# `Soundcheck.Check.Warning.UndefinedMountHook`
[🔗](https://github.com/leonqadirie/soundcheck/blob/v0.1.0/lib/soundcheck/check/warning/undefined_mount_hook.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 `high` and works with any version of Elixir.

## Explanation

Flags `on_mount` hooks whose module has no `on_mount/4` clause that
accepts the hook's argument. LiveView calls `Module.on_mount(arg,
params, session, socket)` before every mount, so such a hook raises
`FunctionClauseError` in every LiveView it guards. The compiler
already warns when the module or its `on_mount/4` is missing.

Before:

    live_session :admin, on_mount: [{MyAppWeb.UserAuth, :require_admin}] do
      live "/admin", AdminLive
    end

    # MyAppWeb.UserAuth defines on_mount(:require_authenticated, ...) only

After:

    def on_mount(:require_admin, _params, session, socket) do
      # ...
    end

The check reads hooks from `on_mount/1` calls in LiveViews and
LiveComponents and from the `:on_mount` option of a router's
`live_session`, and the hook module's compiled `on_mount/4` clauses.
A bare module hook passes `:default`. A clause counts as accepting
unless its first pattern is a different atom; hooks whose argument
is not a literal are skipped.

Confidence: high. Tier: semantic (run `mix compile` before `mix credo`).

## 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"]`.

### `:strict`

  Unused.

*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*