I was trying to declare a window property when I came across this problem. What is the difference between the following two code snippets?
<script>
window.prop=undefined;
alert(window.prop);//undefined
alert(prop);//undefined
</script>
And
<script>
window.prop;
alert(window.prop);//undefined
alert(prop);//ReferenceError: prop is not defined
</script>
As per my understanding they are doing the same thing.
With window.prop=undefined;, you actually declare the variable on the window object.
window.prop; just returns the value.
Example:
a = {};
a.prop = undefined;
console.log(a); // Object {prop: undefined}
a = {};
a.prop;
console.log(a); // Object {}
In the first example, prop actually exists on a (or window, in your case), meaning it can be logged.
In the second example, it doesn't exist. When accessing normal objects, a.prop just returns undefined. However, when the object is window, it throws a reference error when accessing undefined variables on window.
The first line of the second script block actually evaluates window.prop returning it's value (or in this case that it is undefined)
In the first example, window.prop is defined and the value undefined is assigned to it.
However, in the second example, the window.prop property is undefined, which means that it doesn't really exist, and then JavaScript throws a ReferenceError.
This article on MDN describes undefined value in JavaScript.
undefined is a value. The point is that, if you assign it to a property of any object, this property turns to exist with the value undefined. However, if the property doesn't exist, then when you try to access this property it also returns undefined.