Uncaught TypeError: Cannot read property 'x' of undefined

Question

Iam getting error above for this code (line 9):

pair = function(x,y){
   this.x = x;
   this.y = y;
}

alpha = function(){
   this.field = new pair(0,0);
   this.fun = function(){
      console.log(this.field.x);
   }
}

function beta(para){
   para();
}

beta(new alpha().fun);

but the call like:

new alpha().fun();

works fine.

Could someone explain what's going on in this case?

Answer 1

That's because the function isn't called with the right context (this).

You can use bind to ensure it's correct :

this.fun = (function(){
  console.log(this.field.x);
}).bind(this);

You could also use the closure to store the value of this :

alpha = function(){
   var a = this;
   this.field = new pair(0,0);
   this.fun = function(){
      console.log(a.field.x);
   }
}