eslint/utils';

 import { AST_NODE_TYPES } from '@typescript-eslint/utils';
 import * as tsutils from 'ts-api-utils';
 import * as ts from 'typescript';

 import {
  createRule,
  getParserServices,
  isFunction,
  isTypeAnyType,
  isTypeFlagSet,
  isTypeUnknownType,
  nullThrows,
  NullThrowsReasons,
 } from '../util';

 type MessageId = 'uselessDefaultAssignment' | 'uselessUndefined';

 export default createRule<[], MessageId>({
  name: 'no-useless-default-assignment',
  meta: {
    type: 'suggestion',
    docs: {
      description: 'Disallow default values that will never be used',
      recommended: 'strict',
      requiresTypeChecking: true,
    },
    fixable: 'code',
    messages: {
      uselessDefaultAssignment:
        'Default value is useless because the {{ type }} is not optional.',
      uselessUndefined:
        'Default value is useless because it is undefined. Optional {{ type }}s are already undefined by default.',
    },
    schema: [],
  },
  defaultOptions: [],
  create(context) {
    const services = getParserServices(context);
    const checker = services.program.getTypeChecker();
    const compilerOptions = services.program.getCompilerOptions();
    const isNoUncheckedIndexedAccess = tsutils.isCompilerOptionEnabled(
      compilerOptions,
      'noUncheckedIndexedAccess',
    );

    function canBeUndefined(type: ts.Type): boolean {
      if (isTypeAnyType(type) || isTypeUnknownType(type)) {
        return true;
      }
      return tsutils
        .unionConstituents(type)
        .some(part => isTypeFlagSet(part, ts.TypeFlags.Undefined));
    }

    function getPropertyType(
      objectType: ts.Type,
      propertyName: string,
    ): ts.Type | null {
      const symbol = objectType.getProperty(propertyName);
      if (!symbol) {
        if (isNoUncheckedIndexedAccess) {
          return null;
        }
        return objectType.getStringIndexType() ?? null;
      }
      return checker.getTypeOfSymbol(symbol);
    }

    function getArrayElementType(
      arrayType: ts.Type,
      elementIndex: number,
    ): ts.Type | null {
      if (checker.isTupleType(arrayType)) {
        const tupleArgs = checker.getTypeArguments(arrayType);
        if (elementIndex < tupleArgs.length) {
          return tupleArgs[elementIndex];
        }
      }

      if (isNoUncheckedIndexedAccess) {
        return null;
      }

      return arrayType.getNumberIndexType() ?? null;
    }

    function checkAssignmentPattern(node: TSESTree.AssignmentPattern): void {
      if (
        node.right.type === AST_NODE_TYPES.Identifier &&
        node.right.name === 'undefined'
      ) {
        const type =
          node.parent.type === AST_NODE_TYPES.Property ||
          node.parent.type === AST_NODE_TYPES.ArrayPattern
            ? 'property'
            : 'parameter';
        reportUselessDefault(node, type, 'uselessUndefined');
        return;
      }

      const parent = node.parent;

      if (
        parent.type === AST_NODE_TYPES.ArrowFunctionExpression ||
        parent.type === AST_NODE_TYPES.FunctionExpression
      ) {
        const paramIndex = parent.params.indexOf(node);
        if (paramIndex !== -1) {
          const tsFunc = services.esTreeNodeToTSNodeMap.get(parent);
          if (ts.isFunctionLike(tsFunc)) {
            const contextualType = checker.getContextualType(
              tsFunc as ts.Expression,
            );
            if (!contextualType) {
              return;
            }

            const signatures = contextualType.getCallSignatures();
            const params = signatures[0].getParameters();
            if (paramIndex < params.length) {
              const paramSymbol = params[paramIndex];
              if ((paramSymbol.flags & ts.SymbolFlags.Optional) === 0) {
                const paramType = checker.getTypeOfSymbol(paramSymbol);
                if (!canBeUndefined(paramType)) {
                  reportUselessDefault(
                    node,
                    'parameter',
                    'uselessDefaultAssignment',
                  );
                }
              }
            }
          }
        }
        return;
      }

      if (parent.type === AST_NODE_TYPES.Property) {
        const propertyType = getTypeOfProperty(parent);
        if (!propertyType) {
          return;
        }

        if (!canBeUndefined(propertyType)) {
          reportUselessDefault(node, 'property', 'uselessDefaultAssignment');
        }
      } else if (parent.type === AST_NODE_TYPES.ArrayPattern) {
        const sourceType = getSourceTypeForPattern(parent);
        if (!sourceType) {
          return;
        }

