Add example of middleware for javascript

examples/middleware/echo.js

@@ -0,0 +1,57 @@
+#!/usr/bin/env node
+const readline = require("readline");
+const StringDecoder = require("string_decoder").StringDecoder
+
+const rl = readline.createInterface({
+  input: process.stdin
+});
+
+var ignoreIds = new Set();
+var ignoreAddresses = "/api";
+const decoder = new StringDecoder("utf8");
+
+function convertHexString(hex) {
+  var bytes = [];
+  for (var i = 0; i < hex.length - 1; i += 2) {
+    bytes.push(parseInt(hex.substr(i, 2), 16));
+  }
+  return decoder.write(Buffer.from(bytes));
+}
+
+function log(output) {
+	console.error("===================");
+	console.error(output);
+}
+
+function shouldOutputLine(request) {
+  const components = request.split("\n");
+  const header = components[0].split(" ");
+  const type = parseInt(header[0]);
+  const tag = header[1];
+
+  if (type === 3) {
+    return true;
+  }
+  if (type === 1) {
+    // Check if it's oauth
+    const endpoint = components[1].split(" ")[1];
+    if (!endpoint.startsWith(ignoreAddresses)) {
+      ignoreIds.add(tag);
+      return false;
+    }
+  } else if (type === 2) {
+    if (ignoreIds.has(tag)) {
+      ignoreIds.delete(tag);
+      return false;
+    }
+  }
+  return true;
+}
+
+rl.on("line", (input) => {
+  const str = convertHexString(input);
+  console.log(input);
+  if (shouldOutputLine(str)) {
+    log(str);
+	}
+});