Javascript1

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Announcements

Minesweeper demo code begins here.


General Information

This course is currently running November 2013-January 2014, Tuesdays 11:00-1:00.

Part 1 (November-December) will offer students a solid foundation in Javascript as a general-purpose programming language. Part 2 (January) will explore Javascript's integration in web browsers with HTML, CSS, and the Document Object Model (DOM).

Instructor: Dan Bauer (email: dsbauer at gmail)


Course Outline

Week 1: Values and Expressions

  • Firebug Console/Interpreter
    • The console allows a dialogue with Javascript interpreter: you make one (possibly compound) statement, it replies with one value.
  • Expressions
    • An expression (EXPR) can be:
      • a primitive (e.g. 1)
      • a variable (e.g. x)
      • an operator combining smaller EXPR's (e.g. x+1, 2*(x+1))
  • Operators have:
    • a keyword or symbol (OP)
    • a syntax template (e.g. _ OP _)
      • unary prefix: OP_
      • unary postfix: _OP
      • binary infix: _OP_
      • ternary infix: _OP_OP_
    • one or more inputs _ ("operands")
    • one output ("return value")
      • may be undefined
    • possible side-effects (e.g. change in variable's value)
  • Variables have:
    • a name/identifier
    • a value (initially undefined)
      • each value has a type (primitive or reference)
    • a scope
  • Other (non-expression) Statements
    • var NAME;
    • var NAME = EXPR;
    • if (EXPR) {EXPR;}
    • if (EXPR) {EXPR;} else {EXPR;}
  • Primitive Types
    • undefined
    • Number
      • same type for integers and floats (fractional)
      • special values: NaN ("not a number"), Infinity
    • String
      • wrapped in either " " or ' '
      • may contain opposite quote or other special characters
        • \t (tab)
        • \n (newline)
      • may be empty ("")
    • Boolean (true/false)
      • false-ish values: false,undefined,null,0,"",NaN
      • (some) true-ish values: true,1,"false","undefined","0",{} (empty object)

Some Sample Operators:

operator input type example result result type side effects
typeof NUM typeof 1 "number" STR
STR typeof "word" "string"
BOOL typeof true "boolean"
,

(sequence)

any "a", "b", "c" "c" type of

last value

(4,"3"),("two",1) 1
+

(plus)

NUM 1+1 2 NUM
+

(concatenate)

STR "1"+"1" "11" STR
mixed 1+"2" "12" STR
- * / % NUM 3-1 2 NUM
STR "3"-"1" 2
mixed "3"-1 2
< > <= >=

(comparison)

NUM 1<2 true BOOL
STR "b"<"a" false
==

(equality)

NUM 1==2 false BOOL
STR "a"=="aa" false
mixed 1=="1" true
mixed 0==false true
===

(identity)

NUM 1===2 false BOOL
STR "abc"==="abc" true
mixed 1==="1" false
mixed 1===true false
!

(not)

any !1 false BOOL
!"" true
&&

(and)

any null && "red" null first false-ish

or last input

1 && "green" "green"
||

(or)

any "yes" || null "yes" first true-ish

or last input

"" || 0 0
?:

(conditional)

any true? "yes": "no" "yes" any
variable assignment operators x is now...
=

(assignment)

any x=1 1 same as input 1
x = "blue" "blue" "blue"
+=

(incremental assignment)

NUM x=0; x+=2; 2 NUM 2
STR x="he"; x+="llo"; "hello" STR "hello"
mixed x=""; x+=2; "2" STR "2"
++

(post-increment)

NUM x=0; x++; 0 NUM 1
STR x=""; x++; 0 1
++

(pre-increment)

NUM x=0; ++x; 1 NUM 1
STR x=""; ++x; 1 1

Week 2: Loops and Functions

Nov.12 in-class scratchpad 

/*
 * This is a JavaScript Scratchpad.
 *
 * Enter some JavaScript, then Right Click or choose from the Execute Menu:
 * 1. Run to evaluate the selected text (Ctrl+R),
 * 2. Inspect to bring up an Object Inspector on the result (Ctrl+I), or,
 * 3. Display to insert the result in a comment after the selection. (Ctrl+L)
 */
 
//function countdown() {
var countdown = function (start) {
    var list="";
    // generate string of numbers from 10 to 0
    for (var i=start; i>0; i--) {
      list += (i + " ");
      }
    return list;
  }

countdown
var alias=countdown;
alias(7);
alias(8);

/*
8 7 6 5 4 3 2 1 
*//*
7 6 5 4 3 2 1 
*/

alias = 7;


/*
10 9 8 7 6 5 4 3 2 1 
*/
/*
function countdown() {
    var list="";
    // generate string of numbers from 10 to 0
    for (var i=10; i>0; i--) {
      list += (i + " ");
      }
  }
*/
countdown();
/*
10 9 8 7 6 5 4 3 2 1 
*/
/*
10 9 8 7 6 5 4 3 2 1 
*/
/*
undefined
*/

function countdown() {}

function plus(x,y) {
  return x+y;
  }
  
plus(1,2);

/*
3
*/

Loops

Loops have:

  • some form of counter ("iterator"), a variable which changes with each cycle of the loop
  • an initialization (INIT)
  • a counter increment/decrement (CHANGE)
  • a repeated ACTION
  • a continuation condition (COND)
    • any COND expression will be treated as a Boolean, either true-ish or false-ish
    • the ACTIONs and CHANGE occur only when COND is true-ish

Loop Statements:

Loop keyword Pattern Example
while 
 INIT;
 while(COND) {
   ACTION;
   ...
   CHANGE;
 }

 var i=0;
 while (i<10) {
   alert(i);
   i++;
 }
for 
 for (INIT; COND; CHANGE) {
   ACTION;
   ...
 }

 for (var i=0; i<10; i++) {
  alert(i);
 }
  • Sometimes the COND includes the CHANGE, e.g.:
 for (var i=0; (i++)<10; ) {...}
 for (var i=10; --i; ) {...}
  • ACTIONs can be any statements, including nested loops

Functions

Functions:

  • are mini-programs, independent modules which can be reused in multiple contexts
  • are a kind of Object, which can be referenced by multiple names
  • have parameters which can change for each call/use
  • return one value of any type (or undefined if unspecified)
    • the keyword return specifies the returned value and exits the function immediately
  • create a separate space ("call object","execution context", or "scope") for their local variables and parameters
    • Each call creates a separate scope.
    • Scopes last as long as needed, then destroyed automatically.

