Lightning Components Basics : Attributes, Expressions and Value Providers

Attributes
Attributes are just like apex class member variables which holds some value and can be used in performing any operation.
Same like apex class member variable, you need to define name of attribute, datatype of attribute and you can also define default value which is optional.
Below is syntax to define attribute:

        <aura:attribute name="firstname" type="string" default="Sunil"/>

Other attributes are:

• access- Indicates whether the attribute can be used outside of its own namespace. Possible values are public (default), and global, and private.
• required - If it is required to specify the value of attribute. The default is false.
• description - Specify the purpose and usuage of attribute.

Supported values for type are Boolean, date, datetime, decimal, double, integer, long, string, collections (list,set,map), standard and custom objects and user defined data types(wrapper class).

Please refer below URL for more information on basic data types:

Component Attributes Types
                                   
Note:
Salesforce recommend using type="Map" instead of type="Object" to avoid some deserialization issues on the server. For example, when an attribute of type="Object" is serialized to the server, everything is converted to a string. Deep expressions, such as v.data.property can throw an exception when they are evaluated as a string on the server. Using type="Map" avoids
these exceptions for deep expressions, and other deserialization issues.

Checking for Types
To determine a variable type, use "typeof" or a standard JavaScript method instead. The "instanceof" operator is unreliable due to the potential presence of multiple windows or frames.

Expression

Expression are kind of formula which can be used within expression delimiters (“{!” and “}”)in expressions, you can specify attributes or different operators to give you output.

          <aura:attribute name="msg" type="String"/>  
            <p>{!'Hello! ' + v.msg}</p>

Value Providers

Value providers helps you to access attributes values and you can use them either in component markup or in JavaScript functions. Using value providers, you can either set or get attributes values.

        <aura:attribute name="msg" type="String"/>  
            <p>{!'Hello! ' + v.msg}</p>

• Way to find attribute value in controller.js:

               var msgValue = component.get("v.msg")

• How to set attribute value in controller.js

              component.set("v.msg","Sunil Kumar");