Added parallel implementation

Author: yangsu

assignments/bonus-parallel.js

@@ -0,0 +1,70 @@
+/**
+ * @param hash is an hash of names to functions
+ * @param callback is the final function
+ */
+function parallel(hash, callback) {
+  var i = 0;
+  var count = 0;
+  var retval = {};
+  var cb = function(err, key, data) {
+    if (err) {
+      callback(err);
+      // replace callback with noop so the final
+      // callback will only be executed once
+      callback = function() {};
+    } else {
+      i += 1;
+      retval[key] = data;
+      if (i === count) {
+        callback(null, retval);
+      }
+    }
+  };
+
+  // Count the number of functions
+  var key;
+  for (key in hash) {
+    if (hash.hasOwnProperty(key)) {
+      count += 1;
+    }
+  }
+
+  // Dispatch functions
+  for (key in hash) {
+    if (hash.hasOwnProperty(key)) {
+      hash[key](function(err, data) {
+        cb(err, key, data);
+      });
+    }
+  }
+}
+
+// example with non-erroring tasks
+parallel({
+  first: function(callback) {
+    callback(null, {
+      piece: 'of data'
+    });
+  },
+  second: function(callback) {
+    callback(null, {
+      piece: 'of data'
+    });
+  }
+}, function(err, data) {
+  console.log(err, data);
+  // > null { first: { piece: 'of data' }, second: { piece: 'of data' } }
+});
+
+parallel({
+  first: function(callback) {
+    callback(null, {
+      piece: 'of data'
+    });
+  },
+  second: function(callback) {
+    callback('well shit');
+  }
+}, function(err, data) {
+  console.log(err, data);  // > well shit undefined
+});