Function-specific Operators:

operator input type example result result type side effects 
 function NAME(PARAMS){BODY}
 function(PARAMS){BODY}

(define function)

function add(x,y) {return x+y;} a function Function creates Function object,

creates (or redefines) var. NAME if included 

 NAME(ARGS)

(call function)

NAME: Function,

ARGS: any

add(1,2) 3 any depends on function body

Exercises (Set 1)

1) Write a function to calculate, for a group of N people where everyone shakes the hand of everyone else, how many total handshakes there are.

Solution... 

function numPairs(setSize) {
   return setSize*(setSize-1)/2;
}

Write another function to enumerate these interactions, returning a single string describing them all. People may be identified by number. For example, for N=3, it might return: "#1 meets #2. #1 meets #3. #2 meets #3."
(Hint: you may want a second "helper" function to handle a single interaction.)

Solution... 

 function writePair(personA,personB) {
    return "#"+personA+" meets #"+personB+".  ";
 }

 function enumeratePairs(numPeople) {
    var result="";
    for (var personA=1; personA<=numPeople; personA++) {
        for (var personB=personA+1; personB<=numPeople; personB++) {
            result+=writePair(personA,personB);
        }
    }
    return result;
 }

2) Write a function to decide whether a given integer is prime. You may need some of these functions/operators:

  • function Math.floor(N): truncates any fractional part of a number N (i.e. returns greatest int <=N)
  • function Number.isInteger(N): returns true is N is an integer
  • modulo operator %: x%y returns 0 if x divides evenly by y

Solution... 

 function isPrime(n) {
    if (n==0 || n==1 || !Number.isInteger(n))
        return false;    // excludes 0,1,and non-integers
    // search for number which divides n evenly...
    var sqrt=Math.sqrt(n); // only need to search up to n's square root
    for (var test=2; test<=sqrt; test++) {
        if (Number.isInteger(n/test))
            return false;  //found one; n isn't prime
    }
    return true;//n must be prime
 }

3) Imagine that a deck of playing cards is sorted by rank and suit: first all the Aces, then the Twos, etc, with the Kings last. Within each rank, the suits are in the order Heart, Diamond, Spade, Club. Number each card in order from 0 to 51 (i.e. 0=Ace of Hearts; 51=King of Clubs), and let that ID number represent the corresponding card.

Following that encoding scheme, write a set of functions to compute different features/relations of the cards:

  • rank(card) returns 1-13, representing the card's rank.
  • suit(card) returns 1-4, representing the card's suit.
  • cardNum(rank,suit) returns 0-51, identifying the card of a given rank and suit.
  • color(card) returns "red" or "black".
  • precedes(cardA,cardB) returns true only if cardA is one less in rank than cardB. Assume rank wrap-around (King precedes Ace precedes Two).
  • sameColor(cardA,cardB) returns true only if cardA and cardB have the same color.
  • nextInSuit(cardA) returns cardB, the ID number of the card following cardA in the same suit. Assume wrap-around.
  • prevInSuit(cardB) returns cardA, the ID number of the card preceding cardB in the same suit.

Your functions may call each other. Try to reuse their functionality to avoid duplicating code.

Initially, assume each function is given valid arguments (i.e. the args are integers in the appropriate range). Later, devise a system for dealing with arguments which are invalid in various ways, and rewrite your functions to tolerate such errors whenever possible.

Solution: simple version... 

 function rank(card) { // --> 1..13
    return Math.floor(card/4)+1;
 }

 function suit(card) { // --> 1..4
    return (card%4)+1;
 }

 function cardNum(rank,suit) { // --> 0..51
    return (rank-1)*4 + (suit-1);
 }

 function color(card) { // -->"red,"black"
    var theSuit=suit(card);
    return (theSuit<3)? "red": "black";
 }

 function precedes(cardA,cardB) { //-->false,true
    var diff= rank(cardB)-rank(cardA);
    return diff==1 || diff==-12;
 }

 function sameColor(cardA,cardB) { //-->false,true
    return color(cardA)==color(cardB);
 }

 function nextInSuit(cardA) {//--> 0..51
    nextCard = cardA+4;
    if (nextCard>51) nextCard-=52;
    return nextCard;
 }

 function prevInSuit(cardB) {//--> 0..51
    prevCard = cardB-4;
    if (prevCard<0) prevCard+=52;
    return prevCard;
 }

Solution: error-detecting version... 

