# `Soundcheck.Check.Warning.PossiblyUnsetAssign`
[🔗](https://github.com/leonqadirie/soundcheck/blob/v0.1.0/lib/soundcheck/check/warning/possibly_unset_assign.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 template reads of assigns that `mount/3` does not set on every
path before the first render. LiveView raises `KeyError` on the first
render that reaches such a read.

Before:

    def mount(_params, _session, socket), do: {:ok, socket}

    def handle_event("refresh", _params, socket),
      do: {:noreply, assign(socket, :stats, Stats.load())}

    def render(assigns), do: ~H"<p>{@stats.total}</p>"

After:

    def mount(_params, _session, socket), do: {:ok, assign(socket, :stats, nil)}

    def render(assigns), do: ~H"<p :if={@stats}>{@stats.total}</p>"

The check follows `mount/3` in both the disconnected and the
connected context. A key counts as set before the first render when
`mount/3` sets it on every path, when `handle_params/3` or an
`on_mount` hook may set it, or when it is listed in `assume_set`. Only
reads that run on every render count: reads inside `:if` elements,
EEx `if`/`case`/`for` blocks, and slots are skipped. Keys set only
when connected are left to `AssignSetOnlyWhenConnected` (SC202).

When `mount/3` passes the socket to a function soundcheck does not
model, the check makes no claim. When `handle_params/3` does, or an
`on_mount` hook cannot be read, findings are unverifiable.

Confidence: medium. 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"]`.

### `:assume_set`

  Keys the team's conventions guarantee before the first render.

*This parameter defaults to* `[:current_user, :current_scope, :flash, :live_action, :page_title]`.

### `: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*