Add no-sparse-arrays rule

src/rules/noSparseArraysRule.ts

@@ -0,0 +1,91 @@
+/**
+ * @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 utils from "tsutils";
+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-sparse-arrays",
+        description: "Forbids array literals to contain missing elements.",
+        rationale: "Missing elements are probably an accidentally duplicated comma.",
+        optionsDescription: "Not configurable.",
+        options: null,
+        optionExamples: ["true"],
+        type: "functionality",
+        typescriptOnly: false,
+    };
+    /* tslint:enable:object-literal-sort-keys */
+
+    public static FAILURE_STRING = "Array has a missing element.";
+
+    public apply(sourceFile: ts.SourceFile): Lint.RuleFailure[] {
+        return this.applyWithFunction(sourceFile, walk);
+    }
+}
+
+function walk(ctx: Lint.WalkContext<void>): void {
+    return ts.forEachChild(ctx.sourceFile, function cb(node: ts.Node): void {
+        if (!utils.isArrayLiteralExpression(node)) {
+            if (utils.isBinaryExpression(node) && node.operatorToken.kind === ts.SyntaxKind.EqualsToken) {
+                // Ignore LHS of assignments.
+                traverseExpressionsInLHS(node.left, cb);
+                return cb(node.right);
+            } else {
+                return ts.forEachChild(node, cb);
+            }
+        }
+
+        for (const element of node.elements) {
+            if (utils.isOmittedExpression(element)) {
+                // Node has an empty range, so just use range starting at `element.pos`.
+                ctx.addFailureAt(element.pos, 1, Rule.FAILURE_STRING);
+            } else {
+                ts.forEachChild(element, cb);
+            }
+        }
+    });
+}
+
+/** Traverse the LHS of an `=` expression, calling `cb` embedded default value, but ignoring binding patterns. */
+function traverseExpressionsInLHS(node: ts.Node, cb: (node: ts.Expression) => void): void {
+    switch (node.kind) {
+        case ts.SyntaxKind.ArrayLiteralExpression:
+            for (const e of (node as ts.ArrayLiteralExpression).elements) {
+                traverseExpressionsInLHS(e, cb);
+            }
+            break;
+
+        case ts.SyntaxKind.ObjectLiteralExpression:
+            for (const o of (node as ts.ObjectLiteralExpression).properties) {
+                traverseExpressionsInLHS(o, cb);
+            }
+            break;
+
+        case ts.SyntaxKind.BinaryExpression: {
+            const { left, operatorToken, right } = node as ts.BinaryExpression;
+            if (operatorToken.kind === ts.SyntaxKind.EqualsToken) {
+                traverseExpressionsInLHS(left, cb);
+                cb(right);
+            }
+            break;
+        }
+    }
+}

test/rules/no-sparse-arrays/test.js.lint

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+[1, , 3];
+   ~ [Array has a missing element.]

test/rules/no-sparse-arrays/test.ts.lint

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+[1, , 3];
+   ~ [0]
+
+// Destructuring allowed.
+const [foo, , bar] = [];
+const [{foo: [, bar]}] = [];
+const [foo = [, 1]] = [];
+              ~ [0]
+
+[foo, , bar] = [];
+[{foo: [, bar]}] = [];
+[foo = [, 1]] = [];
+        ~ [0]
+
+[1,,];
+   ~ [0]
+
+[1,,,];
+   ~ [0]
+    ~ [0]
+
+[0]: Array has a missing element.

test/rules/no-sparse-arrays/tslint.json

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+{
+  "rules": {
+    "no-sparse-arrays": true
+  },
+  "jsRules": {
+    "no-sparse-arrays": true
+  }
+}