 // Helper function; ensures num is integer between low, high
 // Returns true if valid, otherwise NaN
 function isValid(num,low,high) { // Possible return values--> NaN, true
    if ((typeof num)!="number") //wrong type
        return NaN;
    if (!Number.isInteger(num)) //non-integer
        return NaN;
    if (num<low || num>high) //out of range
        return NaN;
    return true;
 }

 function rank(card) { // --> 1..13, NaN
    return isValid(card,0,51) &&
        Math.floor(card/4)+1;
 }

 function suit(card) { // --> 1..4, NaN
    return isValid(card,0,51) &&
        (card%4)+1;
 }

 function cardNum(rank,suit) { // --> 0..51, NaN
    return isValid(rank,1,13) &&
            isValid(suit,1,4) &&
            ((rank-1)*4 + (suit-1));
 }

 function color(card) { // -->"red,"black",NaN
    var theSuit=suit(card);
    if (isNaN(theSuit))
        return NaN;
    return (theSuit<3)? "red": "black";
 }

 function precedes(cardA,cardB) { //-->false,true,NaN
    var diff= rank(cardB)-rank(cardA);
    if (isNaN(diff))
        return NaN;
    return diff==1 || diff==-12;
 }

 function sameColor(cardA,cardB) { //-->false,true,NaN
    var colorA=color(cardA), colorB=color(cardB);
    if (Number.isNaN(colorA) || Number.isNaN(colorB))
        // must use Number.isNaN() instead of isNaN(), which returns true for non-numeric strings
        return NaN;
    return colorA==colorB;
 }

 function nextInSuit(cardA) {//--> 0..51,NaN
    nextCard = isValid(cardA,0,51) && cardA+4;
    if (nextCard>51) nextCard-=52;
    return nextCard;
 }

 function prevInSuit(cardB) {//--> 0..51,NaN
    prevCard = isValid(cardB,0,51) && cardB-4;
    if (prevCard<0) prevCard+=52;
    return prevCard;
 }

Week 3: Objects, Arrays, and Methods

Objects

An object is a collection of named "properties" or "members", similar to variables...

Variables vs. Properties
Variables... Properties...
live in a global or function "scope" (execution context) live in an object
need no prefix are (usually) prefixed with their object and an operator (. or [])
must be declared with var are created merely by assigment
cause an error if not declared return undefined if not declared
have naming restrictions may be named with any string
last until their scope expires may be deleted


Object creation syntax:
  • using new operator:
var empty=new Object(); // empty object

var rect=new Object(); //empty object
rect.shape= "rectangle"; //set a property
rect.numsides=4; //set another property
rect.blank= new Object(); //set another property, an nested empty object
  • using object-literal notation (JSON):
var obj = {}; //empty object

var rect = {shape:"rectangle", numsides:4, blank:{}}; // object with 3 initial properties
Property syntax:
  • using the dot operator:
rect.numsides; //4 
rect.blank.innerProp; //undefined
  • using the [] (member) operator:
rect["numsides"];//4
rect[numsides]; //Reference Error (unless numsides is a variable)
Some object-related operators:
operator input type example result result type side effect
var obj={name:"Bubba"}
_._

(dot)

OBJ,PROP obj.name "Bubba" any
_[_]

(index/member/"sub")

OBJ,STR obj["name"] "Bubba" any
delete _._ OBJ,PROP delete obj.name true BOOL

(true if found, else false)

removes property
new _ constructor var obj = new Object(); an object OBJ allocates object space
_ in _ STR,OBJ ("name" in obj) true BOOL

Methods:

  • are functions which are properties of an object.
  • use the keyword this to refer to their object (not themselves)
  • Examples:
    • Math.sqrt()
    • Math.floor()
    • Number.isInteger()

Arrays

An array is an object containing a series of integer-indexed values (i.e. properties whose names are integers).

  • Indexes always begin with 0 (i.e. the first element has index 0)
  • Therefore, the last element always has an index of (length-1)

A "pseudo-array" is any object with some properties named by integers: 

var arr={0:'first', 1:'second', 3:'fourth', length:4};//third is implicit but undefined
  • a length property is optional and must be set manually

A genuine Array is a special class of object which...

  • automatically coordinates its integer-named properties with a special length property.
    • When creating new properties with index >= length, length extends to include it: 
      var arr=['a','b','c'];
arr.length; //3
arr[9]='j';
arr.length; //10
    • When setting length to a greater value, new properties (with value undefined) fill in the end
    • When setting length to a lesser value, properties beyond that new range are deleted
  • has a predefined set of useful methods:
    • push(VAL): modifies array, adding VAL to the end
    • pop(): modifies array, removing last value and returning it
    • join(DELIM): creates string by concatenating array elements with DELIM between them. The converse of join is String.split(DELIM), which divides a string and creates an array.
    • many more...
Array-creation syntax
var arr=new Array(); //empty array
var arr = []; //empty array
var arr = [1,2,3,"four",{},[]]; //array with six elements, including an empty Object and empty Array

Wrapper Objects

Primitives may be manually or automatically wrapped in type-specific objects which allow them properties and built-in methods:

  • Numbers: 
     var four= Object(4); //number wrapper object
 typeof four;//"object"
 four== 4;//true
 four===4;//false
 four.valueOf();//4
 four*3; //12
  • Strings: 
     var xyz= Object("x,y,z"); //string wrapper object
 xyz== "x,y,z";//true
 xyz==="x,y,z";//false
 xyz.length;//5
 xyz[2];//"y"
 xyz.split(",");//array ["x","y","z"]

    String primitives auto-convert to wrapper objects: 

     "a b c".split(" ");//array ["a","b","c"]
 "a b c"[4];//"b"


Exercises (Set 2)

4) Revisit Exercise #1b. Build a data structure which represents all people and their meetings...

a) Assume there are 5 people. Give them each a name. Create an array called people of length 5 holding their names. For these exercises, you may use a global variable for people, but recognize that's a dangerous practice in real programming!

Solution... 

var people=['Alan','Betty','Carl','Dana','Emma'];

b) Rewrite your pair-enumeration function to use your people array. Use the array size to simulate all meetings and substitute people's names for numbers, as in: "Alan meets Betty. Alan meets Carl. Betty meets Carl..."

Solution... 

function writePair(personA,personB) {
   return people[personA]+" meets "+people[personB]+".  ";
}

function enumeratePairs() {
   var result="";
   for (var personA=0; personA<(people.length-1); personA++) {
       for (var personB=personA+1; personB<people.length; personB++) {
           result+=writePair(personA,personB);
       }
   }
   return result;
}

c) Instead of constructing your people array manually, write a function to do it incrementally. Function addPerson(name) should add name to the array each time it's called.

