JS 2, String array methods, If and Switch statements, let and var properties, HTML forms and js validation and setInterval()/setTimeout()

We can use index and array methods on strings:

//We can use the index but can't edit a string like an array
"string"[2]        //"r"
"string"[2] = "e"  //"string"

//We can also use more specific methods
"string".charAt(2)     //"r"
"string".charCodeAt    //114
slice(), substring(), concat() and split() methods on strings

We can't use splice() on strings, slice() doesn't edit the string, it creates a new one:

//slice(x,y), negative values start from the end of the string/array
let str = "Apple, Banana, Kiwi";
str.slice(7 , str.length)    //"Banana, Kiwi"
str.slice(6, -2);            //" Banana, Ki"
str.slice(5)                 //", Banana, Kiwi"

substring( [starting index, number of digits] ) can only be used on strings: 

str.substring(0, 5)          //"Apple"

concat() can work on strings too:

let ora= "orange"
str.concat("and ", ora)    //"Apple, Banana, Kiwi and orange"

split() returns an array from a pattern separator:

//it checks for (" ") and creates an array element from it
//We can limit the number of array elements

let siamo= "are you ready"
siamo.split("", 5)         //['a', 'r', 'e', ' ', 'y']
siamo.split(" ", 5)        //['are', 'you', 'ready']
siamo.split("   ", 5)      //['are you ready']

But arrays and strings are not of the same type:

typeof ["w","e","e","l"] //object
typeof "well"            //string

We can convert each other using split(), toString() and join():

typeof "well".split("") == "w","e","e","l"     //object
["w","e","e","l"].toString()                   //"w,e,e,l"   
["w","e","e","l"].join("")                     //"well"

We use the Typeof operator and Array.isArray() to check the type of data:

Array.isArray("indeed".split(" ") ) //true
Array.isArray("indeed" )            //false

String data contains only 1 value and so is primitive, while Object contains more.

If and Switch

The IF statement executes a block of code if its condition is true:

//else_if for other conditions, _else_ if no condition is true

if(true){
    return console.log("true")
}else if(){
}
else{
    return console.log("false")
}

For multiple conditions we can use logical operators:

//We can't 3<x<10 so we have to repeat the variable

if ((x === 5 || y > 3 || x <= 10) && (loggedIn || userName === 'Steve')) {
  //Run the code
}

if(x>3 && x<10){
  //code here
}

We can also use switch and cases:

Switch (if) statement

We use break to finish each case and default as the else{}.

switch (5) {
    case true:
	return console.log("number real")
	break;
    case false:
	return console.log("number not")
	break;
     default:
	return console.log("sdell")
}

If more cases share the same code.

const month = new Date().getMonth();

switch (month) {
	case 0:
	case 1:
	case 2:
		console.log("Winter");		// January, February, March
		break;
	case 3:
	case 4:
	case 5:
		console.log("Spring");		// April, May, June
		break;
	...
	default:
		console.log("No month");
}

//We use break to close the case and default as the last case

switch (5) {
    case true:
	return console.log("number real")
	break;
    case false:
	return console.log("number not")
	break;
     default:
	return console.log("sdell")
}

//if more than one case shares the same code 
const month = new Date().getMonth();

switch (month) {
	case 0:
	case 1:
	case 2:
		console.log("Winter");		// January, February, March
		break;
	case 3:
	case 4:
	case 5:
		console.log("Spring");		// April, May, June
		break;
	...
	default:
		console.log("No month");
}

Strings, numbers, and objects can have falsy values, which will be false in boolean:

- 0
- '': Empty string
- null
- undefined
- NaN

//typeof null being an object is an old bug
typeof null + " " + typeof undefined        //object, undefined

//null is a primitive value for "empty" variables
let vuoto = null
console.log( vuoto === null )        //true, assigned null value

//undefined is for the absence of a value, or non-existent keys in objects
let name;
console.log( name === undefined)     //true, initialized not assigned variable

Ternary operator is a simplified conditional operator:

//The variable is used for returned values, works for if/else

let greeting = ( condition) ? 
	console.log("code returned for true")
	: 
	console.log("code returned for false");

We can use the nullish coalescing operator for null/undefined values:

//using the ?? we get returned the right value if the left one is null/undefined

let amount = null;
let amount1 = 2

console.log(amount ?? 1)        // 1
console.log(amount1 ?? 1)       // 2

Text filter javascript and DOM exercise

Check this code implementing regex, forms, and setInterval:

Let, Var and differences

In 2015 the let keyword for variables was introduced:

