docs: Custom rule & plugin tutorial

docs/_examples/custom-rule-tutorial-code/.gitignore

@@ -0,0 +1 @@
+node_modules

docs/_examples/custom-rule-tutorial-code/enforce-foo-bar.js

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+/**
+ * @fileoverview Rule to enforce that `const foo` is assigned "bar".
+ * @author Ben Perlmutter
+ */
+
+"use strict";
+
+// The enforce-foo-bar rule definition
+module.exports = {
+    meta: {
+        type: "problem",
+        docs: {
+            description: "Enforce that a variable named `foo` can only be assigned a value of 'bar'."
+        },
+        fixable: "code",
+        schema: []
+    },
+    create(context) {
+        return {
+
+            // Performs action in the function on every variable declarator
+            VariableDeclarator(node) {
+
+                // Check if a `const` variable declaration
+                if (node.parent.kind === "const") {
+
+                    // Check if variable name is `foo`
+                    if (node.id.type === "Identifier" && node.id.name === "foo") {
+
+                        // Check if value of variable is "bar"
+                        if (node.init && node.init.type === "Literal" && node.init.value !== "bar") {
+
+                            /*
+                             * Report error to ESLint. Error message uses
+                             * a message placeholder to include the incorrect value
+                             * in the error message.
+                             * Also includes a `fix(fixer)` function that replaces
+                             * any values assigned to `const foo` with "bar".
+                             */
+                            context.report({
+                                node,
+                                message: 'Value other than "bar" assigned to `const foo`. Unexpected value: {{ notBar }}.',
+                                data: {
+                                    notBar: node.init.value
+                                },
+                                fix(fixer) {
+                                    return fixer.replaceText(node.init, '"bar"');
+                                }
+                            });
+                        }
+                    }
+                }
+            }
+        };
+    }
+};
+

docs/_examples/custom-rule-tutorial-code/enforce-foo-bar.test.js

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+/** 
+ * @fileoverview Tests for enforce-foo-bar.js rule.
+ * @author Ben Perlmutter
+*/
+"use strict";
+
+const {RuleTester} = require("eslint");
+const fooBarRule = require("./enforce-foo-bar");
+
+const ruleTester = new RuleTester({
+  // Must use at least ecmaVersion 2015 because
+  // that's when `const` variable were introduced.
+  parserOptions: { ecmaVersion: 2015 }
+});
+
+// Throws error if the tests in ruleTester.run() do not pass
+ruleTester.run(
+  "enforce-foo-bar", // rule name
+  fooBarRule, // rule code
+  { // checks
+    // 'valid' checks cases that should pass
+    valid: [{
+      code: "const foo = 'bar';",
+    }],
+    // 'invalid' checks cases that should not pass
+    invalid: [{
+      code: "const foo = 'baz';",
+      output: 'const foo = "bar";',
+      errors: 1,
+    }],
+  }
+);
+
+console.log("All tests passed!");

docs/_examples/custom-rule-tutorial-code/eslint-plugin-example.js

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+/** 
+ * @fileoverview Example an ESLint plugin with a custom rule.
+ * @author Ben Perlmutter
+*/
+"use strict";
+
+const fooBarRule = require("./enforce-foo-bar");
+const plugin = { rules: { "enforce-foo-bar": fooBarRule } };
+module.exports = plugin;

docs/_examples/custom-rule-tutorial-code/eslint.config.js

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+/** 
+ * @fileoverview Example ESLint config file that uses the custom rule from this tutorial.
+ * @author Ben Perlmutter
+*/
+"use strict";
+
+// Import the ESLint plugin
+const eslintPluginExample = require("./eslint-plugin-example");
+
+module.exports = [
+    {
+        files: ["**/*.js"],
+        languageOptions: {
+            sourceType: "commonjs",
+            ecmaVersion: "latest",
+        },
+        // Using the eslint-plugin-example plugin defined locally
+        plugins: {"example": eslintPluginExample},
+        rules: {
+            "example/enforce-foo-bar": "error",
+        },
+    }
+]

docs/_examples/custom-rule-tutorial-code/example.js

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+/** 
+ * @fileoverview Example of a file that will fail the custom rule in this tutorial.
+ * @author Ben Perlmutter
+*/
+"use strict";
+
+/* eslint-disable no-unused-vars -- Disable other rule causing problem for this file */
+
+// To see the error in the terminal, run the following command:
+// npx eslint example.js
+
+// To fix the error, run the following command:
+// npx eslint example.js --fix
+
+function correctFooBar() {
+  const foo = "bar";
+}
+
+function incorrectFoo(){
+  const foo = "baz"; // Problem!
+}
+

docs/_examples/custom-rule-tutorial-code/package.json

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+{
+  "name": "eslint-plugin-example",
+  "version": "1.0.0",
+  "description": "ESLint plugin for enforce-foo-bar rule.",
+  "main": "eslint-plugin-example.js",
+  "keywords": [
+    "eslint",
+    "eslintplugin",
+    "eslint-plugin"
+  ],
+  "peerDependencies": {
+    "eslint": ">=8.0.0"
+  },
+  "scripts": {
+    "test": "node enforce-foo-bar.test.js"
+  },
+  "author": "",
+  "license": "ISC",
+  "devDependencies": {
+    "eslint": "^8.36.0"
+  }
+}

docs/src/extend/custom-processors.md

@@ -4,7 +4,7 @@ eleventyNavigation:
     key: custom processors
     parent: create plugins
     title: Custom Processors
-    order: 2
+    order: 3
 
 ---