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JS3

Johan Kohlin

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Client side programming

Lecture #3

slides.com/jkohlin/js3/live

Chapter 1 (loops) +

Chapter 2

what was the thing with the var thing

var foo = 42; 
/* is the same as:
 * var foo;
 * foo = 42;  
 */

// Do not write:
var baz;
var baz = prompt('gimme a number');
// only declare once

What does it mean?
Can we use it alone?
When should we NOT use it?

var ≈ name memory cell

var height; // find an empty memory cell and name it height
height = 187; // now put 187 in the memory cell called height

"RAM memory"

types

3.14159
42
0.01
-12
10000
"anything within double quotes"

'or "single" quotes'

"add a line break \n Like this" 
// either
true
// or.. 
false

typeof

typeof 1337 // number
typeof "42" // string
typeof true // boolean

/* Put parentheses 
 * around expressions
 */
typeof ("20"+1)   // string
typeof ("2" == 2) // boolean

Test the type of a value or expression

Math - Built in functions

Math - random

The world famous recipe for a random number 0-n

var max  = 10;

var random = Math.floor(Math.random() * max + 1);

Only for numbers from 0 to max

Math - random

The famous recipe for any random number interval

var min  = -15;  // ANY number you want
var max  = 10; // a higher number than min.

var random = Math.floor(Math.random() * (max-min+1) + min);

Operators

combining comparison operators

AKA logical operators  - outcome combo is either true or false.

var x = 12;
var y = "12"; 

( x === y || x == y ) 
// true because ONE comparison MUST be true


(x == y && typeof x == typeof y) 
// false, because BOTH comparison MUST be true

&& = AND (2 ampersands)
|| = OR (2 pipe characters)

important

remember !

Expressions are always resolved

to a single value

...before assigned to a variable or passed to a function

var b = 20;
var c = 3;

var a = b * 2 + Math.round( c / 2 ) ;

ALL EXPRESSIONS MUST FIRST RESOLVE TO A VALUE before assigned

-start from the left (if possible)

expressions

var b = 20;
var c = 3;

var a = b * 2 + Math.round( c / 2 ) ;


var b = 20;
var c = 3;

var a = 20 * 2 + Math.round( c / 2 ) ;


var b = 20;
var c = 3;

var a = 20 * 2 + Math.round( c / 2 ) ;


var b = 20;
var c = 3;

var a =   40  + Math.round( c / 2 ) ;
var b = 20;
var c = 3;

var a =   40  + Math.round( c / 2 ) ;
var b = 20;
var c = 3;

var a =   40  + Math.round( c / 2 ) ;
var b = 20;
var c = 3;

var a =   40  + Math.round( c / 2 ) ;
var b = 20;
var c = 3;

var a =   40  + Math.round( 3 / 2 ) ;
var b = 20;
var c = 3;

var a =   40  + Math.round( 3 / 2 ) ;
var b = 20;
var c = 3;

var a =   40  + Math.round(  1.5  ) ;
var b = 20;
var c = 3;

var a =   40  + Math.round(  1.5  ) ;
var b = 20;
var c = 3;

var a =   40  +          2          ;
var b = 20;
var c = 3;

var a =   40  +  2 ;
var b = 20;
var c = 3;

var a =      42    ;
var b = 20;
var c = 3;

var a = 42 ;

COMMUNICATING

Using the browser's built in dialog windows

alert("This is an alert!")

prompt("What's your name")

returns the value as a string

confirm("Do you want to continue?")

returns true or false

// alert does not return anything
alert('just a popup with a message');

//prompt returns a string value
prompt('A popup with a text field'); 

// confirm returns a boolean, true or false
confirm('a popup with a message and \ncancel or OK button'); 
\n in a string will cause a line break

The browser's built in dialog windows

Line breaks demo 

alert("Good morning! \nToday is the first day\nof the rest of your life")

This also works for confirm() and prompt() of course

COMMUNICATING

Using the browser's built in developer console

var msg = "log your variables to console.log() to see their value here";
console.log(msg);
console.error("If your game isn't working you can see why in the console");

