Pro Tips

This is a grab-bag document for useful hints and tips when building and maintaining React applications using helix.

Memoizing components

Performance is critical for a lot of user interfaces. React provides tools to memoize calculations a number of different ways, including whole components.

NOTE: Memoization has its own cost! Do not memoize just because; profile, then optimize!

With class components, you can use the componentShouldUpdate method to tell React whether to call the render method. With function components (as created by defnc), React provides the top-level memo higher-order component that you can wrap your component in.

One thing to note is that React's memo function allows you to pass in a custom comparator, which you can use to determine whether to re-render based on the old props and new props. This is not always required. By passing in no comparator, React will default to doing a referential identity check on each prop. Most often, this is what you want!

If you do want to customize the props comparator, the memo function will receive the JS props objects, not a props map like we expect to be passed to our defnc component.

Helix provides helix.core/memo as a wrapper around React's memo function which handles the coercion to a props map just like defnc does. In the cases where you need to use a custom comparator, use that instead!

Don't use deep equals

On the topic of Memoizing components above, we mentioned custom comparators. One thing we should avoid doing unless absolutely necessary is a deep comparison of objects. Here's an example:

(defnc my-component
  [{:keys [data]}]
  {:wrap [(helix.core/memo =)]}
  (d/div (pr-str data)))

Looks simple and easy! But if our data prop isn't identical to the previous, = will do a recursive walk of the structure until it bottoms out in a value that is equal, or can't possibly be equal (different type, or a primitive like string/number/etc. that aren't equal).

For a component that we're supposedly trying to optimize, this can end up adding a ton of extra work each render! Its worst case scenario is if the data is different but structurally similar, which in my experience tends to be true; the data we keep in state or returned by our servers doesn't tend to change drastically in shape render-to-render.

Instead, it's usually better to compare props using identical? (which React.memo and helix.core/memo do by default) and try and maintain referential identity when data has not changed. For instance, if you're computing data during render, wrap it in use-memo to only re-compute it when its dependent data changes. Same thing for event handlers and other functions you pass in to components.

It can be tempting to add a custom comparator to ignore things like event handlers and other functions passed in as props, however this can often lead to subtle bugs later on where the handler needs to close over a new value or changes in some other way and your component ignores the render triggered by this change. Instead, use-callback is your friend!

Create a custom macro

Helix's defnc macro is the heart of the library. It also is going to be at the heart of how you write your components. Keeping a consistent set of semantics across a codebase you build is a key ingredient to maintainability and onboarding.

Helix exposes experimental and optional features through feature flags. This is preferred over global state which might be used by component libraries, but it makes it cumbersome to add all of these feature flags across your whole app - or have some components which use different feature flags.

That's why it's slightly recommended to create your own defnc macro which you can use to build on and extend for your application. There, you can add any feature flags you want, or even add other default functionality on top of helix like hiccup parsing, certain higher order components, debug statements - whatever your heart desires.

Creating a macro that wraps helix's is simple and easy. See below for an example which turns on the experimental "fast-refresh" feature for all components that use it.

(ns my-app.lib
  (:require
    [helix.core]))


(defmacro defnc [type params & body]
  (let [[docstring params body] (if (string? params)
                                  [params (first body) (rest body)]
                                  [nil params body])
        opts? (map? (first body)) ;; whether an opts map was passed in
        opts (if opts?
               (first body)
               {})
        body (if opts?
               (rest body)
               body)
        ;; feature flags to enable by default
        default-opts {:helix/features {:fast-refresh true}}]
    `(helix.core/defnc ~type ~@(when docstring [docstring]) ~params
       ;; we use `merge` here to allow indidivual consumers to override feature
       ;; flags in special cases
       ~(merge default-opts opts)
       ~@body)))

Components would then use my-app.lib/defnc instead of helix.core/defnc to define components. It has the same API as helix, but with the :fast-refresh feature enabled by default. Nice!