typescript-eslint · JoshuaKGoldberg · Dec 12, 2023 · Nov 19, 2023 · Nov 19, 2023 · Nov 20, 2023
diff --git a/packages/eslint-plugin/docs/rules/no-useless-template-literals.md b/packages/eslint-plugin/docs/rules/no-useless-template-literals.md
@@ -0,0 +1,53 @@
+---
+description: 'Disallow unnecessary template literals.'
+---
+
+> 🛑 This file is source code, not the primary documentation location! 🛑
+>
+> See **https://typescript-eslint.io/rules/no-useless-template-literals** for documentation.
+
+This rule reports template literals that can be simplified to a normal string literal.
+
+## Examples
+
+<!--tabs-->
+
+### ❌ Incorrect
+
+```ts
+const ab1 = `${'a'}${'b'}`;
+const ab2 = `a${'b'}`;
+
+const stringWithNumber = `1 + 1 = ${2}`;
+
+const stringWithBoolean = `${'true is '}${true}`;
+
+const string = 'a';
+const wrappedString = `${string}`;
+
+declare const intersectionWithString: string & { _brand: 'test-brand' };
+const wrappedIntersection = `${intersectionWithString}`;
+```
+
+### ✅ Correct
+
+```ts
+const ab1 = 'ab';
+const ab2 = 'ab';
+
+const stringWithNumber = '1 + 1 = 2';
+
+const stringWithBoolean = 'true is true';
+
+const string = 'a';
+const wrappedString = string;
+
+declare const intersectionWithString: string & { _brand: 'test-brand' };
+const wrappedIntersection = intersectionWithString;
+```
+
+<!--/tabs-->
+
+## Related To
+
+- [`restrict-template-expressions`](./restrict-template-expressions.md)
diff --git a/packages/eslint-plugin/src/configs/all.ts b/packages/eslint-plugin/src/configs/all.ts
@@ -135,6 +135,7 @@ export = {
     'no-useless-constructor': 'off',
     '@typescript-eslint/no-useless-constructor': 'error',
     '@typescript-eslint/no-useless-empty-export': 'error',
+    '@typescript-eslint/no-useless-template-literals': 'error',
     '@typescript-eslint/no-var-requires': 'error',
     '@typescript-eslint/non-nullable-type-assertion-style': 'error',
     'object-curly-spacing': 'off',

diff --git a/packages/eslint-plugin/src/configs/disable-type-checked.ts b/packages/eslint-plugin/src/configs/disable-type-checked.ts
@@ -34,6 +34,7 @@ export = {
     '@typescript-eslint/no-unsafe-enum-comparison': 'off',
     '@typescript-eslint/no-unsafe-member-access': 'off',
     '@typescript-eslint/no-unsafe-return': 'off',
+    '@typescript-eslint/no-useless-template-literals': 'off',
     '@typescript-eslint/non-nullable-type-assertion-style': 'off',
     '@typescript-eslint/prefer-includes': 'off',
     '@typescript-eslint/prefer-nullish-coalescing': 'off',

diff --git a/packages/eslint-plugin/src/configs/recommended-type-checked.ts b/packages/eslint-plugin/src/configs/recommended-type-checked.ts
@@ -41,6 +41,7 @@ export = {
     '@typescript-eslint/no-unsafe-return': 'error',
     'no-unused-vars': 'off',
     '@typescript-eslint/no-unused-vars': 'error',
+    '@typescript-eslint/no-useless-template-literals': 'error',
     '@typescript-eslint/no-var-requires': 'error',
     '@typescript-eslint/prefer-as-const': 'error',
     'require-await': 'off',

diff --git a/packages/eslint-plugin/src/configs/strict-type-checked.ts b/packages/eslint-plugin/src/configs/strict-type-checked.ts
@@ -56,6 +56,7 @@ export = {
     '@typescript-eslint/no-unused-vars': 'error',
     'no-useless-constructor': 'off',
     '@typescript-eslint/no-useless-constructor': 'error',
+    '@typescript-eslint/no-useless-template-literals': 'error',
     '@typescript-eslint/no-var-requires': 'error',
     '@typescript-eslint/prefer-as-const': 'error',
     '@typescript-eslint/prefer-includes': 'error',

diff --git a/packages/eslint-plugin/src/rules/index.ts b/packages/eslint-plugin/src/rules/index.ts
@@ -93,6 +93,7 @@ import noUnusedVars from './no-unused-vars';
 import noUseBeforeDefine from './no-use-before-define';
 import noUselessConstructor from './no-useless-constructor';
 import noUselessEmptyExport from './no-useless-empty-export';
+import noUselessTemplateLiterals from './no-useless-template-literals';
 import noVarRequires from './no-var-requires';
 import nonNullableTypeAssertionStyle from './non-nullable-type-assertion-style';
 import objectCurlySpacing from './object-curly-spacing';
@@ -231,6 +232,7 @@ export default {
   'no-use-before-define': noUseBeforeDefine,
   'no-useless-constructor': noUselessConstructor,
   'no-useless-empty-export': noUselessEmptyExport,
+  'no-useless-template-literals': noUselessTemplateLiterals,
   'no-var-requires': noVarRequires,
   'non-nullable-type-assertion-style': nonNullableTypeAssertionStyle,
   'object-curly-spacing': objectCurlySpacing,

