Function

1. What is a Function?

 

Let's make it simple. It is just one of an object.

A function consists of ‘name’, ‘argument’ and ‘expression’.

Normally we define a function using a literal expression.

function functionName(argument) {
	// express what this funtion does
}

But we can also use a constructor.

Remind this:

- An object has Property and method

- A function is an object

- All objects are created by a constructor

 

First, function has default property 'length' and method 'toString()'.

functionName.length; // 1
functionName.toString(); // literal expression of this function
functionName.constructor; // ƒ Function() { [native code] }

Only an object has property and method, so we can see that function is an object.

Second, when you define a function using the literal expression, internally it calls constructor 'Function()' and returns an instance of Function.

 

You can also define a function using a Function constructor.

var sum = new Function('a', 'b', 'return a + b');
sum; //ƒ anonymous(a,b) { return a + b}
sum(2,4); // 6

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2. What happens when you define a Function?

 

First, it delegates constructor to function

It is the reason that only function can use a keyword 'new' .

Be aware that there is no constructor inside the function.

The function is just delegated authority to call the constructor with keyword 'new'.

 

Second, it creates a Prototype and Prototype link.

What the hell is Prototype?

It is tremendously important in JS, but we will see it in the next post.

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3. Appendix

 

In the previous post, I warned that do not confuse the primitive type 'Number' and 'Number' Object.

Here is why :

var n = 1; // it is the primitive type

Number; // constructor of Number Object
ƒ Number() { [native code] }

As the above result shows us, Number Object is a function.