Solution... 

var people=[];
function addPerson(name) {
   people.push(name);
}

d) Now represent each person with an object. Each object should have a name property. Modify your addPerson function to fill the people array with objects instead of name strings. Change your enumeration function as needed to work with the object array.

Solution... 

function addPerson(name) {
   people.push({name:name});
}
function writePair(indexA,indexB) {
   // retrieve objects by index:
   var personA=people[indexA];
   var personB=people[indexB];
   return personA.name+" meets "+personB.name+".  ";
}

e) Make your data structure remember all the meetings. To each object representing a person P in the people array, add a property friends which stores an array of the names of all people whom P has met. Change your enumerate function to add names to these lists with each meeting.

Solution... 

function addPerson(name) {
  people.push({name:name, friends:[]});
}
function writePair(indexA,indexB) {
  //retrieve objects:
  var personA=people[indexA];
  var personB=people[indexB];
  personA.friends.push(personB.name);
  personB.friends.push(personA.name);
  return personA.name+" meets "+personB.name+".  ";
}

f) Change the friends array of each person to store not the names but the objects of other people. Remember that an object can have multiple references. You need to have only one object per person, but with references to it in both people and multiple friends arrays.

Solution... 

function writePair(indexA,indexB) {
  //retrieve objects:
  var personA=people[indexA];
  var personB=people[indexB];
  personA.friends.push(personB);
  personB.friends.push(personA);
  return personA.name+" meets "+personB.name+".  ";
}

g) Once all meetings have been recorded and all friends lists built, write a function newestFriendOf(personNum) which returns the name of the last person met by person # personNum.

Solution... 

function newestFriendOf(personNum) {
  var person = people[personNum];
  var friend = person.friends[person.friends.length-1];
  return friend.name;

}

h) Turn all of your functions into methods of the people array. When a function needs to refer to the people array, use the keyword this instead.

Solution... 

var people=[];

people.addPerson = function(name) {
  this.push({name:name, friends:[]});
}

people.writePair = function(indexA,indexB) {
  //retrieve objects:
  var personA=this[indexA];
  var personB=this[indexB];
  personA.friends.push(personB);
  personB.friends.push(personA);
  return personA.name+" meets "+personB.name+".  ";
}

people.enumeratePairs = function() {
   var result="";
   for (var personA=0; personA<(this.length-1); personA++) {
       for (var personB=personA+1; personB<this.length; personB++) {
           result+=this.writePair(personA,personB);
       }
   }
   return result;
}

people.newestFriendOf = function(personNum) {
  var person = this[personNum];
  var friend = person.friends[person.friends.length-1];
  return friend.name;
}



Week 4: Practice with Objects, Arrays, and Functions

This week is a break from new content, instead offering more practice with real code using Objects, Arrays, and Functions.

Demo Code

These code samples begin with a portion of Exercise #3 (modelling playing cards) and improve the design incrementally in successive versions. You should understand the differences between versions and the rationale for each change.

Version 0: sample from exercise #3 

//===== Version 0: Global functions with card argument (from Exercise #3)


function rank(cardnum) {
   return Math.floor(cardnum/4)+1;// --> 1..13
}
function suit(cardnum) {
   return cardnum%4+1; // --> 1..4
}
function color(cardnum) {
   return (suit(cardnum)<3)? "red":"black";
}
//etc...

//Testing:
rank(51); // 13 (King)
suit(51); // 4 (Clubs)
color(51); // "black"


Version 1: adding card names 

//===== Version 1: Add card names


function rank(cardnum) {
   return Math.floor(cardnum/4)+1;// --> 1..13
}
function suit(cardnum) {
   return cardnum%4+1; // --> 1..4
}
function color(cardnum) {
   return (suit(cardnum)<3)? "red":"black";
}
//etc...


function name(cardnum) {
   return rankNames[rank(cardnum)] + " of " + suitNames[suit(cardnum)];
}


var rankNames = ["","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen","King"];
var suitNames = ["","Hearts","Diamonds","Spades","Clubs"];

//Testing:
rank(51); //13
suit(51); //4
color(51); // "black"
name(51); // King of Clubs


Version 2a: single controller object 

//======== Version 2: Methods of single object
var cardReader = {}; // "toolkit" object containing all card functions


cardReader.rank = function(cardnum) {
   return Math.floor(cardnum/4)+1;// --> 1..13
}
cardReader.suit = function(cardnum) {
   return cardnum%4+1; // --> 1..4
}
cardReader.color = function(cardnum) {
   return (this.suit(cardnum)<3)? "red":"black";
}


cardReader.name = function(cardnum) {
   return this.rankNames[this.rank(cardnum)] +" of "+ this.suitNames[this.suit(cardnum)];
}


cardReader.rankNames = ["","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen","King"];
cardReader.suitNames = ["","Hearts","Diamonds","Spades","Clubs"];

//Testing:
cardReader.rank(51); //13
cardReader.suit(51); //4
cardReader.color(51); // "black"
cardReader.name(51); // King of Clubs


Version 2b: single controller object (alternate form) 

//======== Version 2b: Methods of single object (object literal/JSON form)
var cardReader = { // "toolkit" object containing all card functions


rank: function(cardnum) {
   return Math.floor(cardnum/4)+1;// --> 1..13
},
suit: function(cardnum) {
   return cardnum%4+1; // --> 1..4
},
color: function(cardnum) {
   return (this.suit(cardnum)<3)? "red":"black";
},


name: function(cardnum) {
   return this.rankNames[this.rank(cardnum)] +" of "+ this.suitNames[this.suit(cardnum)];
},


rankNames: ["","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen","King"],
suitNames: ["","Hearts","Diamonds","Spades","Clubs"]
}; //end cardReader