diff --git a/packages/eslint-plugin/src/rules/member-ordering.ts b/packages/eslint-plugin/src/rules/member-ordering.ts
@@ -396,8 +396,8 @@ function getNodeType(node: Member): MemberKind | null {
       return node.value && functionExpressions.includes(node.value.type)
         ? 'method'
         : node.readonly
-          ? 'readonly-field'
-          : 'field';
+        ? 'readonly-field'
+        : 'field';
     case AST_NODE_TYPES.TSPropertySignature:
       return node.readonly ? 'readonly-field' : 'field';
     case AST_NODE_TYPES.TSIndexSignature:
@@ -555,8 +555,8 @@ function getRank(
     'static' in node && node.static
       ? 'static'
       : abstract
-        ? 'abstract'
-        : 'instance';
+      ? 'abstract'
+      : 'instance';
   const accessibility = getAccessibility(node);
 
   // Collect all existing member groups that apply to this node...
@@ -578,7 +578,7 @@ function getRank(
 
       if (type === 'readonly-field') {
         memberGroups.push(`${accessibility}-decorated-field`);
-        memberGroups.push(`decorated-field`);
+        memberGroups.push('decorated-field');
       }
     }
 
@@ -666,7 +666,8 @@ export default createRule<Options, MessageIds>({
         'Member {{member}} should be declared before member {{beforeMember}}.',
       incorrectGroupOrder:
         'Member {{name}} should be declared before all {{rank}} definitions.',
-      incorrectRequiredMembersOrder: `Member {{member}} should be declared after all {{optionalOrRequired}} members.`,
+      incorrectRequiredMembersOrder:
+        'Member {{member}} should be declared after all {{optionalOrRequired}} members.',
     },
     schema: [
       {

diff --git a/packages/eslint-plugin/src/rules/no-mixed-enums.ts b/packages/eslint-plugin/src/rules/no-mixed-enums.ts
@@ -23,7 +23,7 @@ export default createRule({
       requiresTypeChecking: true,
     },
     messages: {
-      mixed: `Mixing number and string enums can be confusing.`,
+      mixed: 'Mixing number and string enums can be confusing.',
     },
     schema: [],
     type: 'problem',

diff --git a/packages/eslint-plugin/src/rules/no-redundant-type-constituents.ts b/packages/eslint-plugin/src/rules/no-redundant-type-constituents.ts
@@ -197,8 +197,10 @@ export default createRule({
       requiresTypeChecking: true,
     },
     messages: {
-      literalOverridden: `{{literal}} is overridden by {{primitive}} in this union type.`,
-      primitiveOverridden: `{{primitive}} is overridden by the {{literal}} in this intersection type.`,
+      literalOverridden:
+        '{{literal}} is overridden by {{primitive}} in this union type.',
+      primitiveOverridden:
+        '{{primitive}} is overridden by the {{literal}} in this intersection type.',
       overridden: `'{{typeName}}' is overridden by other types in this {{container}} type.`,
       overrides: `'{{typeName}}' overrides all other types in this {{container}} type.`,
     },

