# node/handle-callback-err
> require error handling in callbacks

In Node.js, a common pattern for dealing with asynchronous behavior is called the callback pattern.
This pattern expects an `Error` object or `null` as the first argument of the callback.
Forgetting to handle these errors can lead to some really strange behavior in your application.

```js
function loadData (err, data) {
    doSomething(); // forgot to handle error
}
```

## 📖 Rule Details

This rule expects that when you're using the callback pattern in Node.js you'll handle the error.

### Options

The rule takes a single string option: the name of the error parameter. The default is `"err"`.

Examples of **incorrect** code for this rule with the default `"err"` parameter name:

```js
/*eslint handle-callback-err: "error"*/

function loadData (err, data) {
    doSomething();
}

```

Examples of **correct** code for this rule with the default `"err"` parameter name:

```js
/*eslint handle-callback-err: "error"*/

function loadData (err, data) {
    if (err) {
        console.log(err.stack);
    }
    doSomething();
}

function generateError (err) {
    if (err) {}
}
```

Examples of **correct** code for this rule with a sample `"error"` parameter name:

```js
/*eslint handle-callback-err: ["error", "error"]*/

function loadData (error, data) {
    if (error) {
       console.log(error.stack);
    }
    doSomething();
}
```

#### Regular Expression

Sometimes (especially in big projects) the name of the error variable is not consistent across the project,
so you need a more flexible configuration to ensure that the rule reports all unhandled errors.

If the configured name of the error variable begins with a `^` it is considered to be a regexp pattern.

* If the option is `"^(err|error|anySpecificError)$"`, the rule reports unhandled errors where the parameter name can be `err`, `error` or `anySpecificError`.
* If the option is `"^.+Error$"`, the rule reports unhandled errors where the parameter name ends with `Error` (for example, `connectionError` or `validationError` will match).
* If the option is `"^.*(e|E)rr"`, the rule reports unhandled errors where the parameter name matches any string that contains `err` or `Err` (for example, `err`, `error`, `anyError`, `some_err` will match).

## 🔎 Implementation

- [Rule source](../../lib/rules/handle-callback-err.js)
- [Test source](../../tests/lib/rules/handle-callback-err.js)
