HEEx Templates - Phoenix LiveView

HEEx (HTML + EEx) is Phoenix LiveView’s template language that combines HTML with embedded Elixir. It provides HTML-aware interpolation, component support, compile-time validation, and smart change tracking.

The ~H Sigil

HEEx templates are created using the ~H sigil:

def render(assigns) do
  ~H"""
  <div>
    <h1>{@title}</h1>
    <p>Welcome, {@name}!</p>
  </div>
  """
end

The ~H sigil requires an assigns variable to be in scope. This variable must be a map containing all the data your template needs.

External Template Files

For larger templates, use .html.heex files instead of inline templates:

lib/my_app_web/live/
  ├── user_live.ex
  └── user_live.html.heex

If you don’t define render/1, LiveView automatically looks for a .html.heex file with the same name.

Interpolation

Basic Interpolation

Use {...} for HTML-aware interpolation:

<p>Hello, {@name}!</p>
<h1>{@page_title}</h1>
<span>{calculate_total(@items)}</span>

In Attributes

<div class={@css_class}>
<img src={@avatar_url} alt={@user.name}>
<a href={~p"/users/#{@user.id}"}>

String Interpolation in Attributes

<div class={"btn btn-#{@type}"}>
<span id={"user-#{@user.id}-status"}>

Special Attribute Values

true
false/nil
Lists (class only)

Renders the attribute without a value:

<input required={true}>
<!-- Renders: <input required> -->

Omits the attribute entirely:

<input required={false}>
<!-- Renders: <input> -->

<div class={["btn", @active && "active", @size]}>
<!-- Automatically filters nil/false -->

class and style attributes render as empty strings instead of being omitted when false/nil, which has the same effect but enables rendering optimizations.

Dynamic Attributes

Pass multiple attributes as a map or keyword list:

<div {@dynamic_attrs}>
  Content
</div>

assign(socket, :dynamic_attrs, [
  class: "card",
  id: "main",
  "data-user-id": @user.id
])

The expression inside {@dynamic_attrs} must be a keyword list or map with atom keys.

Block Expressions

For multi-line Elixir code, use <%= ... %>:

<%= if @show_greeting? do %>
  <p>Hello, {@name}!</p>
<% end %>

<%= for item <- @items do %>
  <div>{item.title}</div>
<% end %>

<%= case @status do %>
  <% :active -> %>
    <span class="badge-green">Active</span>
  <% :inactive -> %>
    <span class="badge-gray">Inactive</span>
<% end %>

Use <%= %> when the block produces output, and <% %> for control flow keywords like end, else, etc.

Special Attributes

:if - Conditional Rendering

Conditionally render elements:

<div :if={@user}>Welcome, {@user.name}!</div>

<p :if={@show_message}>This is a message</p>

<button :if={@can_edit?}>Edit</button>

Works on any HTML element, component, or slot:

<.button :if={@show_save}>Save</.button>

<:action :if={@can_delete}>
  <button>Delete</button>
</:action>

:for - List Iteration

Iterate over collections:

<ul>
  <li :for={item <- @items}>{item.name}</li>
</ul>

<.card :for={post <- @posts} title={post.title}>
  {post.body}
</.card>

With Pattern Matching

<div :for={{key, value} <- @map}>
  {key}: {value}
</div>

<div :for={%{name: name, age: age} <- @users}>
  {name} is {age} years old
</div>

:key for Change Tracking

By default, LiveView tracks items by index. Use :key for better change tracking:

<li :for={item <- @items} :key={item.id}>
  {item.name}
</li>

Why use :key?

Without :key, inserting an item at the beginning causes all subsequent items to be re-rendered. With :key, only the new item is sent to the client.

:key has no effect when using streams (Phoenix.LiveView.stream/4).

Combining :if and :for

<div :for={user <- @users} :if={user.active?}>
  {user.name}
</div>

<!-- Equivalent to: -->
<%= for user <- @users, user.active? do %>
  <div>{user.name}</div>
<% end %>

HEEx’s :for does not support multiple generators in one expression. For that, use EEx blocks:

<%= for x <- @list1, y <- @list2 do %>
  <div>{x} - {y}</div>
<% end %>

:let - Slot Variables

Capture values passed from components:

<.form :let={f} for={@form}>
  <.input field={f[:name]} />
</.form>

<.list :let={item} entries={@items}>
  <strong>{item.title}</strong>
</.list>

See the Components guide for more on slots and :let.

