--- /dev/null
+/**
+ * @fileoverview Restrict usage of duplicate imports.
+ * @author Simen Bekkhus
+ */
+"use strict";
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+/**
+ * Returns the name of the module imported or re-exported.
+ * @param {ASTNode} node A node to get.
+ * @returns {string} the name of the module, or empty string if no name.
+ */
+function getValue(node) {
+ if (node && node.source && node.source.value) {
+ return node.source.value.trim();
+ }
+
+ return "";
+}
+
+/**
+ * Checks if the name of the import or export exists in the given array, and reports if so.
+ * @param {RuleContext} context The ESLint rule context object.
+ * @param {ASTNode} node A node to get.
+ * @param {string} value The name of the imported or exported module.
+ * @param {string[]} array The array containing other imports or exports in the file.
+ * @param {string} messageId A messageId to be reported after the name of the module
+ *
+ * @returns {void} No return value
+ */
+function checkAndReport(context, node, value, array, messageId) {
+ if (array.indexOf(value) !== -1) {
+ context.report({
+ node,
+ messageId,
+ data: {
+ module: value
+ }
+ });
+ }
+}
+
+/**
+ * @callback nodeCallback
+ * @param {ASTNode} node A node to handle.
+ */
+
+/**
+ * Returns a function handling the imports of a given file
+ * @param {RuleContext} context The ESLint rule context object.
+ * @param {boolean} includeExports Whether or not to check for exports in addition to imports.
+ * @param {string[]} importsInFile The array containing other imports in the file.
+ * @param {string[]} exportsInFile The array containing other exports in the file.
+ *
+ * @returns {nodeCallback} A function passed to ESLint to handle the statement.
+ */
+function handleImports(context, includeExports, importsInFile, exportsInFile) {
+ return function(node) {
+ const value = getValue(node);
+
+ if (value) {
+ checkAndReport(context, node, value, importsInFile, "import");
+
+ if (includeExports) {
+ checkAndReport(context, node, value, exportsInFile, "importAs");
+ }
+
+ importsInFile.push(value);
+ }
+ };
+}
+
+/**
+ * Returns a function handling the exports of a given file
+ * @param {RuleContext} context The ESLint rule context object.
+ * @param {string[]} importsInFile The array containing other imports in the file.
+ * @param {string[]} exportsInFile The array containing other exports in the file.
+ *
+ * @returns {nodeCallback} A function passed to ESLint to handle the statement.
+ */
+function handleExports(context, importsInFile, exportsInFile) {
+ return function(node) {
+ const value = getValue(node);
+
+ if (value) {
+ checkAndReport(context, node, value, exportsInFile, "export");
+ checkAndReport(context, node, value, importsInFile, "exportAs");
+
+ exportsInFile.push(value);
+ }
+ };
+}
+
+module.exports = {
+ meta: {
+ type: "problem",
+
+ docs: {
+ description: "disallow duplicate module imports",
+ category: "ECMAScript 6",
+ recommended: false,
+ url: "https://eslint.org/docs/rules/no-duplicate-imports"
+ },
+
+ schema: [{
+ type: "object",
+ properties: {
+ includeExports: {
+ type: "boolean",
+ default: false
+ }
+ },
+ additionalProperties: false
+ }],
+ messages: {
+ import: "'{{module}}' import is duplicated.",
+ importAs: "'{{module}}' import is duplicated as export.",
+ export: "'{{module}}' export is duplicated.",
+ exportAs: "'{{module}}' export is duplicated as import."
+ }
+ },
+
+ create(context) {
+ const includeExports = (context.options[0] || {}).includeExports,
+ importsInFile = [],
+ exportsInFile = [];
+
+ const handlers = {
+ ImportDeclaration: handleImports(context, includeExports, importsInFile, exportsInFile)
+ };
+
+ if (includeExports) {
+ handlers.ExportNamedDeclaration = handleExports(context, importsInFile, exportsInFile);
+ handlers.ExportAllDeclaration = handleExports(context, importsInFile, exportsInFile);
+ }
+
+ return handlers;
+ }
+};