diff --git a/packages/eslint-plugin/src/rules/no-useless-template-literals.ts b/packages/eslint-plugin/src/rules/no-useless-template-literals.ts
@@ -0,0 +1,97 @@
+import type { TSESTree } from '@typescript-eslint/utils';
+import { AST_NODE_TYPES } from '@typescript-eslint/utils';
+import * as ts from 'typescript';
+
+import {
+  createRule,
+  getConstrainedTypeAtLocation,
+  getParserServices,
+  isTypeFlagSet,
+} from '../util';
+
+type MessageId = 'noUselessTemplateLiteral';
+
+export default createRule<[], MessageId>({
+  name: 'no-useless-template-literals',
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'Disallow unnecessary template literals',
+      recommended: 'recommended',
+      requiresTypeChecking: true,
+    },
+    messages: {
+      noUselessTemplateLiteral:
+        'Template literal expression is unnecessary and can be simplified.',
+    },
+    schema: [],
+  },
+  defaultOptions: [],
+  create(context) {
+    const services = getParserServices(context);
+
+    function isUnderlyingTypeString(expression: TSESTree.Expression): boolean {
+      const type = getConstrainedTypeAtLocation(services, expression);
+
+      const isString = (t: ts.Type): boolean => {
+        return isTypeFlagSet(t, ts.TypeFlags.StringLike);
+      };
+
+      if (type.isUnion()) {
+        return type.types.every(isString);
+      }
+
+      if (type.isIntersection()) {
+        return type.types.some(isString);
+      }
+
+      return isString(type);
+    }
+
+    return {
+      TemplateLiteral(node: TSESTree.TemplateLiteral): void {
+        if (node.parent.type === AST_NODE_TYPES.TaggedTemplateExpression) {
+          return;
+        }
+
+        const hasSingleStringVariable =
+          node.quasis.length === 2 &&
+          node.quasis[0].value.raw === '' &&
+          node.quasis[1].value.raw === '' &&
+          node.expressions.length === 1 &&
+          node.expressions[0].type === AST_NODE_TYPES.Identifier &&
+          isUnderlyingTypeString(node.expressions[0]);
+
+        if (hasSingleStringVariable) {
+          context.report({
+            node,
+            messageId: 'noUselessTemplateLiteral',
+          });
+
+          return;
+        }
+
+        const allowedChars = ['\r', '\n', "'", '"'];
+
+        const hasStringWithAllowedChars = node.quasis.some(quasi => {
+          return new RegExp(`[${allowedChars.join('')}]`).test(quasi.value.raw);
+        });
+
+        if (hasStringWithAllowedChars) {
+          return;
+        }
+
+        const allAreLiterals = node.expressions.every(expression => {
+          return expression.type === AST_NODE_TYPES.Literal;
+        });
+
+        if (allAreLiterals) {
+          context.report({
+            node,
+            messageId: 'noUselessTemplateLiteral',
+          });
+        }
+      },
+    };
+  },
+});
diff --git a/packages/eslint-plugin/src/rules/prefer-literal-enum-member.ts b/packages/eslint-plugin/src/rules/prefer-literal-enum-member.ts
@@ -12,7 +12,8 @@ export default createRule({
       requiresTypeChecking: false,
     },
     messages: {
-      notLiteral: `Explicit enum value must only be a literal value (string, number, boolean, etc).`,
+      notLiteral:
+        'Explicit enum value must only be a literal value (string, number, boolean, etc).',
     },
     schema: [
       {

diff --git a/packages/eslint-plugin/tests/docs.test.ts b/packages/eslint-plugin/tests/docs.test.ts
@@ -82,16 +82,16 @@ describe('Validating rule docs', () => {
       test(`${ruleName}.md must next have a blockquote directing to website`, () => {
         expect(tokens[2]).toMatchObject({
           text: [
-            `🛑 This file is source code, not the primary documentation location! 🛑`,
-            ``,
+            '🛑 This file is source code, not the primary documentation location! 🛑',
+            '',
             `See **https://typescript-eslint.io/rules/${ruleName}** for documentation.`,
-            ``,
+            '',
           ].join('\n'),
           type: 'blockquote',
         });
       });
 
-      test(`headers must be title-cased`, () => {
+      test('headers must be title-cased', () => {
         // Get all H2 headers objects as the other levels are variable by design.
         const headers = tokens.filter(tokenIsH2);
 
@@ -130,7 +130,7 @@ describe('Validating rule metadata', () => {
   }
 
   for (const [ruleName, rule] of rulesData) {
-    describe(`${ruleName}`, () => {
+    describe(ruleName, () => {
       it('`name` field in rule must match the filename', () => {
         // validate if rule name is same as url
         // there is no way to access this field but its used only in generation of docs url

diff --git a/packages/eslint-plugin/tests/rules/array-type.test.ts b/packages/eslint-plugin/tests/rules/array-type.test.ts
@@ -2162,14 +2162,14 @@ describe('schema validation', () => {
   // https://github.com/typescript-eslint/typescript-eslint/issues/6852
   test("array-type does not accept 'simple-array' option", () => {
     if (areOptionsValid(rule, [{ default: 'simple-array' }])) {
-      throw new Error(`Options succeeded validation for bad options`);
+      throw new Error('Options succeeded validation for bad options');
     }
   });
 
   // https://github.com/typescript-eslint/typescript-eslint/issues/6892
   test('array-type does not accept non object option', () => {
     if (areOptionsValid(rule, ['array'])) {
-      throw new Error(`Options succeeded validation for bad options`);
+      throw new Error('Options succeeded validation for bad options');
     }
   });
 });
diff --git a/packages/eslint-plugin/tests/rules/ban-types.test.ts b/packages/eslint-plugin/tests/rules/ban-types.test.ts
@@ -504,7 +504,7 @@ let baz: object = {};
     },
     {
       code: noFormat`let a: Foo<   F   >;`,
-      output: `let a: Foo<   T   >;`,
+      output: 'let a: Foo<   T   >;',
       errors: [
         {
           messageId: 'bannedTypeMessage',

diff --git a/packages/eslint-plugin/tests/rules/comma-spacing.test.ts b/packages/eslint-plugin/tests/rules/comma-spacing.test.ts
@@ -439,7 +439,7 @@ ruleTester.run('comma-spacing', rule, {
           data: { loc: 'before' },
         },
         {
-          messageId: `missing`,
+          messageId: 'missing',
           column: 16,
           line: 1,
           data: { loc: 'after' },