Here We Know about JavaScript from Beginning...

1. What is JavaScript?

=> JavaScript is a Synchronous Single Threaded language

• Single-threaded => JavaScript runs one command at a time.
• Synchronous => It Means JavaScripit runs command in Specific order, i.e. It runs next line when it finish current line.

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2. How does JavaScript Work?

• Everything in JavaScript happens inside Execution Context. -Execution Context contains two types of functionality:-

  • I) Memory => Everything here we kept in key value pair, and it is also known as `variable environment.

  such as:- a : 10;

  • II) Code => In this box we write code that runs from up to down, it is also known as Thread of Execution.

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3. What is Hoisting in JavaScript?

=> JavaScript Hoisting refers to the process whereby the interpreter appears to move the declaration of functions, variables or classes to the top of their scope, before execution of the code.

• Hoisting allows the function to be safely used in code before they have been declared.
• In JavaScript variables can be used before they have been declared.

Example:-

The let and const keyword in Hoisting.

=> Variable declared with let and const are also hoisted, but they cannot be accessed before their declaration and initialized and this is also called the temporal dead zone.

• this is happening because var is global scope but let and const is block scope, and they all are hoisted to the top of their scope.

• To avoid the Temporal dead zone always declare and initialize the variable at the top of the code.

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4. How does function work in JavaScript?

• Function is one of the fundamental blocks in javascript.
• A Function is similar to a procedure - a set of statements that performs a task or calculates a value.
• function should take some input and return am output, where there is some relationship between the input and output.

=> For declare function, we have to follow some keywords:-

• The name of the function(we always have to remember that we should give a name of the function, otherwise we cannot invoke that function).
• A list of parameters to the function, is enclosed in parentheses and separated by commas.
• The JavaScript statements that define the function, enclosed in brackets, {...}.

function square(num){
return num*num
}

• Here input is num and output is num*num and the relationship between them is that num return square value of itself.

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5. What is window in JavaScript?

• The window object is supported by all the browsers, it represents the browser's window.
• All JavaScript objects, functions, variables automatically become members of the window object.
• Global variables are the properties of the window object.
• Global functions are the methods of the window object.
• Even the HTML document object(DOM) is the property of the window object.

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6. What is the this keyword in JavaScript?

• In JavaScript this keyword refers to the object it belongs to.

It has different values depending on where it is used

• In method this refers to the owner object.

const person = {
  firstName: "John",
  lastName: "Doe",

  fullName: function () {
    return this.firstName + " " + this.lastName;
  },
};
let fullname = person.fullName()
console.log('fullname:', fullname)

// output => fullname: John Doe

• Alone this refers to the global object and In function also refers to the global object.

let x = this;

// output => {} or [object Window]

• In function, in strict mode, this is undefined.

"use strict";
function myfunc() {
  return this;
}

let result = myfunc();
console.log('result:', result)

// output => result: undefined

• Methods like call() and apply() can refer this to any object.

function Emp(id, name) {
  this.id = id;
  this.name = name;
}
function PermanentEmp(id, name) {
  Emp.call(this, id, name);
}
function TemporaryEmp(id, name) {
  Emp.call(this, id, name);
}
var p_emp = new PermanentEmp(101, "John Martin");
var t_emp = new TemporaryEmp(201, "Duke William");
console.log('p_emp:', p_emp)
console.log('t_emp:', t_emp)

/*
OUTPUT :-
p_emp: PermanentEmp { id: 101, name: 'John Martin' }
t_emp: TemporaryEmp { id: 201, name: 'Duke William' }
*/

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7. Difference between undefined and not defined.

=> undefined

• It works like when we declare a variable in code but did not assign the value before printing the variable name.
• It is stored in the memory heap.

=> not defined

• It works like when we did not declare a variable but tried to call the variable name.
• It does not store in the memory heap.
• It gives a Reference Error.

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8. What is the scope of JavaScript?

=> Scope means where you can access a specific variable or function in our code.

There is Two types of Scope in JavaScript

1. Global Scope
2. Local Scope

Global Scope

• When we start writing code in JavaScript documents, we are already in the global scope.
• The variable is in the Global scope if it is defined outside of a function.

var name = "Abhishek";
console.log(name) // output = Abhishek
function callName(){
console.log(name)
}
callName() // output = Abhishek

Here Name is in Global Scope

Local Scope

• Variable or function defined inside of a function are in Local Scope nad they have different scope for every call of that function.
• We can use the Same name of the variable in different functions.

function getName(){
var name = "Abhishek"
}

console.log(name) // it gives name is not defined

getName() // output = Abhishek

Here name is in Local Scope that's why when we try to console name outside the function it gives us `ReferenceError: name is not defined, and we try to call the function it returns the value of name.

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9. What is Lexical Scope in JavaScript?

• Lexical Scope means that in nested function, the inner function has access to the variable and other resources of their parent function.
• That means the child function is lexically bound to the execution context of their parent function.

function parent() {
  var role = "gurdian";
// name and like is not accessible here
  function child() {
    var name = "Alex";
// like is not accessible here
// role is accessible here
    function grandchild() {
      var like = 15;
// role and like is accessible here
    }
  }
}

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10. What is Reference Error in JavaScript?

=> The Reference error object represents an error when the variable does not exist or haven't been initialized in the current scope is referenced.

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11. What is a syntax error in JavaScript?

=> A syntax error occurs when a programmer writes an incorrect line of code.

• If the `syntax error occurs in the compiler, then the code does not work.
• Most Syntax error involves missing punctuation and misspelled name.

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12. What is type error or when does it occur?

• when an operand or argument passed to a function is incompatible with the expected by the operator or function.
• When trying to modify a value that can not be changed.
• when trying to use a value in an inappropriate way.

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13. Difference between var , let and const ?

=> var

• It is the oldest keyword in javascript to declare a variable name.
• It is Global and functional scoped, it can access from anywhere. if we assign var variable in global scope we can access it inside the function, if we declare a var variable inside the function then we can not access it in the global scope.

var a = 10;
function checkscope(){
console.log(a)
}
checkscope() // output = 10

function checkscope(){
var a = 12;
}
console.log(a) // output = ReferenceError: a is not defined

• var variable is re-declareable and updateable.

var a = 90;
console.log(a) // 90
var a = "Something"
console.log(a) // Something

a= 1;
console.log(a) // 1

=> let

• It is an improved version of the var variable.
• The Scope of the let variable is Block Scoped. It can nor accessible outside a particular block.

let a = 10;
    function f() {
        let b = 9
        console.log(b); // 9
        console.log(a); // 10
    }
    f();

let a = 10;
    function f() {
        let a = 9

        console.log(a); // 9
    }
    f();
console.log(a); // 10

• It can be updated but can not be re-declared.

  let a = 8;
  a = 10;
  console.log(a); //10
  

let a = 8;
let a = 10;
console.log(a);  // SyntaxError: Identifier 'a' has already been declared

-It can be declared without initialization.

=> const

• The Scope of the const variable is Block Scope.
• It cannot be updated or re-declared into the scope.
• It cannot be declared without initialization.