test.js

var Foo = function () {
    if(this instanceof Foo){
        this.a = true;
        this.b = 'hello';
        this.c = 5;
    } else {
        return new Foo();
    }
};

Foo.prototype.log = function(text) {
    console.log('text is ' + text);
}


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

var bar = new Foo();
var bar1 = Foo();

console.log(bar.b === 'hello');
console.log(bar1.b === 'hello');
console.log(bar instanceof Foo);

console.log(bar1 instanceof Foo);

bar.log('123');

bar1.log('123');