What is reason of using ' [ ] ' notation to access javascript object members instead of dot notation? [duplicate]

First it can be dynamic with a string:

 var key = 'name';
 person[key];

Second, it also supports other symbols that aren't supported

 var key = 'first-name';
 person[key];

Here's a use case (having impossible property name)

var obj = {};
> undefined
obj['first name'] = 'Ivan';
> "Ivan"
obj['first name']
> "Ivan"
obj.first\ name
> SyntaxError: Unexpected token ILLEGAL
obj.first name
> SyntaxError: Unexpected identifier

Here is a very good reason for using the brackets, where we're passing in the property name as a parameter:

function showProperty(propertyName) {
    alert(person[propertyName]);
}

showProperty('name');
showProperty('age');

Here is another good reason, where we can access the 'headers' property of request using dot-notation, but we can not access the 'Content-Type' property of request.headers using dot-notation, because it contains a -.

request.headers['Content-Type'] = 'text/html';

It's very usefully to use this notation in methods, where you send your attribute name as parameter for example.

var Obj = function(){
    this.abc = function(){
        alert("abc!!!");
    };
    this.qqq = function(){
        alert("QQQ!!!");
    };
}

var showAlert = function(param){
    var myObj = new Obj();
    myObj[param]();
};

showAlert('abc');