packages/react-server/core/renderMiddleware.js
function writeElements(res, elements) {

	// Pick up where we left off.
	var start = RLS().nextElement||(RLS().nextElement=0);

	for (var i = start; i < elements.length; RLS().nextElement = ++i){

		// If we haven't rendered the next element yet, we're done.
		if (elements[i] === ELEMENT_PENDING) break;

		// Got one!
		writeElement(res, elements[i], i);

		// Free for GC.
		elements[i] = ELEMENT_ALREADY_WRITTEN;

		if (PageUtil.PageConfig.get('isFragment')) continue;

		if (RLS().haveBootstrapped) {

			// We've already bootstrapped, so we can immediately tell the
			// client controller to wake the new element we just sent.
			wakeElementRange(res, i, i);
		} else if (i === elements.length - 1) {

			// Page didn't emit `<TheFold/>`.  Now we're done.
			// This wakes everything up through `i`.
			bootstrapClient(res, i);
		}
	}

	// It may be a while before we render the next element, so if we just
	// wrote anything let's send it down right away.
	if (i !== start) flushRes(res);
}
function writeElements(res, elements) {

	// Pick up where we left off.
	var start = RLS().nextElement||(RLS().nextElement=0);

	for (var i = start; i < elements.length; RLS().nextElement = ++i){

		// If we haven't rendered the next element yet, we're done.
		if (elements[i] === ELEMENT_PENDING) break;

		// Got one!
		writeElement(res, elements[i], i);

		// Free for GC.
		elements[i] = ELEMENT_ALREADY_WRITTEN;

		if (PageUtil.PageConfig.get('isFragment')) continue;

		if (RLS().haveBootstrapped) {

			// We've already bootstrapped, so we can immediately tell the
			// client controller to wake the new element we just sent.
			wakeElementRange(res, i, i);
		} else if (i === elements.length - 1) {

			// Page didn't emit `<TheFold/>`.  Now we're done.
			// This wakes everything up through `i`.
			bootstrapClient(res, i);
		}
	}

	// It may be a while before we render the next element, so if we just
	// wrote anything let's send it down right away.
	if (i !== start) flushRes(res);
}
Write as many elements out in a row as possible and then flush output. We render elements as their data becomes available, so they might fill in out-of-order.