typescript-eslint · skondrashov · Oct 20, 2024 · Oct 20, 2024 · Nov 3, 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 +1,74 @@
+---
+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 has several idioms used to convert values to certain types. With TypeScript, it is possible to see at build time if the value is already of that type, making the conversion unnecessary.
+Performing unnecessary conversions increases visual clutter and harms code readability, so it's generally best practice to remove them if they don't change the type of an expression.
+This rule reports when a type conversion idiom is identified which does not change the runtime type of an expression.
+
+## 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 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',
             );