Function Invocation in JavaScript

There are a few ways how we can invoke a function in JavaScript language. In this blog post, I am going to talk about the different types of function invocation in JavaScript.

##1. Invoke as A Function This is the most straight forward way of how functions can be invoked in JavaScript. The example below shows how functions can be declared and then invoked as a function.

1
2
3
4
5
6
7
8
9
10
11
12
13
function hello() //method 1
{
  return "called hello";
}

var hi = function() //method 2
{
  return "called hi";
}

//invoke as a function
hello();
hi(); 

##2. Invoke as A Method In JavaScript, an object can have a property that is assigned to a function. The function will then be invoked by referencing it through that property; i.e: invoke as a method of that object.

1
2
3
4
5
var objectA = {}; //declaring an object
objectA.myFunction = function(){}; //create a myFunction property and assign it to a function.

//invoke as a method
objectA.myFunction();

##3. Invoke as A Constructor Functions can be used as a constructor in JavaScript. The new keyword is used to invoke a function as a constructor.

1
2
3
4
5
6
7
function Person(name)
{
  this.name = name;
}   

//invoke as a constructor
var person1 = new Person('Nancy');

When a function is declared as a constructor, the following occurs:

• A new object is created.
• this value of the constructor is assigned to the new object.
• Any properties available are added to the new object.
• Return the new object.

##4. Invoke using apply() and call() methods The apply() and call() methods exist for every function in JavaScript. The apply() method is used by passing two parameters which are the object to be used as the function context and an array of values to be used as the invocation arguments. As for the call() method, an argument list is passed instead of an array.

1
2
3
4
//using .apply() by passing an object and an array
myFunction.apply(object1,[]);
//using .call() by passing an object with an argument list
myFunction.call(object1,1,2,3);

All the above types of invocation are common in the JavaScript programming world. Knowing the different types of function invocation are crucial in understanding how this works in JavaScript. I will explain how this works in my next coming blog post. Stay tune!