# Function

## Overview

`JavaScript functions` have key properties like `constructor`, which specifies the function that creates an object's prototype, and `length`, which indicates the number of expected arguments. These properties allow developers to inspect and manage function behaviors.

Methods such as `apply()`, `bind()`, `call()`, and `toString()` provide flexible ways to manipulate function contexts, invoke functions with specific arguments, and retrieve their source code.

Practical examples demonstrate how these features enhance function utility in `Servoy applications`.

Each function in javascript is actually a Function instance.

For more information see: [Function (MDN)](https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function).

## Properties Summarized

| Type                                                         | Name                        | Summary                                                     |
| ------------------------------------------------------------ | --------------------------- | ----------------------------------------------------------- |
| [Function](/reference/servoycore/dev-api/js-lib/function.md) | [constructor](#constructor) | Specifies the function that creates an object's prototype.  |
| [Number](/reference/servoycore/dev-api/js-lib/number.md)     | [length](#length)           | Specifies the number of arguments expected by the function. |

## Methods Summarized

| Type                                                     | Name                    | Summary                                                                                                                                                                                                 |
| -------------------------------------------------------- | ----------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| void                                                     | [apply()](#apply)       | Applies the method of another object in the context of a different object (the calling object); arguments can be passed as an Array object.                                                             |
| void                                                     | [bind()](#bind)         | Creates a new function which, when called, itself calls this function in the context of the provided value, with a given sequence of arguments preceding any provided when the new function was called. |
| void                                                     | [call()](#call)         | Calls (executes) a method of another object in the context of a different object (the calling object); arguments can be passed as they are.                                                             |
| [String](/reference/servoycore/dev-api/js-lib/string.md) | [toString()](#tostring) | Returns a string representing the source code of the function.                                                                                                                                          |

## Properties Detailed

### constructor

Specifies the function that creates an object's prototype.

**Type**\
[Function](/reference/servoycore/dev-api/js-lib/function.md)

**Sample**

```js
function Tree(name) {
	this.name = name;
}
theTree = new Tree("Redwood");
console.log("theTree.constructor is " + theTree.constructor);
```

### length

Specifies the number of arguments expected by the function.

**Type**\
[Number](/reference/servoycore/dev-api/js-lib/number.md)

**Sample**

```js
function addNumbers(x, y){
	if (addNumbers.length == 2) {
		return (x + y);
	} else
		return 0;
}
```

## Methods Detailed

### apply()

Applies the method of another object in the context of a different object (the calling object); arguments can be passed as an Array object.

**Returns:** void

**Sample**

```js
function book(name, author) {
	this.name = name;
	this.author = author;
}

function book_with_topic(name, author, topic) {
	this.topic = topic;
	book.apply(this, arguments);
}
book_with_topic.prototype = new book();

var aBook = new book_with_topic("name","author","topic");
```

### bind()

Creates a new function which, when called, itself calls this function in the context of the provided value, with a given sequence of arguments preceding any provided when the new function was called.

**Returns:** void

**Sample**

```js
var x = 9,
	module = {
		getX: function() {
		     return this.x;
		},
		x: 81
	};
//  "module.getX()" called, "module" is "this", "module.x" is returned
module.getX(); // > 81
//  "getX()" called, "this" is global, "x" is returned
getX(); // > 9
//  store a reference with "module" bound as "this"
var boundGetX = getX.bind(module);
//  "boundGetX()" called, "module" is "this" again, "module.x" is returned
boundGetX(); // > 81
```

### call()

Calls (executes) a method of another object in the context of a different object (the calling object); arguments can be passed as they are.

**Returns:** void

**Sample**

```js
function book(name) {
	this.name = name;
}

function book_with_author(name, author) {
	this.author = author;
	book.call(this, name);
}
book_with_author.prototype = new book();

var aBook = new book_with_author("name","author");
```

### toString()

Returns a string representing the source code of the function. Overrides the Object.toString method.

**Returns:** [String](/reference/servoycore/dev-api/js-lib/string.md)

**Sample**

```js
function printHello() {
	return "Hello";
}
application.output(printHello.toString());
```

***


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