        const elementIndex = parent.elements.indexOf(node);
        const elementType = getArrayElementType(sourceType, elementIndex);
        if (!elementType) {
          return;
        }

        if (!canBeUndefined(elementType)) {
          reportUselessDefault(node, 'property', 'uselessDefaultAssignment');
        }
      }
    }

    function getTypeOfProperty(node: TSESTree.Property): ts.Type | null {
      const objectPattern = node.parent as TSESTree.ObjectPattern;
      const sourceType = getSourceTypeForPattern(objectPattern);
      if (!sourceType) {
        return null;
      }

      const propertyName = getPropertyName(node.key);
      if (!propertyName) {
        return null;
      }

      return getPropertyType(sourceType, propertyName);
    }

    function getSourceTypeForPattern(pattern: TSESTree.Node): ts.Type | null {
      const parent = nullThrows(
        pattern.parent,
        NullThrowsReasons.MissingParent,
      );

      if (parent.type === AST_NODE_TYPES.VariableDeclarator && parent.init) {
        const tsNode = services.esTreeNodeToTSNodeMap.get(parent.init);
        return checker.getTypeAtLocation(tsNode);
      }

      if (isFunction(parent)) {
        const paramIndex = parent.params.indexOf(pattern as TSESTree.Parameter);
        const tsFunc = services.esTreeNodeToTSNodeMap.get(parent);
        const signature = nullThrows(
          checker.getSignatureFromDeclaration(tsFunc),
          NullThrowsReasons.MissingToken('signature', 'function'),
        );
        const params = signature.getParameters();
        return checker.getTypeOfSymbol(params[paramIndex]);
      }

      if (parent.type === AST_NODE_TYPES.AssignmentPattern) {
        return getSourceTypeForPattern(parent);
      }

      if (parent.type === AST_NODE_TYPES.Property) {
        return getTypeOfProperty(parent);
      }

      if (parent.type === AST_NODE_TYPES.ArrayPattern) {
        const arrayPattern = parent;
        const arrayType = getSourceTypeForPattern(arrayPattern);
        if (!arrayType) {
          return null;
        }
        const elementIndex = arrayPattern.elements.indexOf(
          pattern as TSESTree.DestructuringPattern,
        );
        return getArrayElementType(arrayType, elementIndex);
      }

      return null;
    }

    function getPropertyName(
      key: TSESTree.Expression | TSESTree.PrivateIdentifier,
    ): string | null {
      switch (key.type) {
        case AST_NODE_TYPES.Identifier:
          return key.name;
        case AST_NODE_TYPES.Literal:
          return String(key.value);
        default:
          return null;
      }
    }

    function reportUselessDefault(
      node: TSESTree.AssignmentPattern,
      type: 'parameter' | 'property',
      messageId: MessageId,
    ): void {
      context.report({
        node: node.right,
        messageId,
        data: { type },
        fix(fixer) {
          // Remove from before the = to the end of the default value
          // Find the start position (including whitespace before =)
          const start = node.left.range[1];
          const end = node.range[1];
          return fixer.removeRange([start, end]);
        },
      });
    }

    return {
      AssignmentPattern: checkAssignmentPattern,
    };
  },
 });
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,53 @@
		---
		description: 'Disallow default values that will never be used.'
		---

		import Tabs from '@theme/Tabs';
		import TabItem from '@theme/TabItem';

		> 🛑 This file is source code, not the primary documentation location! 🛑
		>
		> See https://typescript-eslint.io/rules/no-useless-default-assignment for documentation.

		[Default parameters](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters) and [default values](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#default_value) are only used if the parameter or property is `undefined`.
		That can happen when a value is missing, or when one is provided and set to `undefined`.
		If a non-`undefined` value is guaranteed to be provided, then there is no need to define a default.

		## Examples

		<Tabs>
		<TabItem value="❌ Incorrect">

		```ts
		function Bar({ foo = '' }: { foo: string }) {
		return foo;
		}

		const { foo = '' } = { foo: 'bar' };

		const [foo = ''] = ['bar'];

		[1, 2, 3].map((a = 42) => a + 1);
		```

		</TabItem>
		<TabItem value="✅ Correct">

		```ts
		function Bar({ foo = '' }: { foo?: string }) {
		return foo;
		}

		const { foo = '' } = { foo: undefined };

		const [foo = ''] = [undefined];

		[1, 2, 3, undefined].map((a = 42) => a + 1);
		```