//Testing:
cardReader.rank(51); //13
cardReader.suit(51); //4
cardReader.color(51); // "black"
cardReader.name(51); // King of Clubs


Version 3: single maker object, multiple instance objects 

//===== Version 3: Card as object (w. own methods & cached data fields)
// Each card is now an object, created by function makeCard
function makeCard(cardnum) {
   var card = {};


   //computed initially:
   card.rank = Math.floor(cardnum/4)+1;
   card.suit = cardnum%4+1;


   //deferred:
   card.color = function() {
       return (this.suit<3)? "red":"black";
   }


   card.name = function() {
       return (rankNames[this.rank] +" of "+ suitNames[this.suit]);
   }


   var rankNames = ["","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen","King"];
   var suitNames = ["","Hearts","Diamonds","Spades","Clubs"];

   
   return card;
}

//Testing:
var card51 = makeCard(51);
card51.rank; //13
card51.suit; //4
card51.color(); //"black"
card51.name(); //King of Clubs


Version 4: multiple instance objects sharing arrays 

//===== Version 4: Card as object (as before), but arrays are shared


function makeCard(cardnum) {
   var card = {};


   //computed initially:
   card.rank = Math.floor(cardnum/4)+1;
   card.suit = cardnum%4+1;


   //deferred:
   card.color = function() {
       return (this.suit<3)? "red":"black";
   }


   card.name = function() {
       return (makeCard.rankNames[this.rank] +" of "+ makeCard.suitNames[this.suit]);
   }

    
   return card;
}


//Arrays created AFTER function, attached to it:
makeCard.rankNames = ["","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen","King"];
makeCard.suitNames = ["","Hearts","Diamonds","Spades","Clubs"];

//Testing:
var card51 = makeCard(51);
card51.rank; //13
card51.suit; //4
card51.color(); //"black"
card51.name(); //King of Clubs


Week 5: Constructors, Prototypes, and Inheritance

Constructors

A constructor is simply a function whose purpose is to initialize an object (usually empty) to behave as an instance of the "class" the constructor represents. There are no "class" objects in JS, but constructors are the closest approximation.

Think of a constructor as a temporary method which is attached to an object only for one execution, during which it creates and sets the appropriate object properties.

A constructor has:

  • no creation of its intended object (done beforehand by the new operator);
  • no return value;
  • the keyword this, referring to its temporary "owner" object;
  • by convention, a name which is a noun phrase starting with capital letter (e.g. Card, BananaCremePie, PresidentsOfMars)

Some predefined constructors:

  • Object
  • Array
  • Function
  • Number
  • String

The new operator

Constructors are normally called with a special operator, new, which creates an empty object and applies the constructor to it. The expression new CTOR(args) does (approximately) the following steps:

  1. create an empty object OBJ
  2. link OBJ to CTOR's prototype (see #Prototypes)
  3. temporarily reinterpret the word this to refer to OBJ
  4. executes CTOR(args), which initializes OBJ using this
  5. restore the previous meaning of this
  6. return OBJ as the expression's value

Using predefined constructors: 

var obj;
obj = new Object(); //--> return empty Object, i.e. {}
obj = new Array(); //-->empty Array, i.e. []
obj = new Array(1,2,3); //--> initialized Array [1,2,3]
obj = new Array(3); //--> Array of 3 undefineds: [undefined,undefined,undefined], NOT [3]
obj = new Function('x','y','return x+y;');//--> function(x,y) {return x+y;}
obj = new String("hello"); //wrapper obj containing "hello", NOT primitive string "hello"
obj = new Number(12); //wrapper obj containing 12, NOT primitive number 12


Constructors without new

If new is omitted, some predefined constructors return primitives: 

obj = String("hello");// primitive string "hello"
obj = Number(12);// primitive number 12

Others return objects: 

obj = Object(5);//wrapper obj containing 5
obj = Array(1,2,3); //Array object [1,2,3]

But such behavior is atypical; in general, you should always use new with constructors!

Except for special cases, calling a constructor without using new means that its reference this points to the wrong object! Usually it points to the global object, so that constructors will wrongly create global properties: 

 function BlueSquare() {this.color="blue"; this.sides=4;} //a constructor
 obj = new BlueSquare();// CORRECT, returns object: {color:"blue", sides:4}
 obj = BlueSquare(); // WRONG...
 obj; //--> undefined, function has no return val
 color; //--> "blue"
 sides; //--> 4

Constructors as Classes

Constructors unify their instances (the objects they initialize) into a family or "class":

  • All instances have a built-in constructor property which remembers the constructor which made them. It can be a convenient way to check the subtype of an object:
obj = new Card();
obj.constructor == Card; //true
  • Alternatively, the operator instanceof returns true for certain combinations of instance and constructor:
obj instanceof Card;        //true
obj instanceof Object;      //true (everything is an Object)
Card instanceof Object;     //true
Function instanceof Object; //true
Card instanceof Function;   //true (every Ctor is a Function)
Object instanceof Function; //true
Function instanceof Function; //true
  • Properties added to a constructor can store resources (e.g. Arrays) shared by all its instances (as can #Prototypes);
  • Methods added to a constructor can compute class information without needing any particular instance. For example:
 function Tower(hgt) { //constructor makes towers, remembers highest
   this.height=hgt;
   if (hgt > Tower.highestEver()) {
     Tower.highestEver = function() {return hgt;} //revise definition
   }
 }
 Tower.highestEver = function() {return 0;} //initial definition


Demo code, part 2

Continuing improvements to cards demo...

