Learn how to create and use function plugs in Phoenix for middleware functionality.

What’s a plug?

In Phoenix, a plug is like middleware: it receives a request connection (conn), does something with it, and passes it along.

How to make a function plug

• A function plug takes two arguments: conn (connection) and opts (optional params)
• Example: Create a check_promo_code plug that checks for a promo code in the URL query
• If a valid promo code is found, assign a variable (e.g., promo: true) to the conn, making it accessible in your views
• Use assign(conn, :key, value) to attach data; this can be rendered conditionally on the frontend using HEEx templates
• Plugs are configured in the endpoint or the router and run at each request step

Elixir patterns

• You can pattern match directly on conn.params for cleaner, more idiomatic code
• Use multiple function heads for different match cases (e.g., promo code present vs. not present)

Custom plug parameters

• You can pass options with your plug and access them via the opts argument

Frontend

• Use @promo in HEEx to display a message if a valid promo code exists, and render conditionally with :if

Summary

This lesson covers building a custom function plug in Phoenix, assigning values to requests, conditionally rendering frontend content with HEEx, and using Elixir pattern matching for clean plug code.