typescript-eslint · JoshuaKGoldberg · May 5, 2025 · Oct 20, 2024 · Oct 20, 2024 · Nov 3, 2024
diff --git a/eslint.config.mjs b/eslint.config.mjs
@@ -148,6 +148,7 @@ export default tseslint.config(
         'error',
         { allowConstantLoopConditions: true, checkTypePredicates: true },
       ],
+      '@typescript-eslint/no-unnecessary-type-conversion': 'error',
       '@typescript-eslint/no-unnecessary-type-parameters': 'error',
       '@typescript-eslint/no-unused-expressions': 'error',
       '@typescript-eslint/no-unused-vars': [

diff --git a/packages/eslint-plugin/docs/rules/no-unnecessary-type-conversion.mdx b/packages/eslint-plugin/docs/rules/no-unnecessary-type-conversion.mdx
@@ -0,0 +1,79 @@
+---
+description: 'Disallow conversion idioms when they do not change the type or value of the expression.'
+---
+
+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-unnecessary-type-conversion** for documentation.
+
+JavaScript provides several commonly used idioms to convert values to a specific type:
+
+- Primitive coercion (e.g. `Boolean(value)`, `String(value)`): using a built-in primitive function
+- String concatenation (e.g. `value + ''`): turning a value into a string
+- Unary coercion (e.g. `+value`, `!!value`): using a built-in operator
+- The `.toString()` method defined on many types
+
+These conversions are unnecessary if the value is already of that type.
+
+## Examples
+
+<Tabs>
+<TabItem value="❌ Incorrect">
+
+```ts
+String('123');
+'123'.toString();
+'' + '123';
+'123' + '';
+
+Number(123);
++123;
+~~123;
+
+Boolean(true);
+!!true;
+
+BigInt(BigInt(1));
+
+let str = '123';
+str += '';
+```
+
+</TabItem>
+<TabItem value="✅ Correct">
+
+```ts
+function foo(bar: string | number) {
+  String(bar);
+  bar.toString();
+  '' + bar;
+  bar + '';
+
+  Number(bar);
+  +bar;
+  ~~bar;
+
+  Boolean(bar);
+  !!bar;
+
+  BigInt(1);
+
+  bar += '';
+}
+```
+
+</TabItem>
+</Tabs>
+
+## When Not To Use It
+
+If you don't care about having no-op type conversions in your code, then you can turn off this rule.
+If you have types which are not accurate, then this rule might cause you to remove conversions that you actually do need.
+
+## Related To
+
+- [no-unnecessary-type-assertion](./no-unnecessary-type-assertion.mdx)
+- [no-useless-template-literals](./no-useless-template-literals.mdx)
diff --git a/packages/eslint-plugin/src/configs/eslintrc/all.ts b/packages/eslint-plugin/src/configs/eslintrc/all.ts
@@ -98,6 +98,7 @@ export = {
     '@typescript-eslint/no-unnecessary-type-arguments': 'error',
     '@typescript-eslint/no-unnecessary-type-assertion': 'error',
     '@typescript-eslint/no-unnecessary-type-constraint': 'error',
+    '@typescript-eslint/no-unnecessary-type-conversion': 'error',
     '@typescript-eslint/no-unnecessary-type-parameters': 'error',
     '@typescript-eslint/no-unsafe-argument': 'error',
     '@typescript-eslint/no-unsafe-assignment': '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
@@ -34,6 +34,7 @@ export = {
     '@typescript-eslint/no-unnecessary-template-expression': 'off',
     '@typescript-eslint/no-unnecessary-type-arguments': 'off',
     '@typescript-eslint/no-unnecessary-type-assertion': 'off',
+    '@typescript-eslint/no-unnecessary-type-conversion': 'off',
     '@typescript-eslint/no-unnecessary-type-parameters': 'off',
     '@typescript-eslint/no-unsafe-argument': 'off',
     '@typescript-eslint/no-unsafe-assignment': 'off',

diff --git a/packages/eslint-plugin/src/configs/flat/all.ts b/packages/eslint-plugin/src/configs/flat/all.ts
@@ -112,6 +112,7 @@ export default (
       '@typescript-eslint/no-unnecessary-type-arguments': 'error',
       '@typescript-eslint/no-unnecessary-type-assertion': 'error',
       '@typescript-eslint/no-unnecessary-type-constraint': 'error',
+      '@typescript-eslint/no-unnecessary-type-conversion': 'error',
       '@typescript-eslint/no-unnecessary-type-parameters': 'error',
       '@typescript-eslint/no-unsafe-argument': 'error',
       '@typescript-eslint/no-unsafe-assignment': '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
@@ -41,6 +41,7 @@ export default (
     '@typescript-eslint/no-unnecessary-template-expression': 'off',
     '@typescript-eslint/no-unnecessary-type-arguments': 'off',
     '@typescript-eslint/no-unnecessary-type-assertion': 'off',
+    '@typescript-eslint/no-unnecessary-type-conversion': 'off',
     '@typescript-eslint/no-unnecessary-type-parameters': 'off',
     '@typescript-eslint/no-unsafe-argument': 'off',
     '@typescript-eslint/no-unsafe-assignment': 'off',

diff --git a/packages/eslint-plugin/src/rules/adjacent-overload-signatures.ts b/packages/eslint-plugin/src/rules/adjacent-overload-signatures.ts
@@ -79,7 +79,7 @@ export default createRule({
           return {
             ...getNameFromMember(member, context.sourceCode),
             callSignature: false,
-            static: !!member.static,
+            static: member.static,
           };
         case AST_NODE_TYPES.TSCallSignatureDeclaration:
           return {

