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Static functions vs. instance methods

Before we get to Algorithm #3 (accelerate towards the mouse), we need to cover one more rather important aspect of working with vectors and the PVector object: the difference between using static functions and instance methods.
Forgetting about vectors for a moment, take a look at the following code:
var x = 0;
var y = 5;
x = x + y;
Pretty simple, right? x has the value of 0, we add y to it, and now x is equal to 5. We could write the corresponding code pretty easily based on what we’ve learned about PVector.
var v = new PVector(0,0);
var u = new PVector(4,5);
The vector v has the value of (0,0), we add u to it, and now v is equal to (4,5). Easy, right?
Let’s take a look at another example of some simple math:
var x = 0;
var y = 5;
var z = x + y;
x has the value of 0, we add y to it, and store the result in a new variable z. The value of x does not change in this example, and neither does y! This may seem like a trivial point, and one that is quite intuitive when it comes to mathematical operations with numbers. However, it’s not so obvious with mathematical operations in PVector. Let’s try to write the code based on what we know so far.
var v = new PVector(0,0);
var u = new PVector(4,5);
var w = v.add(u); // Don’t be fooled; this is incorrect!!!
The above might seem like a good guess, but it’s just not the way the PVector object works. If we look at the definition of add()...
PVector.prototype.add = function(v) {
    this.x = this.x + v.x;
    this.y = this.y + v.y;
...we see that this code does not accomplish our goal. First, it does not return a new PVector (there is no return statement) and second, it changes the value of the PVector upon which it is called. In order to add two PVector objects together and return the result as a new PVector, we must use the "static" add() function.
A "static" function is a function that is defined on an object, but it doesn't change properties of the object. So why even define it on the object? Typically, it has something to do with the object, so it is logical to attach it to it. It treats the object more like a namespace. For example, all the static functions on PVector perform some sort of manipulation on passed in PVector objects and always return back some value. We could define those functions globally as well, but this way, we avoid global functions and have better ways of grouping related functionality.
Let's contrast. Here's how we use the add() instance method:
That line of code would modify v, so we wouldn't need to save a return value. Conversely, here's how we use the add() static function:
var w = PVector.add(v, u);
If we didn't save the result of that function into a variable, that line of code would be useless, because the static version doesn't change the objects themselves. PVector's static functions allow us to perform generic mathematical operations on PVector objects without having to adjust the value of one of the input PVectors.
Here's how we would write the static version of add():
PVector.add = function(v1, v2) {
  var v3 = new PVector(v1.x + v2.x, v1.y + v2.y);
  return v3;
There are several differences here:
  • We define the function directly on the object, not on its prototype
  • We never access the this keyword inside the function
  • We return a value from the function
The PVector object has static versions of add(), sub(), mult(), and div(). It also has additional static functions that don't exist as instance methods, like angleBetween(), dot(), and cross(). We'll find ourselves using these functions as we continue making programs with PVector.
This "Natural Simulations" course is a derivative of "The Nature of Code" by Daniel Shiffman, used under a Creative Commons Attribution-NonCommercial 3.0 Unported License.

Want to join the conversation?

  • winston default style avatar for user Nicholas J.
    I think I understand, but I'm not completely sure. Please correct my summary if something seems off.

    Instance methods alter their object variable while static functions return the value after manipulation without altering. Is that the only major difference between the two types aside from implementation details?

    The other question is about calling directly on the object not the prototype. This means I have to use the actual object name, in this case PVector.add(v, u), and not an instance of the object like velocity.add or vector1.add.
    (35 votes)
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    • starky ultimate style avatar for user jdsutton
      Instance methods don't necessarily alter variables. It's simply a function that gets "shipped out", so to speak, with each instance of that object you create. They are relative to the individual object, hence the keyword this.

      A static method belongs to the class itself. You can call it without instantiating any members of that class. That's why the method is called from the class name and not from a specific instance.
      (31 votes)
  • hopper jumping style avatar for user abhishek994sharma
    var v = new PVector(1,5);
    var u = PVector.mult( v , 2);
    var w = PVector.sub( v , u);

    why only in first line "new" is used before <i>Pvector</i>??
    (12 votes)
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    • piceratops ultimate style avatar for user Dan Fletcher
      The line
      var v = new PVector(1,5);
      is creating a new PVector object

      The lines
       var u = PVector.mult(v, 2);
      var w = PVector.sub(v, u);

      are using the static functions included with PVector
      and then storing the result which is a "new PVector" inside of a variable.

      remember that when we use

      we are changing the value of v. There is no value that is actually returned so it can't be stored inside of another variable.

      on the other hand
       var u = PVector.mult(v, 2);

      is a function, that takes two parameters, and returns a PVector object, storing it inside of "u"

      The new is used inside of the function itself.
      (14 votes)
  • mr pink red style avatar for user Bob Everton
    I can't seem to get the first step in the static challenge. Here's my code. Any help.