//the main difference is in the scope of the variable

var z = 'hello';
var z = 'world';     //will allow to re-declare the variable
let j = 'hello';
let j = 'world';     //Syntax error on re-using the let keyword to re-assign

//Contrary to var global scope, let is limited to its block of code
let greeter = "hey hi";  
let times = 5;  
if (times > 3) {  
   var hello = "Say Hello JavaTpoint";   
   console.log(hello) 
}  
console.log(hello)    //it shouldn't be declared, error when using let

//var can be called out of its block, which is prone to errors

The different scope can make the var accidentally change its value:

//here for example when calling the cb to see how the var=i changes in the for loop
//BUT being var global scoping it will change to 5 before the .push 

var callbacks = [];
(function() {
  var i;
  for (i = 0; i < 5; i++) {
    console.log("when does " + i + " change")
    callbacks.push( 
      function() { return i; } 
      );
  }
  console.log("is this " + i)
})();

console.log(callbacks.map( 
  function(cb) { return cb(); }  //resulting in [5, 5, 5, 5, 5]
  )
);

//If we had let the for loop would have increased and pushed the [1,2,3,4,5] instead

Regex and string matching

We use Regular Expression Search Methods to search patterns in strings:

//text pattern between // and letters for modifiers 
let pattern = /Lorem/i        //i is for case insensitive

vipsum.search(pattern)    //will return the index of the first match 
vipsum.search(/loreM/i)   //we can also put directly

To search more specific patterns with regex we can use:

//We get an array of the matched [letters] in the text
vipsum.match(/[qvgk]/g)    //['g', 'q', 'g', 'q', 'g', 'v', 'v', 'q']

//First match between 2 values (if we use g then it's the same as [])
vipsum.match( /[q|x]/ )    // ['m', index: 4, input: 'Lorem ...']

//We can also check for whitespaces (\s includes newlines)/new lines (\n only lines)
vipsum.match( /\s/g )    //(62)[' ', ' ', ' ', ' ', ' ', ' ', '\n',...]
vipsum.match( /\n/g )    //(4)['\n', '\n', '\n', '\n']

//Match the string /s+/ case insensitive and on the global string
vipsum.match( /s+/ig )    //(10)['s', 's', 's', 'SSS', 's', 's', 's', 's', 's', 's']

We use the test() method to true/false if a pattern is present on the string, while exec() to return a pattern index match and input:

//The regex goes first [pattern].test/exec(html) 
const primo= /lorem/i;
console.log( primo.test(html) );    //true 

//input being the entire string is taken upon
let yull = /ipsum/gi.exec(html)
console.log(yull)                  //['ipsum', index: 6, input: 'Lorem ipsum ... ']

Then we have the .search() and .match() :

//.search() will return the index of the match if present
let visit = "Plant a tree!";
let n = visit.search(/plants/i);
console.log(n)                    //if no match then -1 return

//.match will return an array like exec.() but can contain multiple matches
console.log( "best is best".match(/best/g) )    //[best, best]

//Here we have html.search/match(___) when pattern is in ()

About variables used as new regex patterns:

//We can get a value from the input to then create a new RegExp
let oltre = document.getElementById("cosa").value 

const regex =  new RegExp( "(" + oltre + ")", 'ig');    // /regex/ig
const kok = html.innerHTML.split( regex );

About stricter filters:

//We can use renex on a split() method (for white spaces for example)
let spaced = html.innerHTML.split( /(\s)/ )

//while if we want to check if the input is only white spaces we do this
primo.match(/^\s*$/)

HTML forms and JS validation

We use forms to collect user input:

<Form> is a container for inputs/labels and the action attribute allows us to redirect the form after submission:

<form id="formula" method="post" action="https://www.ilpost.it/">
  <label for="unon">First</label>
  <input type="text" id="uno" name="unon"> <br>

  <input type="radio" id="html" name="fav_language" value="HTML">
  <label for="html">HTML</label><br>
  <input type="radio" id="css" name="fav_language" value="CSS">
  <label for="css">CSS</label><br>

  <input type="checkbox" id="vehicle1" name="vehicle1" value="Bike">
  <label for="vehicle1"> I have a bike</label><br>
  <input type="checkbox" id="vehicle2" name="vehicle2" value="Car">
  <label for="vehicle2"> I have a car</label><br>

  <input type="submit" value="submit">
</form>
form with input labels

For the check status and value of radio/checkbox buttons, we use the attributes .checked and value:

Radio and checkbox buttons submit

In a form we can put label and input in a different order:

