Javascript | Important Questions & Functionality

 MYSQL | Important Questions & Functionality

1. Difference Between var , let and const variables

var	let	const
The scope of a var variable is functional or global scope.	The scope of a let variable is block scope.	The scope of a const variable is block scope.
It can be updated and re-declared in the same scope.	It can be updated but cannot be re-declared in the same scope.	It can neither be updated or re-declared in any scope.
It can be declared without initialization.	It can be declared without initialization.	It cannot be declared without initialization.
It can be accessed without initialization as its default value is “undefined”.	It cannot be accessed without initialization otherwise it will give ‘referenceError’.	It cannot be accessed without initialization, as it cannot be declared without initialization.
These variables are hoisted.	These variables are hoisted but stay in the temporal dead zone untill the initialization.	These variables are hoisted but stays in the temporal dead zone until the initialization.

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2. Loop

JavaScript supports different kinds of loops:

1. for - loops through a block of code a number of times
2. for/in - loops through the properties (Keys) of an object
3. for/of - loops through the values of an iterable object
4. while - loops through a block of code while a specified condition is true
5. do/while - also loops through a block of code while a specified condition is true

1. for : loops through a block of code a number of times

Syntax:

for (expression 1; expression 2; expression 3) {
  // code block to be executed
}

Example:

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

<script>

let text = "";

for (let i = 0; i < 5; i++) {
  text += "The number is " + i + "<br>";
}

document.getElementById("demo").innerHTML = text;

</script>

Output:

The number is 0
The number is 1
The number is 2
The number is 3
The number is 4

2. for/in : loops through the properties (Keys) of an object

Syntax:

for (key in object) {
  // code block to be executed
}

Example:

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

<script>

const person = {fname:"John", lname:"Doe", age:25};

let text = "";
for (let x in person) {
  text += person[x];
}

document.getElementById("demo").innerHTML = txt;

</script>

Output:

John Doe 25

3. for/of : The for of statement loops through the values of any iterable object:

Syntax:

for (variable of array) {
  // code block to be executed
}

Example:

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

<script>

const cars = ["BMW", "Volvo", "Mini"];

let text = "";
for (let x of cars) {
  text += x;
}

document.getElementById("demo").innerHTML = text;

</script>

Output:

BMW
Volvo
Mini

4. while : Loops can execute a block of code as long as a specified condition is true.

Syntax:

while (condition) {
  // code block to be executed
}

Example:

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

<script>

let text = "";

let i = 0;

while (i < 10) {
  text += "The number is " + i;
  i++;
}

document.getElementById("demo").innerHTML = text;

</script>

Output:

The number is 0
The number is 1
The number is 2
The number is 3
The number is 4
The number is 5
The number is 6
The number is 7
The number is 8
The number is 9

5. do/while : also loops through a block of code while a specified condition is true

Syntax:

do {
  // code block to be executed
}
while (condition);

Example:

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

<script>

let text = ""

let i = 0;

do {
  text += "The number is " + i;
  i++;
}
while (i < 10);

document.getElementById("demo").innerHTML = text;

</script>

Output:

The number is 0
The number is 1
The number is 2
The number is 3
The number is 4
The number is 5
The number is 6
The number is 7
The number is 8
The number is 9

2. Some Important Array Function and Method

1. Array.forEach(): The forEach() method calls a function (a callback function) once for each array element.

<script>

const numbers = [45, 4, 9, 16, 25];

let sum = 0;

numbers.forEach(myFunction);

document.getElementById("demo").innerHTML = sum;

function myFunction(value, index, array) {

  sum += value ; 

}

</script>

2. Array.join(): The join() method returns an array as a string. The join() method does not change the original array. Any separator can be specified. Separator is optional and the default is comma (,).

Syntax:

array.join(separator)

Example:

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

<script>

const fruits = ["Banana", "Orange", "Apple", "Mango"];

let text = fruits.join();        //output: Banana,Orange,Apple,Mango

let text = fruits.join(" and "); //output: Banana and Orange and Apple and Mango

document.getElementById("demo").innerHTML = text;

</script>

3. Ajax Form Submission

$(document).ready(function(){

    //Create Form

    $('#createStatsForm').submit(function(event){

      event.preventDefault();

      $("#submitBtn").attr('disabled', true);

      var form=$("#createStatsForm")[0];

      var data= new FormData(form);

           

      //data.append('header', headerValue); //append data by append function

           

      $.ajax({

        type: "POST",

        url: "{{url('dashboard')}}",

        headers: {

          "X-CSRF-TOKEN": "{{csrf_token()}}"

        },

        data: data,

        processData: false,

        contentType: false,

        success: function(data){

          if(data.status == 200){

            closeModel();

            console.log(data);

          }

          else if(data.status == 400){

            showError(data.message, "showMessage");

            if($("#submitBtn").length) {

              $("#submitBtn").attr('disabled', false);

            }

          }

          else{

            console.log(data);

          }

        },

        error: function(e){

          console.log(e.responseText);

        }

      });

    });

3. Add Event Listener in Ajax

       

        document.getElementById(element_id).onkeyup = function() {function_name()};  //first way

        document.getElementById(element_id).addEventListener("keyup", function_name());  //second way

2. Break and Continue

1. Break Statement: You have already seen the break statement used in an earlier chapter of this tutorial. It was used to "jump out" of a switch() statement.

The break statement can also be used to jump out of a loop.

Example:

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

<script>

let text = "";

for (let i = 0; i < 10; i++) {
  if (i === 3) { break; }
  text += "The number is " + i + "<br>";
}

document.getElementById("demo").innerHTML = text;

</script>

Output:

The number is 0
The number is 1
The number is 2

1. Continue Statement: The continue statement breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.

Example: 

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

<script>

let text = "";

for (let i = 0; i < 10; i++) {

  if (i === 3) { continue; }

  text += "The number is " + i + "<br>";

}

document.getElementById("demo").innerHTML = text;

</script>

Output:

The number is 0
The number is 1
The number is 2
The number is 4
The number is 5
The number is 6
The number is 7
The number is 8
The number is 9