# preserve-caught-error

> Require the caught error as the `cause` of a new error thrown in `catch`

When a `catch` block throws a new error, the one it caught is gone, along with its message and stack trace. The log says "Could not load the config" but not why. Pass the caught error as `cause` to keep both.

The rule checks `throw new Error()` in a `catch` block, and the other built-in classes such as `TypeError` and `AggregateError`. A `throw` in a function inside the block isn't checked.

The report comes with a suggestion that adds `{ cause: error }`.

This rule is on by default.

{/* Generated by src/codegen/lint-rule-docs.ts. Do not edit this section. */}

## Configure

On by default as a warning. To turn it off:

```toml bunfig.toml
[lint.rules]
"preserve-caught-error" = "off" # or "error"
```

Every option, set to its default:

```toml bunfig.toml
[lint.rules]
"preserve-caught-error" = ["warn", {
  requireCatchParameter = false,
  errorClassNames = [], # list of strings or objects
}]
```

{/* End of the generated section. */}

## Examples

Reported:

{/* bad */}
```js
try {
  loadConfig();
} catch (error) {
  throw new Error("Could not load the config");
}
```

The `cause` must be the caught error itself, not a part of it:

{/* bad */}
```js
try {
  loadConfig();
} catch (error) {
  throw new Error("Could not load the config", { cause: error.message });
}
```

Destructuring the parameter loses the rest of the error:

{/* bad */}
```js
try {
  loadConfig();
} catch ({ message }) {
  throw new Error(`Could not load the config: ${message}`);
}
```

Not reported:

{/* good */}
```js
try {
  loadConfig();
} catch (error) {
  throw new Error("Could not load the config", { cause: error });
}

try {
  saveConfig();
} catch (error) {
  report(error);
  throw error;
}
```

## Options

| Option | Default | What it does |
| --- | --- | --- |
| `requireCatchParameter` | `false` | Also report a new error thrown in a `catch` that has no parameter. |
| `errorClassNames` | `[]` | Your own error classes to check as well. |

{/* bad {"options": [{"requireCatchParameter": true}]} */}
```js
try {
  loadConfig();
} catch {
  throw new Error("Could not load the config");
}
```

Each `errorClassNames` entry is the name of a class that takes `{ cause }` as its second argument, like `Error`:

{/* bad {"options": [{"errorClassNames": ["ConfigError"]}]} */}
```js
try {
  loadConfig();
} catch (error) {
  throw new ConfigError("Could not load the config");
}
```

If `{ cause }` is a different argument, make the entry an object that says which one, counting from 1:

{/* bad {"options": [{"errorClassNames": [{"name": "HttpError", "argumentPosition": 3}]}]} */}
```js
try {
  await fetchUser(id);
} catch (error) {
  throw new HttpError(502, "Could not fetch the user");
}
```

{/* good {"options": [{"errorClassNames": [{"name": "HttpError", "argumentPosition": 3}]}]} */}
```js
try {
  await fetchUser(id);
} catch (error) {
  throw new HttpError(502, "Could not fetch the user", { cause: error });
}
```