    var v = new PVector(1,5);
    var w = 2;
    PVector.mult = function(u,w) {
    var u = new PVector(v.x.mult.w,v.y.mult.w);
    return u; };
    (7 votes)
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  • hopper jumping style avatar for user Aaron Nelson
    So, the way I was thinking is that there is no use for a static function that does not return a value. Then I thought about passing things by reference as opposed to by value. Can you do this sort of thing in JavaScript?
    (6 votes)
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  • piceratops ultimate style avatar for user RowanH
    In the Mouse stalker challenge, I have worked out I need to use this code:
    var maxDir = new PVector(width - 1, height - 1);//maximum vector to mouse
    var maxMag = maxDir.mag();

    But why this allowed? maxDir.mag() looks like an instance method to me. Why does it return a value? I was expecting to use something like var maxMag = PVector.mag(maxDir);
    (6 votes)
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    • old spice man green style avatar for user Bob Lyon
      It's hard for others to explain your misunderstandings...

      mag is one of many methods associated with PVector instances. It returns the magnitude (or length) of the PVector instance. Lengths are simple numbers, not vectors.

      Oddly enough, PVector does supply a mag static function. It's mechanically generated in the same manner as get, add, sub, etc. It is pretty useless in that it always returns a clone of its first argument, the same as invoking PVector.get
      (4 votes)
  • aqualine ultimate style avatar for user Curiosity
    I am having some trouble figuring out the Static function challenge.
    Could I see someone's spin off, to see what I did wrong?
    (2 votes)
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    • purple pi purple style avatar for user scottiegazelle
      I have been having trouble with this one, but it seems to be related to the brower. I've found that reloading the entire page seems to make things I'm fairly certain of actually pass, though I'm stymied at the end and can't tell if it's an error or a code problem.

      For the first step, however, I used:

      var u = PVector.mult(v, 2);
      (11 votes)
  • leaf blue style avatar for user costas
    I m trying to understand the implication of" 'We define the function directly on the object, not on its prototype'. Then, a second related question is why instances of PVector don't inherit these static methods too.

    There seem to be (at least) 3 ways of adding methods to an object (call it Vector):
    1. var Vector = function(x, y) {
    this. x = x,
    this.y = y,
    this.add = function(a, b) ......};};
    2. Vector.add = function(a, b) .....
    3. Vector.prototype.add = function(a, b) .....

    Do they do the same thing? I guess though only the first way (1.) makes the method automatically inheritable in any object based on Vector. For way 3, you have to call Vector's prototype into the new object's prototype. What does 2. do ? I thought it might be equivalent to 1. which would make it automatically inheritable, or not?
    (6 votes)
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    • old spice man green style avatar for user Bob Lyon
      Way 1 and way 3 are similar, but way 2 has nothing to do with this object. Way 2 is deployed as a way to keep from "polluting the global namespace". One could have written var VectorAdd(a, b) {... and been done, but by hanging another property (a static function) onto Vector one avoids another var and signals that the function is intimately associated with the Vector function.

      The non-static functions that reference this object are "methods". If all their functionality is derived from this, then the two ways are functionally equivalent. However, the storage requirements differ. By placing the method in the prototype (way 3) one guarantees that a single copy of the method is shared by all constructed objects. If one defines the method within the constructor function (way 1), then each object gets its own unique copy of that function. For example,

          var a = new Vector(3, 4);
      var b = new Vector(12, 5);
      prinltn(a.add === b.add);

      prints false with way 1 and true with way 3. So until you know and the uses of "closures", place your methods on the prototype property.
      (2 votes)
  • duskpin ultimate style avatar for user Farisch
    It was not really clear to me what the difference is between defining the function directly on the object or defining it on its its prototype. What happens on the background and why is this difference so important?
    (4 votes)
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    • old spice man green style avatar for user Bob Lyon
      Defining another function on the object constructor is no big deal. For example. we could have had something like
      * Return a new PVector that is the
      * summation of PVectors a and b.
      var addPVectors = function(a, b) {
      But instead we use PVector.add is do exactly the same thing. The latter does not pollute the global name space with a bunch more names. It also makes it clear the function is naturally provided by the one, same library that creates & handles PVectors. Furthermore the names of the static functions are easily guessed given the names of the objects' methods.

      No big deal. Just good software engineering.
      (3 votes)
  • hopper cool style avatar for user Josiah Okoduwa
    Help me please I am stuck on Step 3
    var v = new PVector(1,5);
    var u = PVector.mult(v,2);
    var w = PVector.sub(v,u);

    Oh noes keep going every time I make one argument So I make two arguments and it tells me to do one argument and OhNoes pops up
    (3 votes)
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  • hopper cool style avatar for user Aarav Dekate
    I need help with the third step
    (3 votes)
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