Is It Proper To Put Code In Slides For Technical Presentation?

Im doing a technical presentation and the topic is the following:

Minimizing Real-Time Data Stream Latency: Node.js and Asynchronous Operations

I've done some very thorough research on the topic and I want to add some code blocks to a few slides to visually show how to do Asynch Programming in Node.js as well as to visually show on pros and cons.

Is it appropriate to add code blocks to some slides? The presentation is for 30-40 minutes. If it is appropriate, how many slides is to much. I want to keep it to about 10-12 slides.

Thanks.