//we can separate or embed the tags
<form id="formula" method="post">
    <input type="radio" id="html" name="fav_language" value="HTML">
    <label for="html">HTML</label><br>
    
    <input type="checkbox" id="vehicle1" name="vehicle1" value="Bike">
    <label for="vehicle1"> I have a bike</label><br>

    <label for="c1"> 
        <input type="checkbox" name="color" value="red" id="c1">Red
    </label>
    <label for="c2">
        <input type="checkbox" name="color" value="green" id="c2"> Green
    </label>
    <label for="c3">
        <input type="checkbox" name="color" value="blue" id="c3">Blue
    </label>

    <input type="submit" value="submit">
</form>

In javascript, we check the check state/value of a single button or queryselect() many with the same attribute.

const formato = document.getElementById('formula');

let gippin = document.querySelector("#html")
let dess = document.querySelector("#vehicle1")

//.checked will return true/false if the button was clicked
//.value will return the value attribute even if not clicked
formato.addEventListener("submit", (event) => {
   event.preventDefault();

   console.log( gippin.checked )                 
   console.log( dess.value)                     

    let checkboxes = document.querySelectorAll('input[name="color"]:checked');
    let values = [];

    checkboxes.forEach((checkbox) => {
        values.push(checkbox.value);
        console.log( checkbox.value )
    });

}) 
//we need the queryselector() inside the event listener, will push
//all the values of the checked buttons in an array
Checkbox/radio buttons form submit

We use select to create a dropdown list.

Select in the DOM and JS

The <select> in the HTML will be:

<label for="weather">Select the weather type today: </label>
<select id="weather">
    <option value="">--Make a choice--</option>
    <option value="black">Sunny</option>
    <option value="white">Ameno</option>
</select>

<p id="well"></p>

When the select gets used it triggers the "change" event, we also use a js function to update the CSS style. 

//we use the value of the <select> list
const select = document.querySelector('#weather');
const para = document.querySelector('#well');

function setWeather() {
  const choice = select.value;

  function update(bgColor, textColor) {
    para.style.backgroundColor = bgColor;
    para.style.color = textColor;
  }

  switch (choice) {
    case 'black':
      para.textContent = 'Text black on white space';
      update('white','black')
      break;
    case 'white':
      para.textContent = 'white text on black space';
      update('black','white')
      break;
    default:
      para.textContent = '';
  }
}

//We need a default "" case for when the value is ""

To push JS content into HTML DOM we can use AppendChild() or innerHTML.

AppendChild() will record and update its content while innerHTML will reset it.

<input type="text" name="name" id="txt_name" > 
<button onclick="kek()">
    ADD
</button>

<p id="demo"></p>
<p id="demani"></p>

//To push/append an element it needs to be createTextNode()
function kek(){
  let inside = document.getElementById("txt_name").value
  
  let textnode = document.createTextNode(inside+ " ");
  document.getElementById("demo").appendChild(textnode);  //will show past inputs

  let text;
  if (isNaN(inside) || inside < 1 || inside > 10) {
    text = "Input not valid";
  } else {
    text = inside + " I guess";
  }
  document.getElementById("demani").innerHTML = text;  //only current text value
}

We can replace() an innerHTML element using regex.

let ipsum = document.getElementById("lorem")
let gulp =  ipsum.innerHTML.replace(/MagNi/gi, "Picoolo") 
ipsum.innerHTML = `<span >` +  gulp + `</span>`

Setinterval() and SetTimeout()

The setInterval() method calls a function at specified millisecond intervals, it returns an ID that can be used to stop it using clearInterval():

<button class="btn btn-success" onclick="real()">Orario</button>
<button class="btn btn-secondary" onclick="stopped()">ferma</button>
<h3 id="attuale"></h3>

We setInterval() using the new Date() object (that updates itself) to create a clock:

let attuale = document.getElementById("attuale")
let att;

//we create the variable for the setInterval() ID and use it to stop it
function current(){
    const date = new Date();
    attuale.innerHTML = date.toLocaleTimeString();
}

function real(){

    att= setInterval(() => {
        current()
    }, 1000);
}

function stopped(){
    clearInterval(att)
}
setInterval() used with new Dat

The setTimeout() method calls a function once after a number of milliseconds:

To repeat a setTimeout() function :

<h2 id="stime" class="text-center"></h2>

//we need to put the setTimeout() in its own function
//and can be stopped with clearTimeout()
let stime= document.getElementById("stime")

function blue(){
    let arti= new Date()
    stime.innerHTML= arti.toLocaleTimeString();
    setTimeout( blue, 1000 )
}

blue()
new Date() with setTimeout()
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