# `Soundcheck.Check.Warning.UndeclaredComponentAttr`
[🔗](https://github.com/leonqadirie/soundcheck/blob/v0.1.0/lib/soundcheck/check/warning/undeclared_component_attr.ex#L1)

## Basics

> #### This check is disabled by default. {: .neutral}
>
> [Learn how to enable it](`e:credo:config_file.html#checks`) via `.credo.exs`.

This check has a base priority of `normal` and works with any version of Elixir.

## Explanation

Flags `@assign` reads in a function component that declares its
interface with `attr`/`slot` but declares no attr or slot of that name.

Phoenix only puts declared attrs (and their defaults) into a
component's assigns, and warns at call sites that pass undeclared
ones. An undeclared `@key` therefore raises `KeyError` on every
render that does not happen to receive it.

Before:

    attr :title, :string, required: true

    def card(assigns) do
      ~H"<h2>{@title}</h2><p>{@subtitle}</p>"
    end

After:

    attr :title, :string, required: true
    attr :subtitle, :string, default: nil

    def card(assigns) do
      ~H"<h2>{@title}</h2><p>{@subtitle}</p>"
    end

The check skips a component when:

  * it declares no `attr` or `slot` at all (its interface is
    undeclared by design);
  * it is a LiveView or LiveComponent `render/1`, whose assigns come
    from the socket rather than from attrs;
  * its body assigns keys from a non-literal source, such as
    `assign(assigns, some_map)`, `Map.merge/2`, or
    `assigns = normalize(assigns)`, because the set of keys is then
    unknown;
  * its module calls it directly as a function, which bypasses attr
    validation.

Keys assigned in the component body before rendering
(`assign(assigns, :key, value)`, `assign_new/3`, `update/3`) count as
declared.

Confidence: high. Tier: syntactic (works without compiling).

## Check-Specific Parameters

Use the following parameters to configure this check:

### `:excluded_paths`

  Paths to skip (atoms, binaries as path segments, or regexes).

*This parameter defaults to* `[~r/\/test\//, "test"]`.

### `:implicit_assigns`

  Assign keys never reported (Phoenix-provided or global by convention).

*This parameter defaults to* `[:inner_block, :rest, :__changed__, :__given__]`.

### `:strict`

  Report unverifiable findings too.

*This parameter defaults to* `false`.

## General Parameters

Like with all checks, [general params](`e:credo:check_params.html`) can be applied.

Parameters can be configured via the [`.credo.exs` config file](`e:credo:config_file.html`).

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*Consult [api-reference.md](api-reference.md) for complete listing*