Phoenix LiveView Guidelines

General

• Never use the deprecated liveredirect and livepatch functions, instead always use <.link navigate={href}> and <.link patch={href}> in templates, and pushnavigate and pushpatch in LiveViews
• Avoid LiveComponents unless you have a strong, specific need for them
• LiveViews should be named like AppWeb.WeatherLive, with a Live suffix. When adding routes, the default :browser scope is already aliased with the AppWeb module, so you can just do live "/weather", WeatherLive

LiveView Streams

• Always use LiveView streams for collections instead of assigning regular lists to avoid memory ballooning and runtime termination:

- Basic append: stream(socket, :messages, [newmsg]) - Reset stream: stream(socket, :messages, [newmsg], reset: true) (e.g. for filtering) - Prepend: stream(socket, :messages, [newmsg], at: -1) - Delete: streamdelete(socket, :messages, msg)

• When using streams, the template must:

1. Set phx-update="stream" on the parent element with a DOM id 2. Consume @streams.stream_name and use the id as the DOM id for each child:

<div id="messages" phx-update="stream">
  <div :for={{id, msg} <- @streams.messages} id={id}>
    {msg.text}
  </div>
</div>

• LiveView streams are not enumerable. You cannot use Enum.filter/2 or Enum.reject/2 on them. To filter or refresh, refetch the data and re-stream with reset: true:

def handle_event("filter", %{"filter" => filter}, socket) do
  messages = list_messages(filter)

  {:noreply,
   socket
   |> assign(:messages_empty?, messages == [])
   |> stream(:messages, messages, reset: true)}
end

• Streams do not support counting or empty states. Track counts with a separate assign. For empty states, use Tailwind:

<div id="tasks" phx-update="stream">
  <div class="hidden only:block">No tasks yet</div>
  <div :for={{id, task} <- @streams.tasks} id={id}>
    {task.name}
  </div>
</div>

The empty state only works if it's the only HTML block alongside the stream for-comprehension.

• When updating an assign that should change content inside streamed items, you MUST re-stream the items along with the updated assign:

def handle_event("edit_message", %{"message_id" => message_id}, socket) do
  message = Chat.get_message!(message_id)
  edit_form = to_form(Chat.change_message(message, %{content: message.content}))

  {:noreply,
   socket
   |> stream_insert(:messages, message)
   |> assign(:editing_message_id, String.to_integer(message_id))
   |> assign(:edit_form, edit_form)}
end

• Never use the deprecated phx-update="append" or phx-update="prepend" for collections

JavaScript Interop

• When using phx-hook="MyHook" and that JS hook manages its own DOM, you must also set phx-update="ignore"
• Always provide a unique DOM id alongside phx-hook

Inline Colocated JS Hooks

Never write raw embedded <script> tags in HEEx. Instead, always use colocated js hook script tags:

<input type="text" name="user[phone_number]" id="user-phone-number" phx-hook=".PhoneNumber" />
<script :type={Phoenix.LiveView.ColocatedHook} name=".PhoneNumber">
  export default {
    mounted() {
      this.el.addEventListener("input", e => {
        let match = this.el.value.replace(/\D/g, "").match(/^(\d{3})(\d{3})(\d{4})$/)
        if(match) {
          this.el.value = `${match[1]}-${match[2]}-${match[3]}`
        }
      })
    }
  }
</script>

• Colocated hooks are automatically integrated into the app.js bundle
• Colocated hook names MUST ALWAYS start with a . prefix (e.g., .PhoneNumber)

External phx-hook

External JS hooks must be placed in assets/js/ and passed to the LiveSocket constructor:

const MyHook = {
  mounted() { ... }
}
let liveSocket = new LiveSocket("/live", Socket, {
  hooks: { MyHook }
});

Pushing Events Between Client and Server

Use push_event/3 to push events/data to the client for a phx-hook to handle. Always return or rebind the socket:

socket = push_event(socket, "my_event", %{...})

# or return directly:
def handle_event("some_event", _, socket) do
  {:noreply, push_event(socket, "my_event", %{...})}
end

Receive in JS hook with this.handleEvent:

mounted() {
  this.handleEvent("my_event", data => console.log("from server:", data));
}

Push from client to server with reply:

mounted() {
  this.el.addEventListener("click", e => {
    this.pushEvent("my_event", { one: 1 }, reply => console.log("got reply:", reply));
  })
}

Server handles with reply:

def handle_event("my_event", %{"one" => 1}, socket) do
  {:reply, %{two: 2}, socket}
end

LiveView Tests

• Use Phoenix.LiveViewTest module and LazyHTML (included) for assertions
• Form tests are driven by rendersubmit/2 and renderchange/2
• Split test cases into small, isolated files. Start with simpler content-existence tests, then add interaction tests
• Always reference key element IDs from your templates in tests for element/2, has_element/2, etc.
• Never test against raw HTML; always use element/2, has_element/2, and similar
• Focus on testing outcomes rather than implementation details
• Be aware that Phoenix.Component functions may produce different HTML than expected. Test against output structure
• When facing test failures with selectors, debug with LazyHTML:

html = render(view)
document = LazyHTML.from_fragment(html)
matches = LazyHTML.filter(document, "your-complex-selector")
IO.inspect(matches, label: "Matches")

Form Handling

Creating a Form from Params

def handle_event("submitted", params, socket) do
  {:noreply, assign(socket, form: to_form(params))}
end

When passing a map to to_form/1, it assumes string keys. You can specify a name:

def handle_event("submitted", %{"user" => user_params}, socket) do
  {:noreply, assign(socket, form: to_form(user_params, as: :user))}
end

Creating a Form from Changesets

%YourApp.Users.User{}
|> Ecto.Changeset.change()
|> to_form()

In the template:

<.form for={@form} id="todo-form" phx-change="validate" phx-submit="save">
  <.input field={@form[:field]} type="text" />
</.form>

Always give the form an explicit, unique DOM ID.

Avoiding Form Errors

Always use a form assigned via to_form/2 in the LiveView, and the <.input> component in the template:

<%!-- ALWAYS do this (valid) --%>
<.form for={@form} id="my-form">
  <.input field={@form[:field]} type="text" />
</.form>

Never do this:

<%!-- NEVER do this (invalid) --%>
<.form for={@changeset} id="my-form">
  <.input field={@changeset[:field]} type="text" />
</.form>

• You are FORBIDDEN from accessing the changeset in the template
• Never use <.form let={f} ...>, always use <.form for={@form} ...> and drive all form references from the form assign