10 Important topics of JavaScript

important topics of javaScript

1. Truthy and Falsy values:

In the boolean expression, Truthy values are values that evaluate True and Falsy values are values that evaluate False. Again, in numeric value, falsy values mean 0 (zero) and Truthy values mean 1 (one).

There is six falsy value in JavaScript:

false

null

0

undefined

NaN

""

const a = NaN;
const b = false;
const c = null;
console.log(a); // false
console.log(b); // false
console.log(c); //false

Bellow some truthy values:

true

{ }

[ ]

23

'hello'

newDate()

const a = true;
const b = “hello world”;
const c = 45;
console.log(a); // true
console.log(b); // true
console.log(c); // true

2. Null vs Undefined:

Null means nothing, it is an assigned value. But Undefined means a variable has been declared but not defined.

const a = null;
const b;
console.log(a); // null
console.log(b); // undefined

3. Double equal ( == ) vs Triple equal ( === ):

Double equal (== ) is used for comparing two variables, but it can not check the data type of variable. On the other hand, Triple equal ( === ) is used for comparing two variables and it can check the data type of variable.

const a = ‘hello’;
const b = ‘hello’;
console.log( a == b ); // true
console.log( a === b ); // true
const c = hello;
const d = ‘hello’;
console.log( c == d ); // true
console.log( c === d ); // false

4. Call():

Call and apply ara very similarly. call() is a method that takes the arguments separated by a comma and invokes a function with a given ‘this’ value.

const friend1 = { firstName: ‘Fahim’ , lastName: ‘Ahammed’ };
const friend2 = { firstName: ‘Kuddus’ lastName: ‘Ali’ };
function Friend ( gretting1, gretting2 ) {
console.log( gretting1 + ‘ ‘ + this.firstName + ‘ ‘ + this.lastName + ‘ ‘ + gretting2 );
}
Friend.call( friend1, ‘Hay’, ‘What’s up?’ ); // Hay, Fahim Ahmmed, What’s up?
Friend.call( friend2, ‘Hay’, ‘What’s up?’ ); // Hay, Kuddus Ali, What’s up?

5.apply()

apply() is a method that takes the array of arguments.

const friend1 = { firstName: ‘Fahim’ , lastName: ‘Ahammed’ };
const friend2 = { firstName: ‘Kuddus’ lastName: ‘Ali’ };
function Friend ( gretting1, gretting2 ) {
console.log( gretting1 + ‘ ‘ + this.firstName + ‘ ‘ + this.lastName + ‘ ‘ + gretting2 );
}
Friend.apply( friend1, [ ‘Hay’, ‘What’s up?’ ] ); // Hay, Fahim Ahmmed, What’s up?
Friend.apply( friend2, [ ‘Hay’, ‘What’s up?’ ]); // Hay, Kuddus Ali, What’s up?

6. blind()

blind () is a method that creates a new function. But it does not call the function.

const friend1 = { firstName: ‘Fahim’ , lastName: ‘Ahammed’ };
const friend2 = { firstName: ‘Kuddus’ lastName: ‘Ali’ };
function Friend ( gretting1, gretting2 ) {
console.log( gretting1 + ‘ ‘ + this.firstName + ‘ ‘ + this.lastName + ‘ ‘ + gretting2 );
}
const inviteFriend1 = Friend.blind ( friend1 );
const inviteFriend2 = Friend.blind ( friend2 );
console.log( inviteFriend1 ( ‘Hello’, ‘Whats up? ‘ ));
// Hello, Fahim Ahammed Whats up?
console.log( inviteFriend2 ( ‘Hello’, ‘Whats up? ‘ ));
// Hello,  Kuddus Ali Whats up?

7. Global Variable;

A global variable declared outside a function. It can be accessed any function.

const value = 100;
function a () {
    console.log( value + 50 ) ; // 150
}
function b () {
    console.log( value * 5 ) ; // 500
}

8. this keyword

The this keyword refers to the current object in a method or constructor.

const Person = {
    firstName: ‘Akbor’,
    lastName: ‘Ali’,
    age: ‘23’,
    fullName: function () {
                return this.firstName + “ “ + lastName;
          }
}

9. Private Variable:

A private variable is accessible in the current class or function. It cannot access all functions or classes.

const a = 100;
if ( a < 80 ) {
     const b = 50;
} else {
    const c = 120;
}
console.log( a ); // 100
console.log( b ); // b cannot be accessable
console.log( c ) ; // c cannot be accessable

10. Global Scope:

A global variable declared at the top of a program or outside a function and a global variable has global scope.

const a = “hello”;
function test () {
    console.log(a);
}
test(); // hello

Thank you… :)

Fahim Ahammed Firoz

Fahim Ahammed Firoz

Fahim Ahammed Firoz - Imperial College of Engineering (Rajshahi University Affiliated) - Bangladesh…