JavaScript Objects With Value - valueOf and toString
Written by Mike James   
Friday, 08 March 2013
Article Index
JavaScript Objects With Value - valueOf and toString
Functions v Objects

Functions

Now we come to a tricky area. In JavaScript functions are objects - yes they can have properties and methods. They can therefore have toString and valueOf methods. What this means is that a function can return three different values depending on how it is used.

Consider:

function myFunction() {
 return 1;
};
myFunction.valueOf = function() { return 2; }; myFunction.toString = function() { return "A"; };

After this what does:

alert(myFunction);

display? Answer: "A".

What does

alert(myFunction+0);

display?  Answer: 2.

Finally, what does:

alert(myFunction());

display? Answer: 1.

This also raises the issue of why bother using a function if general objects can return a value?

The reason is fairly obvious but deserves some thought.

Functions Versus Objects

You do have a choice of associating a value with an object as in:

myObject+1;

You also can define a function that does the same thing:

myFunction()+1;

The difference is that the function has a natural way to accept arguments. For example, you can write:

myFunction(10)+1;

but without defining it to be a function you can't write:

myObject(10)+1;

In other words, objects can simply represent a value or a state. That value or state can be manipulated by the methods that the object provides but it cannot be modified while it is being used in an expression.

A function, on the other hand, represents a relationship between input data and the result.

Notice also that, while a function is an object, not all objects are functions and in this sense a function is a "bigger" object.

When should you consider using a value associated with an object?

Some might reply "never" as it isn't a common pattern and could be confusing.

However, if an object represents data or something with state then it is a good approach.

Consider the JavaScript date object:

var d=Date.now();
alert(d);

The toString function returns the number of milliseconds since the date epoch. A Date object is an ideal example of when to use an object as a value.

 

Related Articles

A JavaScript TimeInterval object

Javascript Jems - First class functions

Overriding a JavaScript global function - parseInt

 

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Last Updated ( Friday, 08 March 2013 )
 
 

   
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