add isRegisteredFormat method (#462)

Adding a check  if the format is already registered. It useful, if you are using the method  registerFormat() explicitly, to avoid the error "..format already registered", you can check if the format exists, if not you can register the new one.

README.md

@@ -89,4 +89,6 @@ Archiver ships with out of the box support for TAR and ZIP archives.
 
 You can register additional formats with `registerFormat`.
 
+You can check if format already exists before to register a new one with `isRegisteredFormat`.
+
 _Formats will be changing in the future to implement a middleware approach._

index.js

@@ -63,6 +63,20 @@ vending.registerFormat = function(format, module) {
   formats[format] = module;
 };
 
+/**
+ * Check if the format is already registered.
+ * 
+ * @param {String} format the name of the format.
+ * @return boolean
+ */
+vending.isRegisteredFormat = function (format) {
+  if (formats[format]) {
+    return true;
+  }
+
+  return false;
+};
+
 vending.registerFormat('zip', require('./lib/plugins/zip'));
 vending.registerFormat('tar', require('./lib/plugins/tar'));
 vending.registerFormat('json', require('./lib/plugins/json'));

test/archiver.js

@@ -389,5 +389,13 @@ describe('archiver', function() {
           archive.finalize()
       });        
     });
+
+    describe('#isRegisteredFormat', function () {
+      var isRegisteredFormat = archiver.isRegisteredFormat('zip');
+      it('should return true when the value is present', function () {
+        assert.equal(true, isRegisteredFormat);
+      });
+    });
+
   });
 });