Linter Rule: Prefer each over map when the result is discarded

Rule: erb-prefer-each-over-map

Description

Disallow map and the other collection-building methods in a silent ERB block, where the collection they return goes nowhere.

<% @users.map do |user| %>
`map` builds a new collection that is then discarded. Use `each` instead, or output the result with `<%= %>`. (erb-prefer-each-over-map)
  <p><%= user.name %></p>
<% end %>

`map` builds a new collection that is then discarded. Use `each` instead, or output the result with `<%= %>`.

Rationale

map, select and friends exist to return a new collection. In a silent tag that return value is thrown away, so the method allocates an array for nothing and the code misleads anyone reading it into looking for where the result is used.

When the block renders markup, as it almost always does in a template, each is what was meant. When the intent really was to build and print a collection, the tag should be an output tag.

The rule covers map, flat_map, select, filter, reject and filter_map. each, each_with_index, times and the other iteration methods return their receiver and are not reported.

Output tags are left to erb-no-output-control-flow, which reports <%= @users.map do |user| %> for a different reason: iteration methods return the collection rather than the rendered markup, so the collection is printed on top of the block's output.

A result that is assigned is not discarded, and is not reported:

<% names = @users.map do |user| %>
  <%= user.name %>
<% end %>

Examples

✅ Good

<% @users.each do |user| %>
  <p><%= user.name %></p>
<% end %>

<%= @users.map(&:name).join(", ") %>

<% names = @users.map do |user| %>
  <%= user.name %>
<% end %>

🚫 Bad

<% @users.map do |user| %>
`map` builds a new collection that is then discarded. Use `each` instead, or output the result with `<%= %>`. (erb-prefer-each-over-map)
  <p><%= user.name %></p>
<% end %>

<% @users.select do |user| %>
`select` builds a new collection that is then discarded. Use `each` instead, or output the result with `<%= %>`. (erb-prefer-each-over-map)
  <p><%= user.name %></p>
<% end %>

<% @groups.flat_map do |group| %>
`flat_map` builds a new collection that is then discarded. Use `each` instead, or output the result with `<%= %>`. (erb-prefer-each-over-map)
  <p><%= group.name %></p>
<% end %>

References

• Ruby Style Guide: Each vs Map