diff --git a/packages/eslint-plugin/src/rules/explicit-function-return-type.ts b/packages/eslint-plugin/src/rules/explicit-function-return-type.ts
@@ -169,14 +169,14 @@ export default createRule<Options, MessageIds>({
             }
           }
         }
-        if (!!funcName && !!options.allowedNames.includes(funcName)) {
+        if (!!funcName && options.allowedNames.includes(funcName)) {
           return true;
         }
       }
       if (
         node.type === AST_NODE_TYPES.FunctionDeclaration &&
         node.id &&
-        !!options.allowedNames.includes(node.id.name)
+        options.allowedNames.includes(node.id.name)
       ) {
         return true;
       }

diff --git a/packages/eslint-plugin/src/rules/index.ts b/packages/eslint-plugin/src/rules/index.ts
@@ -75,6 +75,7 @@ import noUnnecessaryTemplateExpression from './no-unnecessary-template-expressio
 import noUnnecessaryTypeArguments from './no-unnecessary-type-arguments';
 import noUnnecessaryTypeAssertion from './no-unnecessary-type-assertion';
 import noUnnecessaryTypeConstraint from './no-unnecessary-type-constraint';
+import noUnnecessaryTypeConversion from './no-unnecessary-type-conversion';
 import noUnnecessaryTypeParameters from './no-unnecessary-type-parameters';
 import noUnsafeArgument from './no-unsafe-argument';
 import noUnsafeAssignment from './no-unsafe-assignment';
@@ -208,6 +209,7 @@ const rules = {
   'no-unnecessary-type-arguments': noUnnecessaryTypeArguments,
   'no-unnecessary-type-assertion': noUnnecessaryTypeAssertion,
   'no-unnecessary-type-constraint': noUnnecessaryTypeConstraint,
+  'no-unnecessary-type-conversion': noUnnecessaryTypeConversion,
   'no-unnecessary-type-parameters': noUnnecessaryTypeParameters,
   'no-unsafe-argument': noUnsafeArgument,
   'no-unsafe-assignment': noUnsafeAssignment,

diff --git a/packages/eslint-plugin/src/rules/member-ordering.ts b/packages/eslint-plugin/src/rules/member-ordering.ts
@@ -488,7 +488,7 @@ function isMemberOptional(node: Member): boolean {
     case AST_NODE_TYPES.PropertyDefinition:
     case AST_NODE_TYPES.TSAbstractMethodDefinition:
     case AST_NODE_TYPES.MethodDefinition:
-      return !!node.optional;
+      return
EED3
 node.optional;
   }
   return false;
 }

diff --git a/packages/eslint-plugin/src/rules/no-duplicate-enum-values.ts b/packages/eslint-plugin/src/rules/no-duplicate-enum-values.ts
@@ -58,9 +58,9 @@ export default createRule({
 
           let value: number | string | undefined;
           if (isStringLiteral(member.initializer)) {
-            value = String(member.initializer.value);
+            value = member.initializer.value;
           } else if (isNumberLiteral(member.initializer)) {
-            value = Number(member.initializer.value);
+            value = member.initializer.value;
           } else if (isStaticTemplateLiteral(member.initializer)) {
             value = member.initializer.quasis[0].value.cooked;
           }

diff --git a/packages/eslint-plugin/src/rules/no-empty-interface.ts b/packages/eslint-plugin/src/rules/no-empty-interface.ts
@@ -97,11 +97,10 @@ export default createRule<Options, MessageIds>({
               def => def.node.type === AST_NODE_TYPES.ClassDeclaration,
             );
 
-          const isInAmbientDeclaration = !!(
+          const isInAmbientDeclaration =
             isDefinitionFile(context.filename) &&
             scope.type === ScopeType.tsModule &&
-            scope.block.declare
-          );
+            scope.block.declare;
 
           const useAutoFix = !(
             isInAmbientDeclaration || mergedWithClassDeclaration

diff --git a/packages/eslint-plugin/src/rules/no-redundant-type-constituents.ts b/packages/eslint-plugin/src/rules/no-redundant-type-constituents.ts
@@ -172,7 +172,7 @@ function describeLiteralTypeNode(typeNode: TSESTree.TypeNode): string {
 }
 
 function isNodeInsideReturnType(node: TSESTree.TSUnionType): boolean {
-  return !!(
+  return (
     node.parent.type === AST_NODE_TYPES.TSTypeAnnotation &&
     isFunctionOrFunctionType(node.parent.parent)
   );

diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-template-expression.ts b/packages/eslint-plugin/src/rules/no-unnecessary-template-expression.ts
@@ -35,9 +35,7 @@ const evenNumOfBackslashesRegExp = /(?<!(?:[^\\]|^)(?:\\\\)*\\)/;
 // '\\\\$' <- true
 // '\\\\\\$' <- false
 function endsWithUnescapedDollarSign(str: string): boolean {
-  return new RegExp(`${String(evenNumOfBackslashesRegExp.source)}\\$$`).test(
-    str,
-  );
+  return new RegExp(`${evenNumOfBackslashesRegExp.source}\\$$`).test(str);
 }
 
 export default createRule<[], MessageId>({
@@ -316,10 +314,7 @@ export default createRule<[], MessageId>({
             // \${ -> \${
             // \\${ -> \\\${
             .replaceAll(
-              new RegExp(
-                `${String(evenNumOfBackslashesRegExp.source)}(\`|\\\${)`,
-                'g',
-              ),
+              new RegExp(`${evenNumOfBackslashesRegExp.source}(\`|\\\${)`, 'g'),
               '\\$1',
             );