Eslint check for mocking node core modules

.eslintrc.yaml

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+---
+root: true
+parser: "@typescript-eslint/parser"
+plugins:
+  - "@typescript-eslint"
+  - zowe-explorer
+env:
+  node: true
+  es6: true
+rules:
+  zowe-explorer/no-unmocked-core-modules:
+    - error
+    - coreModuleNames:
+        - fs
+      filePathPattern: ".*\\.unit\\.test\\..*"
+parserOptions:
+  sourceType: module

eslint-plugin/README.md

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+# eslint-plugin-zowe-explorer
+
+Custom ESLint Rules for ZOWE Explorer
+
+## Installation
+
+You'll first need to install [ESLint](http://eslint.org):
+
+```
+$ npm i eslint --save-dev
+```
+
+Next, install `eslint-plugin-zowe-explorer`:
+
+```
+$ npm install eslint-plugin-zowe-explorer --save-dev
+```
+
+**Note:** If you installed ESLint globally (using the `-g` flag) then you must also install `eslint-plugin-zowe-explorer` globally.
+
+## Usage
+
+Add `zowe-explorer` to the plugins section of your `.eslintrc` configuration file. You can omit the `eslint-plugin-` prefix:
+
+```json
+{
+  "plugins": ["zowe-explorer"]
+}
+```
+
+Then configure the rules you want to use under the rules section.
+For example to disallow the use of unmocked `fs` module in unit tests
+(whose file name contains `.unit.test.`) specify this rule config
+
+```json
+{
+  "rules": {
+    "zowe-explorer/no-unmocked-core-modules": [
+      "error",
+      { "coreModuleNames": ["fs"], "filePathPattern": ".*\\.unit\\.test\\..*" }
+    ]
+  }
+}
+```
+
+## Supported Rules
+
+- no-unmocked-core-modules

eslint-plugin/docs/rules/no-unmocked-core-modules.md

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+# Disallow the use of unmocked node core module (no-unmocked-core-modules)
+
+Node modules are [automatically mocked by jest](https://jestjs.io/docs/en/manual-mocks#mocking-node-modules). That means that if a file exists in the `__mocks__` directory next to `node_modules`, the mock will be returned instead of the actual module on a `require/import`.
+
+However, node core modules (fs, path, ...) are not automatically mocked and an explicit
+
+```js
+jest.mock("fs");
+```
+
+is required to get a mock instead of the actual module.
+
+## Rule details
+
+If your unit tests mock or modify some of the core modules, you may want to make sure `jest.mock` is called for them so that the actual core module is not modified.
+
+This rule will report an error if a module is imported and `jest.mock` is not called for it.
+
+## Options
+
+- `coreModuleNames` an array with the names of core modules that should be always mocked
+- `filePathPattern` of the unit tests for which this rule should be mandeted as a string, for example `".*\\.unit\\.test\\..*"`

eslint-plugin/jest.config.js

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+module.exports = { testMatch: ["**/tests/**/*.js"] };

eslint-plugin/lib/index.js

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+/**
+ * @fileoverview Custom ESLint Rules for ZOWE Explorer
+ */
+"use strict";
+
+var requireIndex = require("requireindex");
+
+//------------------------------------------------------------------------------
+// Plugin Definition
+//------------------------------------------------------------------------------
+
+// import all rules in lib/rules
+module.exports.rules = requireIndex(__dirname + "/rules");

eslint-plugin/lib/rules/no-unmocked-core-modules.js

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+const isJestMockOf = (name) => (node) => {
+  return (
+    node.type === "ExpressionStatement" &&
+    node.expression.type === "CallExpression" &&
+    node.expression.callee.type === "MemberExpression" &&
+    node.expression.callee.object.type === "Identifier" &&
+    node.expression.callee.object.name === "jest" &&
+    node.expression.callee.property.type === "Identifier" &&
+    node.expression.callee.property.name === "mock" &&
+    node.expression.arguments.length === 1 &&
+    node.expression.arguments[0].type === "Literal" &&
+    node.expression.arguments[0].value === name
+  );
+};
+
+module.exports = {
+  meta: {
+    type: "problem",
+    docs: {
+      description:
+        "disallow the use of unmocked node core modules in unit tests",
+      category: "Unit Tests",
+      recommended: true,
+      url: "",
+    },
+    schema: [
+      {
+        type: "object",
+        properties: {
+          coreModuleNames: {
+            type: "array",
+            items: { type: "string" },
+            default: ["fs"],
+          },
+          filePathPattern: {
+            type: "string",
+            default: ".*\\.test\\..*",
+          },
+        },
+        additionalProperties: false,
+      },
+    ],
+    fixable: "code",
+  },
+  create: (context) => {
+    const {
+      coreModuleNames = ["fs"],
+      filePathPattern = /.*\.ttest\..*/,
+    } = context.options[0];
+    const fileName = context.getFilename();
+    if (!fileName.match(RegExp(filePathPattern))) {
+      // file does not match pattern
+      return {}; // no linting
+    }
+
+    return {
+      ImportDeclaration(node) {
+        const moduleName = node.source.value;
+        if (
+          coreModuleNames.includes(moduleName) && // imports node core module
+          node.parent.type === "Program" &&
+          !node.parent.body.some(isJestMockOf(moduleName)) // does not mock it
+        ) {
+          context.report({
+            node,
+            message: `Use jest.mock('${moduleName}') to mock a node core module`,
+            fix: (fixer) =>
+              fixer.insertTextAfter(
+                node,
+                `
+jest.mock('${moduleName}');`
+              ),
+          });
+        }
+      },
+    };
+  },
+};