Question

The following code :

var obj = {uname:"OdO", age:"22"};  
alert(obj.uname);

results in:

OdO 

Now, using the same concept in a for..in statement :

for (x in obj) {
    document.write(obj.x+"<br>");
}

I expected it to print the following:

OdO  
22 

but it prints :

undefined  
undefined

And to achieve printing looping in the elements, it should be written as an array elements like this:

for (x in obj) {
    document.write(obj[x]+"<br>");
} 

Then, why the first syntax doesn’t work, however it works out of the for..in statement ?

Answer 1

When you write obj.x, this literally looks for a property named “x” in obj — just like obj.size would look for a property named “size”. x is not defined for your objects, so it comes out as nothing. The correct way of writing it — obj[x] — uses the variable x to look up a property in the object. The bracket syntax uses the value inside the brackets to look up the property, while the dot syntax turns the property name into a string. So these two are equivalent:

obj.x
obj["x"]

So when you write x after obj., it converts that x into a string — it’s not a variable anymore.