Function Components

Invoke function components with HTML-like syntax:

Local Components

<.button type="submit">Save</.button>
<.card title="Welcome" />
<.icon name="check" />

Remote Components

<MyApp.Components.button>Click me</MyApp.Components.button>
<Phoenix.Component.link href="/">Home</Phoenix.Component.link>

With Attributes

<.user_card 
  name={@user.name}
  email={@user.email}
  avatar={@user.avatar}
/>

Self-Closing vs Block Form

<!-- Self-closing (no inner content) -->
<.icon name="star" />

<!-- Block form (with inner content) -->
<.button>
  <.icon name="save" /> Save
</.button>

Comments

HEEx Comments

Comments that don’t appear in rendered HTML:

<%!-- This is a HEEx comment -->
<%!-- It won't appear in the HTML sent to the browser -->

HTML Comments

Regular HTML comments are preserved:

<!-- This is an HTML comment -->
<!-- It will appear in the rendered HTML -->

Escaping Curly Braces

If you need literal { or } in text:

<!-- Use HTML entities -->
<p>This is a &lbrace;curly brace&rbrace;</p>

<!-- Or EEx expressions -->
<p>This is a <%= "{" %>curly brace<%= "}" %></p>

Script and Style Tags

Curly brace interpolation is disabled inside <script> and <style> tags:

<script>
  // Use <%= %> for interpolation here
  const userId = <%= @user.id %>;
  const apiUrl = "<%= @api_url %>";
</script>

<style>
  .theme-color {
    color: <%= @theme_color %>;
  }
</style>

Disabling Interpolation

Use phx-no-curly-interpolation to disable {...} in any tag:

<div phx-no-curly-interpolation>
  {This won't be interpolated}
</div>

Code Formatting

HEEx templates support automatic formatting via mix format:

# .formatter.exs
[
  inputs: [
    "*.{heex,ex,exs}",
    "{config,lib,test}/**/*.{heex,ex,exs}"
  ]
]

Format your code:

mix format

Preventing Formatting

Use the noformat modifier:

~H"""
noformat
<div>
    This   wonot    be formatted
</div>
"""

Debug Annotations

Enable debug annotations to see where components are rendered:

# config/dev.exs
config :phoenix_live_view,
  debug_heex_annotations: true,
  debug_attributes: true

Debug Comments

With debug_heex_annotations: true:

<!-- @caller lib/app_web/live/home_live.ex:20 -->
<!-- <AppWeb.Components.header> lib/app_web/components.ex:123 -->
<header class="p-5">
  <!-- @caller lib/app_web/live/home_live.ex:48 -->
  <!-- <AppWeb.Components.button> lib/app_web/components.ex:456 -->
  <button class="px-2 bg-indigo-500">Click</button>
  <!-- </AppWeb.Components.button> -->
</header>
<!-- </AppWeb.Components.header> -->

Debug Attributes

With debug_attributes: true:

<header data-phx-loc="125" class="p-5">
  <button data-phx-loc="458" class="px-2">Click</button>
</header>

These options require mix clean and a full recompile to take effect.

Template Best Practices

Avoid defining variables in templates

Don’t do this:

<% total = @x + @y %>
<p>Total: {total}</p>

Do this instead:

def render(assigns) do
  assigns = assign(assigns, :total, assigns.x + assigns.y)
  ~H"<p>Total: {@total}</p>"
end

Variables disable change tracking!

Keep logic out of templates

Don’t do this:

<p>Price: {if @discount, do: @price * 0.9, else: @price}</p>

Do this instead:

def render(assigns) do
  assigns = assign(assigns, :final_price, calculate_price(assigns))
  ~H"<p>Price: {@final_price}</p>"
end

defp calculate_price(%{discount: true, price: price}), do: price * 0.9
defp calculate_price(%{price: price}), do: price

Never access the assigns variable directly

Don’t do this:

<div {assigns.attrs}>

Do this instead:

<div {@attrs}>

Direct access to assigns disables change tracking.