		</TabItem>
		</Tabs>

		## When Not To Use It

		If you use default values defensively against runtime values that bypass type checking, or for documentation purposes, you may want to disable this rule.
Original file line number	Diff line number	Diff line change
Expand Up		@@ -120,6 +120,7 @@ export = {
		'@typescript-eslint/no-use-before-define':'error',
		'no-useless-constructor':'off',
		'@typescript-eslint/no-useless-constructor':'error',
		'@typescript-eslint/no-useless-default-assignment':'error',
		'@typescript-eslint/no-useless-empty-export':'error',
		'@typescript-eslint/no-wrapper-object-types':'error',
		'@typescript-eslint/non-nullable-type-assertion-style':'error',
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -44,6 +44,7 @@ export = {
		'@typescript-eslint/no-unsafe-return':'off',
		'@typescript-eslint/no-unsafe-type-assertion':'off',
		'@typescript-eslint/no-unsafe-unary-minus':'off',
		'@typescript-eslint/no-useless-default-assignment':'off',
		'@typescript-eslint/non-nullable-type-assertion-style':'off',
		'@typescript-eslint/only-throw-error':'off',
		'@typescript-eslint/prefer-destructuring':'off',
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -39,6 +39,7 @@ export = {
		'@typescript-eslint/no-unsafe-member-access':'error',
		'@typescript-eslint/no-unsafe-return':'error',
		'@typescript-eslint/no-unsafe-unary-minus':'error',
		'@typescript-eslint/no-useless-default-assignment':'error',
		'no-throw-literal':'off',
		'@typescript-eslint/only-throw-error':'error',
		'prefer-promise-reject-errors':'off',
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -68,6 +68,7 @@ export = {
		'@typescript-eslint/no-unused-vars':'error',
		'no-useless-constructor':'off',
		'@typescript-eslint/no-useless-constructor':'error',
		'@typescript-eslint/no-useless-default-assignment':'error',
		'@typescript-eslint/no-wrapper-object-types':'error',
		'no-throw-literal':'off',
		'@typescript-eslint/only-throw-error':'error',
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -134,6 +134,7 @@ export default (
		'@typescript-eslint/no-use-before-define':'error',
		'no-useless-constructor':'off',
		'@typescript-eslint/no-useless-constructor':'error',
		'@typescript-eslint/no-useless-default-assignment':'error',
		'@typescript-eslint/no-useless-empty-export':'error',
		'@typescript-eslint/no-wrapper-object-types':'error',
		'@typescript-eslint/non-nullable-type-assertion-style':'error',
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -51,6 +51,7 @@ export default (
		'@typescript-eslint/no-unsafe-return':'off',
		'@typescript-eslint/no-unsafe-type-assertion':'off',
		'@typescript-eslint/no-unsafe-unary-minus':'off',
		'@typescript-eslint/no-useless-default-assignment':'off',
		'@typescript-eslint/non-nullable-type-assertion-style':'off',
		'@typescript-eslint/only-throw-error':'off',
		'@typescript-eslint/prefer-destructuring':'off',
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -52,6 +52,7 @@ export default (
		'@typescript-eslint/no-unsafe-member-access':'error',
		'@typescript-eslint/no-unsafe-return':'error',
		'@typescript-eslint/no-unsafe-unary-minus':'error',
		'@typescript-eslint/no-useless-default-assignment':'error',
		'no-throw-literal':'off',
		'@typescript-eslint/only-throw-error':'error',
		'prefer-promise-reject-errors':'off',
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -81,6 +81,7 @@ export default (
		'@typescript-eslint/no-unused-vars':'error',
		'no-useless-constructor':'off',
		'@typescript-eslint/no-useless-constructor':'error',
		'@typescript-eslint/no-useless-default-assignment':'error',
		'@typescript-eslint/no-wrapper-object-types':'error',
		'no-throw-literal':'off',
		'@typescript-eslint/only-throw-error':'error',
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -92,6 +92,7 @@ import noUnusedPrivateClassMembers from './no-unused-private-class-members';
		import noUnusedVars from './no-unused-vars';
		import noUseBeforeDefine from './no-use-before-define';
		import noUselessConstructor from './no-useless-constructor';
		import noUselessDefaultAssignment from './no-useless-default-assignment';
		import noUselessEmptyExport from './no-useless-empty-export';
		import noVarRequires from './no-var-requires';
		import noWrapperObjectTypes from './no-wrapper-object-types';
Expand DownExpand Up		@@ -227,6 +228,7 @@ const rules = {
		'no-unused-vars': noUnusedVars,
		'no-use-before-define': noUseBeforeDefine,
		'no-useless-constructor': noUselessConstructor,
		'no-useless-default-assignment': noUselessDefaultAssignment,
		'no-useless-empty-export': noUselessEmptyExport,
		'no-var-requires': noVarRequires,
		'no-wrapper-object-types': noWrapperObjectTypes,
Expand Down