You address arrays by using brackets (), as shown in the following example. The brackets enclose either a numeric value or an expression that evaluates to a whole number.
You generally use the dot operator "." to access an object's properties. For example,
In this case the property name is an identifier. You can also access an object's properties by using the index operator (). In this case you are treating the object as an associative array in which data values are associated with strings. For example,
The index operator is more commonly associated with accessing array elements. When it is used with objects, the index is a string literal that represents the property name.
Notice the important differences between the two ways of accessing object properties.
A property name treated like an identifier (the dot (.) syntax) cannot be manipulated like data.
A property name treated like an index (the braces () syntax) can be manipulated like data.
This difference becomes useful when you do not know what the property names will be until runtime (for example, when you are constructing objects based on user input). To extract all the properties from an associative array, you must use the for…in loop.