add an example of using dollar eq

javascript/ql/src/Security/CWE-089/SqlInjection.inc.qhelp

@@ -69,8 +69,16 @@ object, so this code is vulnerable to a NoSQL injection attack.
 <sample src="examples/NoSqlInjection.js" />
 
 <p>
-To fix this vulnerability, we can check that the user input is a
-literal value and not a query object before using it in a query.
+To fix this vulnerability we can use the <code>$eq</code> operator
+to ensure that the user input is interpreted as a literal value
+and not as a query object:
+</p>
+
+<sample src="examples/NoSqlInjectionFix2.js" />
+
+<p>
+Alternatively check that the user input is a
+literal value and not a query object before using it:
 </p>
 
 <sample src="examples/NoSqlInjectionFix.js" />

javascript/ql/src/Security/CWE-089/examples/NoSqlInjectionFix.js

@@ -11,11 +11,7 @@ app.use(express.urlencoded({ extended: false }));
 
 app.delete("/api/delete", async (req, res) => {
   let id = req.body.id;
-  if (typeof id !== "string") {
-    res.status(400).json({ status: "error" });
-    return;
-  }
-  await Todo.deleteOne({ _id: id }); // GOOD: id is guaranteed to be a string
+  await Todo.deleteOne({ _id: { $eq: id } }); // GOOD: using $eq operator for the comparison
 
   res.json({ status: "ok" });
-});
+});

javascript/ql/src/Security/CWE-089/examples/NoSqlInjectionFix2.js

@@ -0,0 +1,21 @@
+const express = require("express");
+const mongoose = require("mongoose");
+const Todo = mongoose.model(
+  "Todo",
+  new mongoose.Schema({ text: { type: String } }, { timestamps: true })
+);
+
+const app = express();
+app.use(express.json());
+app.use(express.urlencoded({ extended: false }));
+
+app.delete("/api/delete", async (req, res) => {
+  let id = req.body.id;
+  if (typeof id !== "string") {
+    res.status(400).json({ status: "error" });
+    return;
+  }
+  await Todo.deleteOne({ _id: id }); // GOOD: id is guaranteed to be a string
+
+  res.json({ status: "ok" });
+});