variable function undefined error

Your problem is in the way you're declaring functions:

var funcB = function(){
    // function codes
    funcC();
  }

var funcC = function(){
    // function codes
    funcB();
}

When you're declaring funcB, it tries to find funcC, which doesn't exist, yet. This breaks your code, throwing an undefined error.

You can define the functions like this:

function funcB(){
    // function codes
    funcC();
  }

function funcC(){
    // function codes
    funcB();
}

However, if funcB calls funcC, and funcC calls funcB, it doesn't matter how they're declared / initialized, you're going to have an infinite loop, probably resulting in a stack overflow-like error, or a crashing browser (/tab).

Chrome throws this:

RangeError: Maximum call stack size exceeded

To avoid that error, do something like this:

function funcB(calledFromC){
    if(!calledFromC){
        funcC(true);
    }
}

function funcC(calledFromB){
    if(!calledFromB){
        funcB(true);
    }
}

Or shorter:

function funcC(calledFromB){
    !calledFromB && funcB(true);
}

If you would declare functions like this:

var funcA = function(){
   function funcB() {
      funcC();
   }

   function funcC {
      funcB();
   }
}

it would work. Why you ask? Read more about hoisting, scoping, function declarations and function expression here

check fiddle here

of-course it would be an infinite loop, so don't do this, but it would be correct javascript.

edit: if you would like to call them out of the scope (outside of funcA), you need to return them:

 var funcA = (function() {
    var funcB = function () {
        //code here
    };
    var funcC = function () {
        //code here
    };
    return {
        funcB: funcB,
        funcC: funcC
    };
   })();

Now you can do this outside the funcA:

funcA.funcB();
funcA.funcC();

You are probably asking the wrong question as interlinked methods rarely end happily.

The answer to your question really is because of this earlier answer regarding when functions actually exist. In particular read about function hoisting.

var funcA = function() {

  function funcB(){
    // function codes
    funcC();
  }

  function funcC() {
    // function codes
    funcB();
  }
}

funcA.funcB();