javascript var vs this [duplicate]

Answer 1

If you define a variable as var, then its scope is limited to the function (it can't be seen from the outside). In OO terms it's somehow like a private property, without actually being a property at all.

If you define a variable as a property (this.name) then it is accessible from the outside.

Same goes for functions. Functions declared inside the scope of the function, but not assigned to a property, are only visible from the inside. If you assign a function to a property you are able to access that function from the outside (as long as the property keeps pointing to that function).

function Person(){

    // Declared variable, function scope
    var name = "John";          

    // Property        
    this.surname = "Doe";       

    // Assign anonymous function to property
    this.getName = function(){  
        return name;
    }

    // Assign anonymous function to property
    this.getSurname = function(){ 
        return this.surname;
    }

    // Declare function
    function saluteCasually(){      
        console.log("Hi folks!");
    }

    // Declare function
    function salutePolitely(){          
        console.log("Nice to meet you");
    }

    // Assign (not anonymous) function to property
    this.salutePolitely = salutePolitely;

}

var person = new Person();

console.log(person.name);           // undefined
console.log(person.getName());      // "John"
console.log(person.surname);        // "Doe"
console.log(person.getSurname());   // "Doe"

person.saluteCasually(); // Error: person has not a property "saluteCasually".
person.salutePolitely(); // Prints "Nice to meet you";

person.salutePolitely = function(){ // Properties can be messed with from anywhere!
    console.log("Bananas");
}
person.salutePolitely(); // Prints "Bananas";