Passing a prototype's function as parameter without losing the 'this' context

I'm defining a 'class' in JavaScript by means of prototype.

The first time func() runs, it works, but when it's called the second time, through a setTimeout, it fails because this time it has lost the object context, I.E. this doesn't reference the object anymore but now references window.

Is there a way I can overcome this while still using prototype? or do I need instead to use closures to define a 'class'?

function klass(){}

klass.prototype = {
  a: function() {
    console.log( "Hi" );
  },    
  func: function(){
    this.a();
    setTimeout( this.func, 100 );
  }
};

var x = new klass();
x.func();

Use Function.prototype.bind:

setTimeout( this.func.bind(this), 100 );

From Mozilla Developer Network:

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/bind

if (!Function.prototype.bind) {  
  Function.prototype.bind = function (oThis) {  
    if (typeof this !== "function") {  
      // closest thing possible to the ECMAScript 5 internal IsCallable function  
      throw new TypeError("Function.prototype.bind - what is trying to be bound is not callable");  
    }  
  
    var aArgs = Array.prototype.slice.call(arguments, 1),   
        fToBind = this,   
        fNOP = function () {},  
        fBound = function () {  
          return fToBind.apply(this instanceof fNOP  
                                 ? this  
                                 : oThis || window,  
                               aArgs.concat(Array.prototype.slice.call(arguments)));  
        };  
  
    fNOP.prototype = this.prototype;  
    fBound.prototype = new fNOP();  
  
    return fBound;  
  };  
}