Javascript：在成员函数中访问类的成员变量（数组）

Question

I am trying to access a member variable of a class which is an array in member function of a class but getting an error:

Can not read property 'length' of undefined

Class:

function BasicArgs(){
  var argDataType = new Uint8Array(1);
  var argData = new Uint32Array(1);
}

Member function:

BasicArgs.prototype.getByteStreamLength = function(){
  alert(this.argData.length);
  return i;
}

This is one of the example but i have come across this at many places. variables like integer are easily accessible but most of the times problem is with arrays. Help would be appreciated.

Answer 1

You need this to make properties of the object in a constructor.

function BasicArgs(){
    this.argDataType = new Uint8Array(1);
    this.argData = new Uint32Array(1);
}

There's no way for prototyped functions to directly access the variable scope of the constructor function.

And then be sure to use new to invoke the constructor.

var ba = new BasicArgs();

ba.getByteStreamLength();

Answer 2

You can access private variable of function

modified code:

   function BasicArgs(){
      this.argDataType = new Uint8Array(1);
     this.argData = new Uint32Array(1);
    }

    BasicArgs.prototype.getByteStreamLength = function(){
       alert(this.argData.length);
        return i;
    }

Answer 3

Declaring var argData doesn't create a property on the object. It just creates a local variable that goes away as soon as the constructor exits. You need to do

this.argData = new Uint32Array(1)

instead.