Version 5: Card constructor 

//===== Version 5: Card constructor


// makeCard (optional) is a wrapper which calls Card constructor
function makeCard(cardnum) {
   return new Card(cardnum);
}


// Constructor:
function Card(cardnum) {
   this.rank = Math.floor(cardnum/4)+1;
   this.suit = cardnum%4+1;


   this.color = function() {
       return (this.suit<3)? "red":"black";
   }
   this.name = function() {
       return (Card.rankNames[this.rank]
           +" of "
           +Card.suitNames[this.suit]);
   }
   //no return needed
}


Card.rankNames = ["","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen","King"];
Card.suitNames = ["","Hearts","Diamonds","Spades","Clubs"];

//Testing:
var card51 = new Card(51);
card51.color(); //black
card51.name();  //King of Clubs


Version 6: Card constructor w. shared prototype methods 

//===== Version 6: Card constructor + prototype

function makeCard(cardnum) {
    return new Card(cardnum);
}

// Constructor:
function Card(cardnum) {
    this.rank = Math.floor(cardnum/4)+1;
    this.suit = cardnum%4+1;
}

// Shared methods:
Card.prototype.name = function() {
    return (Card.rankNames[this.rank]
	    +" of "
	    +Card.suitNames[this.suit]);
}
Card.prototype.color = function() {
    return (this.suit<3)? "red":"black";
}


Card.rankNames =  ["","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen","King"];
Card.suitNames = ["","Hearts","Diamonds","Spades","Clubs"];


//Testing:
var card51 = new Card(51);
card51.name(); // --> King of Clubs


Version 7: Card subclassing, inheritance 

//==== Version 7: Card subclassing

// Superclass constructor:
function Card(cardnum) {
    this.rank = Math.floor(cardnum/4)+1;
    this.suit = cardnum%4+1;
}

Card.prototype.name = function() {
    return (this.constructor.rankNames[this.rank]
	    +" of "
	    +this.constructor.suitNames[this.suit]);
}
Card.prototype.color = function() {
    return (this.suit<3)? "red":"black";
}

Card.rankNames =  ["","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen","King"];
Card.suitNames = ["","Hearts","Diamonds","Spades","Clubs"];


// Subclass constructor: (Minor Arcana of Tarot deck)
function MinorArcana(cardnum,orientation) {
    Card.call(this,cardnum); //initialize as superclass (Card)
    //add subclass-specific properties:
    this.orientation = orientation; 
}

MinorArcana.rankNames = Card.rankNames.slice(0); //copy all
MinorArcana.rankNames.splice(11,1,"Page","Knight");//replace J w. P+K
MinorArcana.suitNames = ["","Cups","Coins","Swords","Wands"];

MinorArcana.prototype = new Card();
MinorArcana.prototype.constructor = MinorArcana;

//Testing:
var normal42 = new Card(42);
normal42.name(); // --> Jack of Spades
var tarot42  = new MinorArcana(42,"upright");
tarot42.name(); // --> Page of Swords
tarot42.orientation; //--> upright

The call method

Some explanation is needed for this mysterious line in Version 7: 

Card.call(this,cardnum)

Every function has a predefined method (i.e. a member function) named call which executes the function, similarly to the call operator _(_). The difference is that a function's call method expects one additional argument, an object, in the first position, and it calls its owner function as if that function were a method of that object. For example: 

 var dot = {radius:1, color:"white"};
 function paint(newcolor) {
   this.color = newcolor;
 }
 paint.call(dot,"blue");//calls paint as if it belongs to dot (i.e. this==dot)
 dot.color;//--> "blue"


This "temporary ownership" can be used to apply a constructor manually (without new) to initialize an existing object. In the demo line 

Card.call(this,cardnum)

a subclass of Card is using Card as an initializer in the first step of its own initialization.

Advanced Topic: Building a simple application (Minesweeper)

Overview

We're building a simple version of the game Minesweeper. Like many applications, this one will separate the roles of Model and View into two modules:

  • the game engine (Model) will manage the abstract structure and rules of the game;
  • the interface (View) will control the display and user input. If time permits, we'll write more than one interface module to illustrate that the same Model works with both.

Outer HTML file

The modules are combined into a single application by including the javascript (and any CSS) for each in a single HTML file.

minesweeper.html 

 <html>
 <head>
  <meta charset="utf-8">
  <script type="text/javascript" src="mines7.js"></script>
  <script type="text/javascript" src="minesweeper-gui2.js"></script>
  <link type="text/css" rel="StyleSheet" href="minesweeper-gui.css">
 </head>
 <body onload="makeMinefield(10,10,15);">
  <div id="gamespace"></div>
 </body>
 </html>

This file is just a preview; it won't work until we've finished the components.

Game engine/model

Version 1: Basic grid geometry

mines1.js 

// Stage 1: Grid and Cell objects + basic coordinate system
//--- Grid ---
function Grid(w,h,bombs) {
    this.length = w*h;
    this.w = w;
    this.h = h;

    for (var n=0; n<this.length; n++) {
	this[n] = new Cell(n,this);
    }
}

Grid.prototype.XYtoN = function(x,y) {
    return (y*this.w)+x;//-->n
}

Grid.prototype.NtoX = function(n) {
    return n % this.w;//-->x
}

Grid.prototype.NtoY = function(n) {
    return Math.floor(n/this.w);//-->y
}


//--- Cell ---
function Cell(n,grid) {
    this.grid = grid;
    this.n = n;
    this.x = grid.NtoX(n);
    this.y = grid.NtoY(n);
}

Cell.prototype.neighbors = function() { //returns list of grid positions
    // four booleans (true if NOT adjacent to respective grid edge):
    var goW = (this.x>0);
    var goE = (this.x+1 < this.grid.w);
    var goN = (this.y>0);
    var goS = (this.y+1 < this.grid.h);

    var result=[];
    if (goN) {
	var northN = this.n-this.grid.w;
	if (goW) result.push(northN-1);//NW
	result.push(northN);//N
	if (goE) result.push(northN+1);//NE
    }
    if (goW) result.push(this.n-1);//W
    if (goE) result.push(this.n+1);//E
    if (goS) {
	var southN =this.n+this.grid.w;
	if (goW) result.push(southN-1);//SW
	result.push(southN);//S
	if (goE) result.push(southN+1);//SE
    }
    return result;
}

// Start:
function makeMinefield(w,h,bombs) {
    window.minefield=
	new Grid(w,h,bombs);
}
Related Exercise

Implement a similar coordinate system in 3D--- specifically, for a Rubik's Cube. Imagine that each of the 27 smaller cubes (include the center) in a Rubik's Cube is numbered in two different ways:

  • each has a unique number N, from 0 to 26, and
  • each has a unique three-component coordinate (X,Y,Z), each component from 0 to 2, describing its position in the big cube.

