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jTypes: Under the Hood

, 13 Jun 2013 Apache
A detailed look at how jTypes compiles C#-like object definitions into prototype-based instance matrices.
var Person = $$(function($fName, $lName, $age)
    // set the readonly fields (cast the string arguments)
    this.firstName = $$.asString($fName);
    this.lastName  = $$.asString($lName);

    // set the protected field (cast the number arguments)
    this._age = $$.asInt($age);
    'public readonly firstName': '',
    'public readonly lastName': '',

    'protected _age': 0,
    'public getFullName': function()
        // return the concatenated full name
        return this.firstName + ' ' + this.lastName;
    'public virtual triggerOneYearOlder': function()
        // increment the protected field

    'public age':
        'get': function()
            // return the protected field
            return this._age;
        'set': function($v)
            // if the incoming property value is valid, set the protected field
            if ($v > 0)
                this._age = $v;

var Employee = $$(Person, function($fName, $lName, $age, $salary)
    // call the base constructor
    this.__base($fName, $lName, $age);
    // set the protected salary field
    this._salary = $salary;
    'protected _salary': 0,
    'public override triggerOneYearOlder': function()
        // increment the protected age field (by calling the base method)

        // increase the salary by three percent
        this._salary *= 1.03;
    'public salary':
        'get': function()
            // return the salary
            return this._salary;

// instantiate a person object
var p = new Person('John', 'Doe', 30);
// check that the values were set
console.log(p.firstName);// John
console.log(p.lastName);// Doe
console.log(p.age);// 30
// get a protected field
console.log(p._age);// undefined
// set a readonly field (throws an exception)
//p.firstName = 'Jane'; (uncomment to check)
// set an invalid property value (throws if you code it)
p.age = -40;
// check that the field and property didn't change
console.log(p.firstName);// John
console.log(p.age);// 30
// set a valid property value
p.age = 40;
// check that the property did change
console.log(p.age);// 40
// invoke a method
console.log(p.getFullName());// John Doe
// invoke a virtual method
// check that the age was incremented (by the virtual method)
console.log(p.age);// 41

// instantiate an employee object
var e = new Employee(p.firstName, p.lastName, p.age, 75000);
// check that the inherited values were set
console.log(e.firstName);// John
console.log(e.lastName);// Doe
console.log(e.age);// 41
// get an inherited protected field
console.log(e._age);// undefined
// set an inherited readonly field (throws an exception)
//e.firstName = 'Jane'; (uncomment to check)
// check that the field didn't change
console.log(e.firstName);// John
// get a declared field (not inherited)
console.log(e.salary);// 75000
// cast the employee object as a person object
e =;
// check the types of both person objects
console.log(p instanceof Person);// true
console.log(p instanceof Employee);// false
console.log(e instanceof Person);// true
console.log(e instanceof Employee);// true
// check that both person objects don't have the derived property
console.log(p.salary);// undefined
console.log(e.salary);// undefined
// invoke an overridden method
// cast the person object as an employee object
e =;
// check that the age was incremented (by the base method)
console.log(e.age);// 42
// check that the salary increased 3% (by the overridden method)
console.log(e.salary);// 77250

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This article, along with any associated source code and files, is licensed under The Apache License, Version 2.0


About the Author

Nick Gaulin
CEO Gaulinsoft Corporation
United States United States
Nick is the founder of Gaulinsoft, a small software consulting company in Chicago, IL. He graduated from the Illinois Institute of Technology in 2011 with a B.S. in Aerospace Engineering.

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