slides.com/jkohlin/js-2/live

lecture 2

Chapter 2 +

Sneek peak of Chapter 3 (83-85)

my appologies

😔

It was a rough start.

what do you know so far?

JavaScript in HTML

Or in External file

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Document</title>
</head>
<body>
  <h1>Hello, world!</h1>

  <script>
  alert("to be safe, put me here");
  </script>
</body>
</html>
<!DOCTYPE html>
<html lang="en">
<head>
  <title>Document</title>
</head>
<body>
  <h1>Hello, world!</h1>  

  <script src="lab2_kojo.js"></script>
</body>
</html>
alert("to be safe, put me here");

lab2_kojo.js

what's with the var thing

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

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

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"

AKA declare variables

You might see let and const as well

const pi = Math.PI;

if (pi > 3) {
    let raspberry = "pie";
}
pi = raspberry; // double error

// raspberry is not defined
// Identifier 'pi' has already been declared

let will only survive the {code block} where it is declared

const can not have its value changed

var b = 20;
var c = 3;

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

ALL EXPRESSIONS MUST FIRST RESOLVE TO A VALUE

-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 ;
var foo;  // foo was declared
foo = 42; // foo changed value from undefined to 42

foo;           // Nothing is changed. Nothing is done
Math.random()  // Something is done, but it's not saved

ONE STATEMENT PER LINE;

SHOULD ALWAYS CHANGE OR DO SOMETHING

statements

foo;           // Nothing is changed. Nothing is done
Math.random();  // Something is done, but it's not saved
42;
prompt("write something")

The compter executes each statements,

then forgets it ever happened. 

That's why saving to memory is important

not statements

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

comparison operators

AKA Boolean operators  - outcome is either true or false.

var x = 12; // number
var y = "12"; // string

// == makes a loose comparison 
x == y // true

// === makes a strict comparison (value AND type)
x === y // false

// ! means NOT so...
5 != 5 // false, because we are testing if "5 is not equal to 5"

// !== is the strict not equal operator
5 !== "5" // true, because the number 5 is not strictly equal to the string "5" 

// < is the value on the left side less than the right side
12 < "13" // true

// > is the value on the left side greater than the right side
14 > 13 // true

// <= means less than OR equal to
// >= means greater than OR equal to

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
|| = OR (2 pipe characters)

while loop

var counter = 0;

while (counter < 7) {

    counter = counter + 1;
    concole.log('Loop no: ' + counter);

}
// > Loop no: 1
// > Loop no: 2
// > Loop no: 3
// > Loop no: 4
// > Loop no: 5
// > Loop no: 6

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

}

while loop

while (myVariable < 10) {

    myVariable = prompt('guess a number');
    if ( myVariable == 0 ) {
      break;
    }
}

if, else if, else

var foo = 12;
var bar = 30;

if (foo == 10) {
    alert('only runs if foo is 10');

} else if (bar < foo) {
    alert("only runs if foo isn't 10 but more than bar");

} else {
    alert('only runs if none of the above is true');
}

alert('this statement will always run');

 Built in popup functions

// 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

Math - Built in functions

Math - random

The world famous recipe for a random number interval

var min  = 1;  // any number you want
var max  = 10; // a higher number than min.

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

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()

functions

a sneak peak

Functions

reusable save-for-later-code-blocks

Encapsulated scripts that won't run until you tell it to run, or until the user tells it to run. 

 

Functions can be versatile and dynamic by using parameters. That way, one function can do different things.

 

One of the most important part of a modern programming language.

function     (            ) {





}

The function keyword begins a function definition

function bark(            ) {





}

Next we give the function a variable name, like bark

function bark(name, weight) {





}

We call these the parameters
and, separate, them, by, commas

function bark(name, weight) {
  if (weight > 20) {
    alert(name + " says WOOF WOOF");
  } else {
    alert(name + " says woof"):
  }
}

We call this the body of the function

function bark(name, weight) {
  if (weight > 20) {
    alert(name + " says WOOF WOOF");
  } else {
    alert(name + " says woof"):
  }
}
bark('Jane', 21);
bark('John', 19);

Call with Different ARGUMENTS

ARGUMENTS vs parameters

Define with PARAMETER(S)

The first argument you pass to a function will be the value of the first parameter.

parameters / Arguments

See page 83-86

Parameters/arguments

function times(a, b) {
    document.write( a * b );
}

times(25, 4); // will write 100 to the page

By adding parameters to your function it can be used for more than one thing. 

Let's try it out: