1.Null vs Undefined
These are some ways you will get undefined..
a)let name;
console.log(name);//undefined
b)if the function don't return
function add(num1,num2){
console.log(num1+num2);
}
const res = add(5,6);
console.log(res);//11 undefined
const res2 = add(5);
console.log(res2);//undefined
c)Call an object's property that is missing
const movie = {name:'Social Network', director:'david Fincher',year:'2011'};
console.log(movie.cast);//undefined
d)If you set a variable undefined
let a = undefined;
console.log(a);
Null means empty. We can set a variable null.
2.Double equal(==) vs Triple equal(===)
Double equal doesn't check the type of variable whether triple equal checks.
const num1 = 5; //integer
const num2 = "5"; // String
if(num1==num2){
console.log("This is true");
}else{
console.log("This is false");
}
//Output: This is true.
if(num1===num2){
console.log("This is true");
}else{
console.log("This is false");
}
//Output: This is false.
3.Closures vs Block Scope
Scope: Access something from inside of a function. For example,
function add(num1,num2){
const res = num1+num2;
return res;
}
console.log(res);//error, because res is a variable inside the function, we can't access it from outside.
Global scope:When a variable is declared globally, it can be accessed from outside of the function.
let total = 100;
function sub(num3){
return total+num3;
}
console.log(total);//here we can access the total variable from outside of the function.
Block Scope:When you declare a variable inside 2nd brackets. They can't be accessed from outside of the area.
{} =blocked scope.
if(number >10){
let response = 'Happy';
console.log(response);
}
console.log(response); // error
Note: But if you use var to declare a variable. It will not show any error.
if(number >10){
var response = 'Happy';
console.log(response);
}
console.log(response); //no error
Important: It is called hoisting in Javascript when a variable is declared using 'var' inside a scope'{}', it grabs the variable outside of the scope and presents it as a global variable.
Closures: when a function returns another function. A closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time.
function increase(){
let count = 0;
return function(){
count++;
return count;
}
}
const res1 = increase();
console.log(res1);//1
console.log(res1);//2
console.log(res1);//3
const res2 = increase();
console.log(res2);//1
console.log(res2);//2
console.log(res1);//4
4.remove duplicate from an array
const name = [2,2,3,3,4,5,6];
const uniqueName = [];
for(let i =0;i<name.length;i++){
let element = name[i];
let index = uniqueName.indexOf(element);
if(index==-1){
uniqueName.push(element);
}
}
console.log(uniqueName);//[2,3,4,5,6]
5.Reverse a String
function reverseString(str){
const reverse ='';
for (let i=0;i<str.length;i++){
let char = str[i];
reverse = char+reverse;
}
return reverse;
}
const sentence = 'Hello programming community';
const result = reverseString(sentence);
console.log(result);
6.Asynchronous
Javascript works synchronously. By saying that, I mean, javascript code finishes working one line after another. Example...
function movie(){
console.log('Ironman1');
}
console.log('Ironman2');
console.log('Ironman3');
//Output:
Ironman1
Ironman2
Ironman3
But we can make javascript asynchronous. We can put a function or task on hold, by using the setTimeout() method.
function movie(){
console.log('Ironman1');
}
setTimeout(movie);
console.log('Ironman2');
console.log('Ironman3');
//Output:
Ironman2
Ironman3
Ironman1
Note:We can also set a specific time for how long the function will be on hold.
example,
setTimeout(movie,10000);//here time is in mili second.
7.Event loop and Event Buble
8.let const and var
9.Recursive function
10.Call back function
11.CRUD Operation
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