Write four functions to convert between these two systems:

  • function XYZtoN(x,y,z) should convert (X,Y,Z) to N
  • functions NtoX(n), NtoY(n), and NtoZ(n) should each convert N to one component of (X,Y,Z)

There are many solutions!

Version 6: Full Game

mines6.js 

// Stage 1: basic coordinate system
// Stage 2: add mines,count
// Stage 3: add visibility,reveal
// Stage 4: add flags
// Stage 5: add win/lose
// Stage 6: attach UI!

if (typeof gui == "undefined")
    var gui = null;

//--- Grid ---
function Grid(w,h,bombs) {
    this.length = w*h;
    this.w = w;
    this.h = h;
    this.numVisible = 0;

    if (gui) this.token = gui.makeGrid(this,w,h);

    for (var n=0; n<this.length; n++) {
	this[n] = new Cell(n,this);
    }

    //limit bomb density to <1/4
    var maxbombs = Math.floor(this.length/4);
    if (bombs>maxbombs) bombs=maxbombs;

    this.numBombs = bombs;
    this.flagsAvail = bombs;
    while (bombs) {
	n = Math.floor(Math.random()*this.length);
	var cell = this[n];
	if (!cell.isBomb) {
	    cell.placeBomb();
	    bombs--;
	}
    }

}

Grid.prototype.XYtoN = function(x,y) {
    return (y*this.w)+x;//-->n
}

Grid.prototype.NtoX = function(n) {
    return n % this.w;//-->x
}

Grid.prototype.NtoY = function(n) {
    return Math.floor(n/this.w);//-->y
}

Grid.prototype.reveal = function(x,y) {
    var n=this.XYtoN(x,y);
    this[n].reveal();
}

Grid.prototype.revealAll = function() {
    for (var n=0; n<this.length; n++) {
	this[n].reveal(true);
    }
}

Grid.prototype.flag = function(x,y) {
    var n=this.XYtoN(x,y);
    this[n].flag();
}

Grid.prototype.checkWin = function() {
    if (this.flagsAvail==0 && (this.numVisible>= this.length-this.numBombs))
	this.youWin();
}

Grid.prototype.youWin = function() {
    alert("You win!");
}

Grid.prototype.youLose = function() {
    alert("KABOOM!");
    this.revealAll();
}

//--- Cell ---
function Cell(n,grid) {
    this.grid = grid;
    this.n = n;
    this.x = grid.NtoX(n);
    this.y = grid.NtoY(n);
    this.count = 0;
    this.isBomb = false;
    this.isVisible = false;
    this.isFlag = false;

    if (gui) this.token=gui.makeCell(grid.token,this,this.x,this.y);
}

Cell.prototype.neighbors = function() { //returns list of grid positions
    // four booleans (true if NOT adjacent to respective grid edge):
    var goW = (this.x>0);
    var goE = (this.x+1 < this.grid.w);
    var goN = (this.y>0);
    var goS = (this.y+1 < this.grid.h);

    var result=[];
    if (goN) {
	var northN = this.n-this.grid.w;
	if (goW) result.push(northN-1);//NW
	result.push(northN);//N
	if (goE) result.push(northN+1);//NE
    }
    if (goW) result.push(this.n-1);//W
    if (goE) result.push(this.n+1);//E
    if (goS) {
	var southN =this.n+this.grid.w;
	if (goW) result.push(southN-1);//SW
	result.push(southN);//S
	if (goE) result.push(southN+1);//SE
    }
    return result;
}

Cell.prototype.placeBomb = function() {
    this.isBomb = true;
    var neighbors = this.neighbors();
    for (var i=0; i<neighbors.length; i++)
	this.grid[neighbors[i]].count++;
}

Cell.prototype.reveal = function(skipCheck) {
    if (this.isVisible || this.isFlag) return;
    this.isVisible = true;
    this.grid.numVisible++;
    if (gui) gui.reveal(this.token,this.count,this.isBomb);

    if (!skipCheck) {
	if (this.isBomb) {
	    this.grid.flagsAvail--;
	    this.grid.youLose();
	    return;
	}
	this.grid.checkWin();
    }	

    if (this.count==0) {
	var neighbors = this.neighbors();
	for (var i=0; i<neighbors.length; i++)
	    this.grid[neighbors[i]].reveal();
    }

}

Cell.prototype.flag = function() {
    if (this.isVisible) return;
    //toggle on or off:
    if (!this.isFlag) {//no flag
	if (this.grid.flagsAvail==0) return;//none available
	this.grid.flagsAvail--;
    } else {//has flag, remove it
	this.grid.flagsAvail++;
    }
    this.isFlag = !this.isFlag;

    if (gui) gui.flag(this.token,this.isFlag);
    this.grid.checkWin();
}

// Start:
function makeMinefield(w,h,bombs) {
    window.minefield=
	new Grid(w,h,bombs);
}

Version 7: Bonus Variant: Wrap-around grid

mines7.js 

// Stage 1: basic coordinate system
// Stage 2: add mines,count
// Stage 3: add visibility,reveal
// Stage 4: add flags
// Stage 5: add win/lose
// Stage 6: attach UI!
// Stage 7: subclass Cell for wrap-around

if (typeof gui == "undefined")
    var gui = null;