Don't perform data loading in templates

Never do this:

<%= for user <- Repo.all(User) do %>
  {user.name}
<% end %>

Always load in callbacks:

def mount(_params, _session, socket) do
  {:ok, assign(socket, :users, Repo.all(User))}
end

Common Patterns

Conditional Classes

<div class={[
  "card",
  @active && "card-active",
  @large && "card-lg",
  @color
]}>

Rendering Lists with Empty State

<div :if={@items == []}>
  <p>No items found</p>
</div>

<div :if={@items != []}>
  <div :for={item <- @items}>
    {item.name}
  </div>
</div>

Tables

<table>
  <thead>
    <tr>
      <th>Name</th>
      <th>Email</th>
      <th>Actions</th>
    </tr>
  </thead>
  <tbody>
    <tr :for={user <- @users}>
      <td>{user.name}</td>
      <td>{user.email}</td>
      <td>
        <button phx-click="edit" phx-value-id={user.id}>Edit</button>
      </td>
    </tr>
  </tbody>
</table>

Forms

<.form for={@form} phx-submit="save" phx-change="validate">
  <.input field={@form[:name]} label="Name" required />
  <.input field={@form[:email]} label="Email" type="email" />
  
  <div class="actions">
    <.button type="submit">Save</.button>
    <.link href={~p"/cancel"}>Cancel</.link>
  </div>
</.form>

Loading States

<div :if={@loading} class="spinner">
  Loading...
</div>

<div :if={!@loading}>
  <div :for={item <- @items}>
    {item.title}
  </div>
</div>

Error Messages

<div :if={@errors != []} class="alert alert-error">
  <p :for={error <- @errors}>{error}</p>
</div>

<div :if={@show_modal} class="modal-backdrop" phx-click="close_modal">
  <div class="modal" phx-click-away="close_modal">
    <div class="modal-header">
      <h2>{@modal_title}</h2>
      <button phx-click="close_modal">×</button>
    </div>
    <div class="modal-body">
      {@modal_content}
    </div>
  </div>
</div>

Compilation and Performance

HEEx templates are compiled to efficient Elixir code:

Static parts are computed once at compile time
Dynamic parts are only re-computed when their assigns change
Change tracking minimizes data sent over the wire
Diffs are computed automatically and sent to the client

What Gets Tracked?

<div id={"user-#{@user.id}"}>
  {@user.name}
</div>

If only @user.name changes:

@user.id is not re-evaluated
Only the @user.name text is sent to the client
The id attribute is not sent again

Summary

Use ~H sigil for inline templates or .html.heex files for external templates
Interpolate with {@assign} for values and {expression} for computed values
Use :if, :for, and :key for control flow
Invoke components with <.component> syntax
Avoid variables and direct data access in templates
Keep complex logic in functions, not templates
Use :key with :for for efficient list updates
Enable debug annotations during development

Getting Started

Core Concepts

Server-Side Features

Client-Side Integration

Testing

Advanced

​The ~H Sigil

​External Template Files

​Interpolation

​Basic Interpolation

​In Attributes

​String Interpolation in Attributes

​Special Attribute Values

​Dynamic Attributes

​Block Expressions

​Special Attributes

​:if - Conditional Rendering

​:for - List Iteration

​With Pattern Matching

​:key for Change Tracking

​Combining :if and :for

​:let - Slot Variables

​Function Components

​Local Components

​Remote Components

​With Attributes

​Self-Closing vs Block Form

​Comments

​HEEx Comments

​HTML Comments

​Escaping Curly Braces

​Script and Style Tags

​Disabling Interpolation

​Code Formatting

​Preventing Formatting

​Debug Annotations

​Debug Comments

​Debug Attributes

​Template Best Practices

​Common Patterns

​Conditional Classes

​Rendering Lists with Empty State

​Tables

​Forms

​Loading States

​Error Messages

​Modal Dialogs

​Compilation and Performance

​What Gets Tracked?

​Summary

Build docs developers (and LLMs) love

The ~H Sigil

External Template Files

Interpolation

Basic Interpolation

In Attributes

String Interpolation in Attributes

Special Attribute Values

Dynamic Attributes

Block Expressions

Special Attributes

:if - Conditional Rendering

:for - List Iteration

With Pattern Matching

:key for Change Tracking

Combining :if and :for

:let - Slot Variables

Function Components

Local Components

Remote Components

With Attributes

Self-Closing vs Block Form

Comments

HEEx Comments

HTML Comments

Escaping Curly Braces

Script and Style Tags

Disabling Interpolation

Code Formatting

Preventing Formatting

Debug Annotations

Debug Comments

Debug Attributes

Template Best Practices

Common Patterns

Conditional Classes

Rendering Lists with Empty State

Tables

Forms

Loading States

Error Messages

Modal Dialogs

Compilation and Performance

What Gets Tracked?

Summary