Reject reserved names in morgan.token()

README.md

@@ -210,6 +210,10 @@ morgan.token('type', function (req, res) { return req.headers['content-type'] })
 Calling `morgan.token()` using the same name as an existing token will overwrite that
 token definition.
 
+The names `token`, `format`, and `compile` are reserved by morgan itself and cannot be
+used as custom token names. Attempting to register a token with one of these names will
+throw a `TypeError`.
+
 The token function is expected to be called with the arguments `req` and `res`, representing
 the HTTP request and HTTP response. Additionally, the token can accept further arguments of
 its choosing to customize behavior.

index.js

@@ -19,6 +19,17 @@ module.exports.compile = compile
 module.exports.format = format
 module.exports.token = token
 
+/**
+ * Names that must not be registered as custom tokens, because token()
+ * stores callbacks on the morgan export object itself.
+ * @private
+ */
+var RESERVED_TOKEN_NAMES = {
+  compile: true,
+  format: true,
+  token: true
+}
+
 /**
  * Module dependencies.
  * @private
@@ -585,6 +596,10 @@ function recordStartTime () {
  */
 
 function token (name, fn) {
+  if (Object.prototype.hasOwnProperty.call(RESERVED_TOKEN_NAMES, name)) {
+    throw new TypeError('morgan.token cannot use reserved name "' + name + '"')
+  }
+
   // wrap the token so its string output is always escaped for line-oriented
   // logs, regardless of whether the format is a string or a function
   morgan[name] = function tokenValue () {

test/morgan.js

@@ -1830,6 +1830,36 @@ describe('morgan.compile(format)', function () {
   })
 })
 
+describe('morgan.token(name, fn)', function () {
+  describe('arguments', function () {
+    describe('name', function () {
+      it('should reject reserved names', function () {
+        assert.throws(morgan.token.bind(morgan, 'token', function () {}), /reserved name/)
+        assert.throws(morgan.token.bind(morgan, 'format', function () {}), /reserved name/)
+        assert.throws(morgan.token.bind(morgan, 'compile', function () {}), /reserved name/)
+      })
+
+      it('should not corrupt morgan.token after rejecting a reserved name', function () {
+        assert.throws(morgan.token.bind(morgan, 'token', function () {}), /reserved name/)
+
+        // morgan.token itself must still be a working function
+        assert.strictEqual(typeof morgan.token, 'function')
+
+        // and registering a normal token afterwards must still work
+        morgan.token('my-custom-token', function () { return 'custom-value' })
+        assert.strictEqual(typeof morgan['my-custom-token'], 'function')
+        assert.strictEqual(morgan['my-custom-token']({}, {}), 'custom-value')
+      })
+
+      it('should allow non-reserved names', function () {
+        morgan.token('some-other-token', function () { return 'value' })
+        assert.strictEqual(typeof morgan['some-other-token'], 'function')
+        assert.strictEqual(morgan['some-other-token']({}, {}), 'value')
+      })
+    })
+  })
+})
+
 function after (count, callback) {
   var args = new Array(3)
   var i = 0