Explicitly handle the case where a callback is called more than once.

README.md

@@ -25,9 +25,9 @@ resolver.resolve(function(error, results) {
   results[3];
 };
 var something = new Something();
-something.on('start', resolver.createCallback());
-something.on('in progress', resolver.createCallback());
-something.on('end', resolver.createCallback());
+something.once('start', resolver.createCallback());
+something.once('in progress', resolver.createCallback());
+something.once('end', resolver.createCallback());
 something.run(resolver.createCallback());
 ````
 

index.js

@@ -72,6 +72,9 @@ class Resolver {
         clearTimeout(self.timeout);
         self.resolveFunction(error, self.results);
       }
+      else if (self.results.length > self.callbacks.length) {
+        throw new Error('Callback called more than once.');
+      }
     };
     self.callbacks.push(callback);
     return callback;

package.json

@@ -1,6 +1,6 @@
 {
   "name": "multiple-callback-resolver",
-  "version": "1.2.3",
+  "version": "1.2.4",
   "description": "Provides a function for generating callback functions and calls a provide callback once all callbacks have completed or an error after a configurable timeout.",
   "main": "index.js",
   "scripts": {

test/resolver.js

@@ -61,4 +61,21 @@ describe('multiple callback resolver', function() {
     one();
     two();
   });
+  it('should gracefully handle calling a callback more than once', function(done) {
+    var resolver = new Resolver();
+    var one = resolver.createCallback();
+    var two = resolver.createCallback();
+    resolver.resolve(function(error) {
+      should.not.exist(error);
+    });
+    one();
+    two();
+    try {
+      two();
+    }
+    catch (error) {
+      should.exist(error);
+      done();
+    }
+  });
 });