tools: decrease code duplication for isString() in lint rules

This commit makes isString() a reusable utility
function for core's custom ESLint rules.

PR-URL: #27719
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

tools/eslint-rules/no-duplicate-requires.js

@@ -4,17 +4,13 @@
  */
 'use strict';
 
-const { isRequireCall } = require('./rules-utils.js');
+const { isRequireCall, isString } = require('./rules-utils.js');
 
 //------------------------------------------------------------------------------
 // Rule Definition
 //------------------------------------------------------------------------------
 
 
-function isString(node) {
-  return node && node.type === 'Literal' && typeof node.value === 'string';
-}
-
 function isTopLevel(node) {
   do {
     if (node.type === 'FunctionDeclaration' ||

tools/eslint-rules/require-common-first.js

@@ -4,7 +4,7 @@
 'use strict';
 
 const path = require('path');
-const { isRequireCall } = require('./rules-utils.js');
+const { isRequireCall, isString } = require('./rules-utils.js');
 
 //------------------------------------------------------------------------------
 // Rule Definition
@@ -15,15 +15,6 @@ module.exports = function(context) {
   const isESM = context.parserOptions.sourceType === 'module';
   const foundModules = [];
 
-  /**
-   * Function to check if a node is a string literal.
-   * @param {ASTNode} node The node to check.
-   * @returns {boolean} If the node is a string literal.
-   */
-  function isString(node) {
-    return node && node.type === 'Literal' && typeof node.value === 'string';
-  }
-
   /**
    * Function to check if the path is a module and return its name.
    * @param {String} str The path to check

tools/eslint-rules/required-modules.js

@@ -4,7 +4,7 @@
  */
 'use strict';
 
-const { isRequireCall } = require('./rules-utils.js');
+const { isRequireCall, isString } = require('./rules-utils.js');
 
 //------------------------------------------------------------------------------
 // Rule Definition
@@ -25,15 +25,6 @@ module.exports = function(context) {
     return {};
   }
 
-  /**
-   * Function to check if a node is a string literal.
-   * @param {ASTNode} node The node to check.
-   * @returns {boolean} If the node is a string literal.
-   */
-  function isString(node) {
-    return node && node.type === 'Literal' && typeof node.value === 'string';
-  }
-
   /**
    * Function to check if the path is a required module and return its name.
    * @param {String} str The path to check

tools/eslint-rules/rules-utils.js

@@ -8,6 +8,10 @@ function isRequireCall(node) {
 }
 module.exports.isRequireCall = isRequireCall;
 
+module.exports.isString = function(node) {
+  return node && node.type === 'Literal' && typeof node.value === 'string';
+};
+
 module.exports.isDefiningError = function(node) {
   return node.expression &&
          node.expression.type === 'CallExpression' &&