Change how Array#reduce works

refs #12247

doc/19-library-reference.md

@@ -708,8 +708,8 @@ Signature:
     function reduce(func);
 
 Reduces the elements of the array into a single value by calling the provided
-function `func` as `func(a, b)` repeatedly where `a` is the previous result of
-function call (null initially) and `b` is an element of the array.
+function `func` as `func(a, b)` repeatedly where `a` and `b` are elements of the array
+or results from previous function calls.
 
 ### <a id="array-filter"> Array#filter
 

lib/base/array-script.cpp

@@ -172,14 +172,17 @@ static Value ArrayReduce(const Function::Ptr& function)
 	if (vframe->Sandboxed && !function->IsSideEffectFree())
 		BOOST_THROW_EXCEPTION(ScriptError("Reduce function must be side-effect free."));
 
-	Value result;
+	if (self->GetLength() == 0)
+		return Empty;
+
+	Value result = self->Get(0);
 
 	ObjectLock olock(self);
-	BOOST_FOREACH(const Value& item, self) {
+	for (size_t i = 1; i < self->GetLength(); i++) {
 		ScriptFrame uframe;
 		std::vector<Value> args;
 		args.push_back(result);
-		args.push_back(item);
+		args.push_back(self->Get(i));
 		result = function->Invoke(args);
 	}