Add no-let-undefined rule

src/rules/noLetUndefinedRule.ts

@@ -0,0 +1,57 @@
+/**
+ * @license
+ * Copyright 2017 Palantir Technologies, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import * as ts from "typescript";
+
+import * as Lint from "../index";
+
+export class Rule extends Lint.Rules.AbstractRule {
+    /* tslint:disable:object-literal-sort-keys */
+    public static metadata: Lint.IRuleMetadata = {
+        ruleName: "no-let-undefined",
+        description: "Forbids a 'let' statement to be initialized to 'undefined'.",
+        hasFix: true,
+        optionsDescription: "Not configurable.",
+        options: null,
+        optionExamples: ["true"],
+        type: "style",
+        typescriptOnly: false,
+    };
+    /* tslint:enable:object-literal-sort-keys */
+
+    public static FAILURE_STRING = "Unnecessary initialization to 'undefined'.";
+
+    public apply(sourceFile: ts.SourceFile): Lint.RuleFailure[] {
+        return this.applyWithWalker(new Walker(sourceFile, this.getOptions()));
+    }
+}
+
+class Walker extends Lint.RuleWalker {
+    public visitVariableDeclaration(node: ts.VariableDeclaration) {
+        if (Lint.isNodeFlagSet(node.parent!, ts.NodeFlags.Let) && isUndefined(node.initializer)) {
+            const fix = this.createFix(this.deleteFromTo(
+                Lint.childOfKind(node, ts.SyntaxKind.EqualsToken)!.pos,
+                node.end));
+            this.addFailureAtNode(node, Rule.FAILURE_STRING, fix);
+        }
+        super.visitVariableDeclaration(node);
+    }
+}
+
+function isUndefined(node: ts.Node | undefined): boolean {
+    return node !== undefined && node.kind === ts.SyntaxKind.Identifier && (node as ts.Identifier).text === "undefined";
+}

test/rules/no-let-undefined/test.js.lint

@@ -0,0 +1,2 @@
+let x = undefined;
+    ~~~~~~~~~~~~~ [Unnecessary initialization to 'undefined'.]

test/rules/no-let-undefined/test.ts.fix

@@ -0,0 +1,10 @@
+let x: string | undefined;
+
+for (let y: number | undefined; y < 2; y++) {}
+
+let z;
+
+const x = undefined;
+
+function f(x: string | undefined = undefined) {}
+

test/rules/no-let-undefined/test.ts.lint

@@ -0,0 +1,13 @@
+let x: string | undefined = undefined;
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [0]
+
+for (let y: number | undefined = undefined; y < 2; y++) {}
+         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [0]
+
+let z;
+
+const x = undefined;
+
+function f(x: string | undefined = undefined) {}
+
+[0]: Unnecessary initialization to 'undefined'.

test/rules/no-let-undefined/tslint.json

@@ -0,0 +1,8 @@
+{
+  "rules": {
+    "no-let-undefined": [true]
+  },
+  "jsRules": {
+    "no-let-undefined": [true]
+  }
+}