Javascript类构造函数调用方法

Question

In Java you could call methods to help you do some heavy lifting in the constructor, but javascript requires the method to be defined first, so I'm wondering if there's another way I could go about this or if I'm forced to call the method that does the heavy lifting after it's been defined. I prefer to keep instance functions contained within the Object/Class, and it feels weird that I would have to have the constructor at the very end of the object/class.

function Polynomials(polyString)
{
    // instance variables
    this.polys = [];
    this.left = undefined;
    this.right = undefined;

    // This does not work because it's not yet declared
    this.parseInit(polyString);

    // this parses out a string and initializes this.left and this.right
    this.parseInit = function(polyString)
    {
        //Lots of heavy lifting here (many lines of code)
    }

    // A lot more instance functions defined down here (even more lines of code)

    // Is my only option to call it here?
}

Answer 1

Here's what I would do:

var Polynomials = function() { 
   // let's use a self invoking anonymous function
   // so that we can define var / function without polluting namespace
   // idea is to build the class then return it, while taking advantage
   // of a local scope.

   // constructor definition
   function Polynomials( value1) (
      this.property1 = value1;
      instanceCount++;
      // here you can use publicMethod1 or parseInit
   }

   // define all the public methods of your class on its prototype.
   Polynomials.prototype = {

     publicMethod1 : function() { /* parseInit()... */ },

     getInstanceCount : function() ( return instanceCount; }

   }

  // you can define functions that won't pollute namespace here
  // those are functions private to the class (that can't be accessed by inheriting classes)
  function parseInit() {
  }

  // you can define also vars private to the class
  //  most obvious example is instance count.
  var instanceCount = 0; 

   // return the class-function just built;
   return Polynomials;

}();

Remarks:

Rq 1:

prototype functions are public methods available for each instance of the class.

var newInstance = new MyClass();
newInstance.functionDefinedOnPrototype(sameValue);

Rq2: If you want truly 'private' variable, you have to got this way:

function Constructor() {
   var privateProperty=12;
   this.functionUsingPrivateProperty = function() {  
    // here you can use privateProperrty, it's in scope
   }
}

Constructor.prototype = {
    // here define public methods that uses only public properties
    publicMethod1 : function() {
        // here privateProperty cannot be reached, it is out of scope.
    }
}

personally, I do use only properties (not private vars), and use the ' ' common convention to notify a property is private. So I can define every public method on the prototype.
After that, anyone using a property prefixed with ' ' must take his/her responsibility , it seems fair. :-)

For the difference between function fn() {} and var fn= function() {} , google or SO for this question, short answer is that function fn() {} gets the function defined and assigned its value in whole scope, when var get the var defined, but its value is only evaluated when code has run the evaluation.

Answer 2

Your 'instance variables' are declared on the 'this' object which if you're looking for a Java equivalent is a bit like making them public. You can declare variables with the var keyword which makes them more like private variables within your constructor function. Then they are subject to 'hoisting' which basically means they are regarded as being declared at the top of your function (or whatever scope they are declared in) even if you write them after the invoking code.

Answer 3

I would create a function declaration and then assign the variable to the function declaration. The reason being that JavaScript will hoist your function declarations.

So you could do this:

function Polynomials(polyString) {
    // instance variables
    this.polys = [];
    this.left = undefined;
    this.right = undefined;

    // this parses out a string and initializes this.left and this.right
    this.parseInit = parseInitFunc;

    // This does not work because it's not yet declared
    this.parseInit(polyString);

    // A lot more instance functions defined down here (even more lines of code)

    function parseInitFunc(polyString) {
        console.log('executed');
    }

    // Is my only option to call it here?
}

That way your code stays clean.

jsFiddle