phoenix-html

Phoenix HTML guidelines

• Phoenix templates always use ~H or .html.heex files (known as HEEx), never use ~E

• Always use the imported Phoenix.Component.form/1 and Phoenix.Component.inputs_for/1 function to build forms. Never use Phoenix.HTML.form_for or Phoenix.HTML.inputs_for as they are outdated

• When building forms always use the already imported Phoenix.Component.to_form/2 (assign(socket, form: to_form(...)) and <.form for={@form} id="msg-form">), then access those forms in the template via @form[:field]

• Always add unique DOM IDs to key elements (like forms, buttons, etc) when writing templates, these IDs can later be used in tests (<.form for={@form} id="product-form">)

• For "app wide" template imports, you can import/alias into the my_app_web.ex's html_helpers block, so they will be available to all LiveViews, LiveComponent's, and all modules that do use MyAppWeb, :html (replace "my_app" by the actual app name)

• Elixir supports if/else but **does NOT support if/else if or if/elsif. Never use else if or elseif in Elixir, always use cond or case for multiple conditionals.

  Never do this (invalid):

  <%= if condition do %>
    ...
  <% else if other_condition %>
    ...
  <% end %>
  

  Instead always do this:

  <%= cond do %>
    <% condition -> %>
      ...
    <% condition2 -> %>
      ...
    <% true -> %>
      ...
  <% end %>
  

• HEEx require special tag annotation if you want to insert literal curly's like { or }. If you want to show a textual code snippet on the page in a <pre> or <code> block you must annotate the parent tag with phx-no-curly-interpolation:

  <code phx-no-curly-interpolation>
    let obj = {key: "val"}
  </code>
  

  Within phx-no-curly-interpolation annotated tags, you can use { and } without escaping them, and dynamic Elixir expressions can still be used with <%= ... %> syntax

• HEEx class attrs support lists, but you must always use list [...] syntax. You can use the class list syntax to conditionally add classes, always do this for multiple class values:

  <a class={[
    "px-2 text-white",
    @some_flag && "py-5",
    if(@other_condition, do: "border-red-500", else: "border-blue-100"),
    ...
  ]}>Text</a>
  

  and always wrap if's inside {...} expressions with parens, like done above (if(@other_condition, do: "...", else: "..."))

  and never do this, since it's invalid (note the missing [ and ]):

  <a class={
    "px-2 text-white",
    @some_flag && "py-5"
  }> ...
  => Raises compile syntax error on invalid HEEx attr syntax
  

• Never use <% Enum.each %> or non-for comprehensions for generating template content, instead always use <%= for item <- @collection do %>

• HEEx HTML comments use <%!-- comment --%>. Always use the HEEx HTML comment syntax for template comments (<%!-- comment --%>)

• HEEx allows interpolation via {...} and <%= ... %>, but the <%= %> only works within tag bodies. Always use the {...} syntax for interpolation within tag attributes, and for interpolation of values within tag bodies. Always interpolate block constructs (if, cond, case, for) within tag bodies using <%= ... %>.

  Always do this:

  <div id={@id}>
    {@my_assign}
    <%= if @some_block_condition do %>
      {@another_assign}
    <% end %>
  </div>
  

  and Never do this – the program will terminate with a syntax error:

  <%!-- THIS IS INVALID NEVER EVER DO THIS --%>
  <div id="<%= @invalid_interpolation %>">
    {if @invalid_block_construct do}
    {end}
  </div>