objects are not ordered.

Instead of identifying elements by index, we identify them by key. So an object is a list of key / value pairs separated by colon (:).

var employee = {
        'name': 'James Taylor',
        'yearOfBirth': 1948,
        'ID': 'SBJ0001',
        'role': 'IT Analyst'
    };

Objects keys are also called properties(屬性)

To access the properties we can use braces or the dot notation:

console.log( employee['ID'] ); //  The console will show 'SBJ0001'

    console.log( employee.ID ); // Same thing using the dot notation

Important: the dot notation only works with properties that follow the variables naming rules.

var test = {
        'property1': 'Some value',
        '2a': 'Some other value',
        09335: 'Another value',
        'hello-world': 'Last value'
    }; 

    // 'property1' can be retrieve恢復 using the dot notation:

    console.log( test.property1 );  // The console will show 'Some value'

    // The other ones can't because they don't follow the naming rules. They can only be retrieved with braces.

    console.log( test['hello-world'] ); // The console will show 'Last value'

• alter values assigned to properties and add new properties.

var employee = {
        'name': 'James Taylor',
        'yearOfBirth': 1948,
        'ID': 'SBJ0001',
        'role': 'IT Analyst'
    };

    employee.role = 'IT Manager';  // alter a value
    employee.passport = 'KV09888';  // add a new property

    console.log(employee); 

    /* The console will show:
    
    {
        'name': 'James Taylor',
        'yearOfBirth': 1948,
        'ID': 'SBJ0001',
        'role': 'IT Manager',
        'passport': 'KV09888'
    }; 
    
    */

Some other 方向 worth remembering:

• Arrays are identified by braces [ ].
• Arrays are ordered lists of values separated by comma（,）.
• Objects are identified by curly braces { }.
• Objects elements are key / value pairs separated by colon(:).
• Objects elements are separated by comma.(,)

Therefore we can conclude the following:

• The courses variable is an array containing 3 elements.
• The elements of courses array are objects.