//--- Grid ---
function Grid(celltype,w,h,bombs) {
    this.length = w*h;
    this.w = w;
    this.h = h;
    this.numVisible = 0;

    if (gui) this.token = gui.makeGrid(this,w,h);

    for (var n=0; n<this.length; n++) {
	this[n] = new celltype(n,this);
    }

    //limit bomb density to <1/4
    var maxbombs = Math.floor(this.length/4);
    if (bombs>maxbombs) bombs=maxbombs;

    this.numBombs = bombs;
    this.flagsAvail = bombs;
    while (bombs) {
	n = Math.floor(Math.random()*this.length);
	var cell = this[n];
	if (!cell.isBomb) {
	    cell.placeBomb();
	    bombs--;
	}
    }

}

Grid.prototype.XYtoN = function(x,y) {
    return (y*this.w)+x;//-->n
}

Grid.prototype.NtoX = function(n) {
    return n % this.w;//-->x
}

Grid.prototype.NtoY = function(n) {
    return Math.floor(n/this.w);//-->y
}

Grid.prototype.reveal = function(x,y) {
    var n=this.XYtoN(x,y);
    this[n].reveal();
}

Grid.prototype.revealAll = function() {
    for (var n=0; n<this.length; n++) {
	this[n].reveal(true);
    }
}

Grid.prototype.flag = function(x,y) {
    var n=this.XYtoN(x,y);
    this[n].flag();
}

Grid.prototype.checkWin = function() {
    if (this.flagsAvail==0 && (this.numVisible>= this.length-this.numBombs))
	this.youWin();
}

Grid.prototype.youWin = function() {
    alert("You win!");
}

Grid.prototype.youLose = function() {
    alert("KABOOM!");
    this.revealAll();
}

//--- Cell ---
function Cell(n,grid) {
    if (!grid) return;
    this.grid = grid;
    this.n = n;
    this.x = grid.NtoX(n);
    this.y = grid.NtoY(n);
//    this.count = 0;
    this.isBomb = false;
    this.isVisible = false;
    this.isFlag = false;

    if (gui) this.token=gui.makeCell(grid.token,this,this.x,this.y);
}

Cell.prototype.neighbors = function() { //returns list of grid positions
    // four booleans (true if NOT adjacent to respective grid edge):
    var goW = (this.x>0);
    var goE = (this.x+1 < this.grid.w);
    var goN = (this.y>0);
    var goS = (this.y+1 < this.grid.h);

    var result=[];
    if (goN) {
	var northN = this.n-this.grid.w;
	if (goW) result.push(northN-1);//NW
	result.push(northN);//N
	if (goE) result.push(northN+1);//NE
    }
    if (goW) result.push(this.n-1);//W
    if (goE) result.push(this.n+1);//E
    if (goS) {
	var southN =this.n+this.grid.w;
	if (goW) result.push(southN-1);//SW
	result.push(southN);//S
	if (goE) result.push(southN+1);//SE
    }
    return result;
}

Cell.prototype.placeBomb = function() {
    this.isBomb = true;
//    var neighbors = this.neighbors();
//    for (var i=0; i<neighbors.length; i++)
//	this.grid[neighbors[i]].count++;
}

Cell.prototype.reveal = function(skipCheck) {
    if (this.isVisible || this.isFlag) return;
    this.isVisible = true;
    this.grid.numVisible++;
    
    if (!skipCheck && this.isBomb) {
        if (gui) gui.reveal(this.token,0,true);
        this.grid.flagsAvail--;
        this.grid.youLose();
        return; 
    }

    var neighbors = this.neighbors();
    var bombCount=0;
    for (var n=0; n<neighbors.length; n++) {
        if (this.grid[neighbors[n]].isBomb)
            bombCount++;
    }
    
    if (gui) gui.reveal(this.token,bombCount,this.isBomb);
    
	if (!skipCheck) this.grid.checkWin();

    if (bombCount==0) {
	for (var i=0; i<neighbors.length; i++)
	    this.grid[neighbors[i]].reveal(true);
    }

}

Cell.prototype.flag = function() {
    if (this.isVisible) return;
    //toggle on or off:
    if (!this.isFlag) {//no flag
	if (this.grid.flagsAvail==0) return;//none available
	this.grid.flagsAvail--;
    } else {//has flag, remove it
	this.grid.flagsAvail++;
    }
    this.isFlag = !this.isFlag;

    if (gui) gui.flag(this.token,this.isFlag);
    this.grid.checkWin();
}

//--- WraparoundCell ---

function WraparoundCell(n,grid) {
    Cell.call(this,n,grid);
}
WraparoundCell.prototype=new Cell(0,null);
WraparoundCell.prototype.constructor = WraparoundCell;

WraparoundCell.prototype.neighbors = function() {
    var w=this.grid.w;
    var len=this.grid.length;
    //delta added to n to obtain neighbor in each dir:
    var goW = (this.x>0)?-1:w-1;
    var goE = (this.x+1 < w)? 1:1-w;
    var goN = (this.y>0)? -w:len-w;
    var goS = (this.y+1 < this.grid.h)? w:w-len;
    var result=[];

    var northN =this.n+goN;
    var southN =this.n+goS;
    result.push(northN+goW);
    result.push(northN);
    result.push(northN+goE);
    result.push(this.n+goW);
    result.push(this.n+goE);
    result.push(southN+goW);
    result.push(southN);
    result.push(southN+goE);

    return result;
}

// Start:
function makeMinefield(w,h,bombs) {
    window.minefield=
	new Grid(Cell,w,h,bombs);
}
function makeWrappingMinefield(w,h,bombs) {
    window.minefield=
	new Grid(WraparoundCell,w,h,bombs);
}

Interface

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