COMMUNICATING

By writing directly to the HTML

<body>
    <h1>Welcome!</h1>
    <script>
    document.write('<p>To this dynamic web page</p>');
    </script>
</body>
<body>
    <h1>Welcome!</h1>
    <p>To this dynamic web page</p>
</body>

write on the HTML page where the script tag is

using document.write(htmlCode)

COMMUNICATING

By manipulating the HTML

var h1Pointer = document.querySelector('h1');

h1Pointer.innerHTML = 'Hello, <b>world!</b>';
<body>
    <h1> <- Everything between these are innerHTML -> </h1>
</body>
<body>
    <h1>Hello, <b>world!</b></h1>
</body>

but more on this later

Code blocks

Fencing one or many statements

var guess = 0
var answer

{
  guess++
  answer = prompt('Guess a number')
}

They don't do much on their own ...

Conditional statements

if (destination == "Råslätt") {
  
    "Take the left path"
  
} else {
  
    "Take the right path"
  
}

The if statement

if (true == true) {
  /* Code in this block
   * Will only run if 
   * the test is true. */
}

A test (boolean expression)

What is the value of points?

after the script has run

var points = 0
var skill = 3

if (skill > 3) {
  points = points + 1
}

console.log("Your current score is " + points)

The else statement

Can only follow an if-statement

var points = 0
var extra = confirm('want a point?') 

if ( extra ) {
  points = points + 1 // only if extra is true
  
} else {
    
  points  = points - 1 // only if extra is false
}

Otherwise

The else if statement

Can only follow an if-statement

if ( extra ) {
  points = points + 1 // only if extra is true
  
  
} else if (points == 0){
    
  alert ("Can't get any lower") 
  /* only if extra is false 
   * and points is 0 */
} else {
  points = points -1
}

OK, well, what about...

Otherwise

The while loop

 

while (true == true) {
  /* Code in this block
   * Will run again and again 
   * as long as the test is true. */
}

A test (boolean expression)

Loops

An if-statements that keeps going until it's false

var points = 0;

if ( points < 5 ) {
  points = points + 1;
  console.log( "you have " + points );
}
var points = 0;

while ( points < 5 ) {
  points = points + 1;
  console.log( "you have " + points );
}

A super simple game

var score = 0;
while ( confirm("your current score is " + score) ) {
  score = score +1;
}

while loop

while (boolean expression is true) {

    we are stuck in here until:
    a) the boolean expression is false, or
    b) we encounter the keyword break

}

A simple guessing game

while (true) {

    guess = prompt('guess a number');
    if ( guess == 7 ) {
      break;
    }

}

you know
while-loops…

 

There's another, bold version of it

do {
  
    kiss();
  
} while( confirm("Want another one?") )

do-while-loop:

The self-confident loop –Do first, ask later. 

BTW

Exercise

all caps need to be in the same direction

0 1 2 3 4 5 6 7 8 9

You can say either

"number 0: switch", "number 2: switch" 

or

"number 3 to 5, switch"

How many times must you yell out 

var current = 0
var students = "011100"

while ( current < students.length ) {
  
  if (students.charAt(current) != "O") {
    
    switch() // made up function...
           
  }

  current++ // short for current = current +1
  
}

If we were to program this instead, we could use loops

How to solve a problem:

1. formulate the problem in your own words

2. Poke it with questions, then answer them?

3. Write pseudocode

var legs = 4 Declare legs and set the value to 4
while(legs > 0) { Loop while legs are more than 0
  legs = legs -1;   Decrease the value of legs by 1
} End loop
if (legs == 0) { If legs is 0 then 
  legs = 4;   set it back to 4
} end if

There is no pseudocode standard, So use any words you like.

The company you end up working for might have a style guide though.

 

3. or a flowchart

https://www.draw.io

Flowcharts though, have a standard.

4. Write javascript

Write the pseudocode for...

a) A night at Akademin

b) How to pass an exam

c) Order a foot long at Subway

set money  to 900

set sober as true

set beers to 0

while money is more than 0

  increase beers by 1

  decrease money by 50

  dance()

  if sober then

    orderShot()

  else    

    dance('on table')

  end if

end while

sleep()