Why declare properties on the prototype for instance variables in JavaScript

Question

I\'m trying to get my head around this black art called JavaScript - and, I must admit, pretty excited about it. I\'ve been looking at code examples, mainly from \"easeljs\"

Answer 1

Yeah, I agree the prototype can be used for default values of properties (variables). The constructor function doesn't need to declare a property; it may be done conditionally.

function Person( name, age ) {
    this.name = name;

    if ( age ) {
        this.age = age;
    }
}

Person.prototype.sayHello = function() {
    console.log( 'My name is ' + this.name + '.' );
};

Person.prototype.sayAge = function() {
    if ( this.age ) {
        console.log( 'I am ' + this.age + ' yrs old!' ); 
    } else {
        console.log( 'I do not know my age!' );
    }
};

Person.prototype.age = 0.7;

//-----------

var person = new Person( 'Lucy' );
console.log( 'person.name', person.name ); // Lucy
console.log( 'person.age', person.age );   // 0.7
person.sayAge();                           // I am 0.7 yrs old!

See how Lucy's age is conditionally declared and initialized.