# isFunction()

## `isFunction()`

Checks if [`any`](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#any) value is a [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions) type or the type obtained from its [`object` class](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/toString#using_tostring_to_detect_object_class) equal to `'function'` and an instance of [`Function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions). It also denies it's a [`class`](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/class) by checking whether the function converted with [`Function.prototype.toString()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/toString) to the [`string`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String) does not contain the word [`class`](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/class) at the beginning.

{% code title="is-function.func.ts" %}

```typescript
const isFunction = <Payload extends object>(
  value: any,
  callback: ResultCallback<any, Payload> = resultCallback,
  payload?: Payload
): value is Function =>
  callback(
    typeof value === 'function' ||
      (typeOf(value) === 'function' && (value as any) instanceof Function)
      ? /class/.test(Function.prototype.toString.call(value).slice(0, 5)) ===
          false
      : false,
    value,
    payload
  );
```

{% endcode %}

### Generic type variables

#### <mark style="color:green;">**`Payload`**</mark>**`extends`**<mark style="color:green;">**`object`**</mark>

The `Payload` generic type variable constrained by [`object`](https://www.typescriptlang.org/docs/handbook/basic-types.html#object) indicates the type of optional parameter [`payload`](/type/type/resultcallback.md#payload-payload) of the supplied [`callback`](#callback-resultcallback-less-than-any-payload-greater-than) function and [`payload`](#payload-payload) optional parameter of the [`isFunction()`](#isfunction) function from which it captures its value.

### Parameters

#### `value: any`

The value of [`any`](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#any) type to check.

#### `callback: ResultCallback<any, Payload>`

A callback `function` of [`ResultCallback`](/type/type/resultcallback.md) type with parameters, the [`value`](#value-any) that has been checked, the [`result`](/type/type/resultcallback.md#result-boolean) of this check, and [`payload`](/type/type/resultcallback.md#payload-payload) of generic type variable [`Payload`](#payloadextendsobject) with optional properties from the provided [`payload`](#payload-payload), to handle them before the [`result`](/type/type/resultcallback.md#result-boolean) return. By default, it uses [`resultCallback()`](/type/helper/resultcallback.md) function.

#### `payload?: Payload`

An optional [`object`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object) of the generic type variable [`Payload`](#payloadextendsobject) is assigned to the [`payload`](/type/type/resultcallback.md#payload-payload) of the given [`callback`](#callback-resultcallback-less-than-any-payload-greater-than) function.

### Return type

#### `value is Function`

The **return type** is a [`boolean`](https://www.typescriptlang.org/docs/handbook/basic-types.html#boolean) as the result of its statement indicating the [`value`](#value-any) is a [`Function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions).

### Returns

The **return value** is a [`boolean`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean) indicating whether the provided [`value`](#value-any) is a [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions).

## Example usage

```typescript
// Example usage.
import { isFunction } from '@angular-package/type';

class Class { x = 5; }
const FUNC = (x: number): any => x + 5;

isFunction(Class); // Returns `false` as `value is typeof Class`, it must not be a `class`.
isFunction(FUNC); // Returns `true` as `value is (x: number) => any`
isFunction(() => 5); // Returns `true` as `value is () => 5`
```

### Parameters `callback` and `payload`

```typescript
// Example usage with callback and payload.
import { isFunction } from '@angular-package/type';

isFunction(() => 5, (result, value, payload) => {
  value // Returns `() => 5`
  if (payload) {
    payload.number // Returns `true`
  }
  return result;
}, { number: true }); // Returns `true` as `value is () => 5`
```


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