Suggest to replace indexOf(..., '') === 0 with _.startsWith. Fixes #11

lib/rules/prefer-startswith.js

@@ -0,0 +1,28 @@
+/**
+ * @fileoverview Rule to check if a call to _.indexOf === 0 should be a call to _.startsWith
+ */
+'use strict';
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = function (context) {
+    var lodashUtil = require('../util/lodashUtil');
+
+    function isExpectingZeroIndex(operator) {
+        if (operator && operator.type === 'BinaryExpression' && operator.right) {
+            return operator.right.value === 0;
+        }
+
+        return false;
+    }
+
+    return {
+        CallExpression: function (node) {
+            if (lodashUtil.isCallToMethod(node, 'indexOf') && isExpectingZeroIndex(node.parent)) {
+                context.report(node, 'Prefer _.startsWith instead of _.indexOf(...,) === 0');
+            }
+        }
+    };
+};

tests/lib/rules/prefer-startswith.js

@@ -0,0 +1,34 @@
+'use strict';
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+var rule = require('../../../lib/rules/prefer-startswith');
+var RuleTester = require('eslint').RuleTester;
+
+// ------------------------------------------------------------------------------
+// Tests
+// ------------------------------------------------------------------------------
+
+var ruleTester = new RuleTester();
+var toErrorObject = require('../testUtil/toErrorObject')
+    .fromMessage('Prefer _.startsWith instead of _.indexOf(...,) === 0');
+
+ruleTester.run('prefer-startswith', rule, {
+    valid: [
+        '_.indexOf(a, b) == 10',
+        '_.indexOf(a, b) === 10',
+        '_.indexOf(a, b) === -1',
+        'if (_.indexOf(a.b.c, b.c)) {}'
+    ],
+    invalid: [
+        '_.indexOf(a, b) === 0',
+        '_.indexOf(a.b.c, "b") == 0',
+        '_.indexOf(a, b) !== 0',
+        '_.indexOf(a.b.c.d, "b") != 0',
+        '_.indexOf(a.b, b.c) === 0',
+        '_.indexOf(a.b.c, b.c) !== 0',
+        'if (_.indexOf(a.b.c, b.c) === 000) {}'
+    ].map(toErrorObject)
+});