# JavaScript is a high-level, versatile, and dynamic programming language primarily.
# Used for creating, interactive and dynamic content on website.
# It enables web developers to add functionallity, interactively, and behaviour to the webpage.
# High-level language
# Interpreted language
# Single Threaded
# Loosely typed
# Dynamic Language
# Synchronous language
They are three ways to declare any variable 1.let 2.const 3.var
- In let keyword redeclaration is not possible.
- In var keyword redeclaration is possible.
- It is used to take constant value.
- We can't perform redeclaration and reinitialization.
- Here the declaration and intialization in the same line.
<!-- !In js we have to types of data types -->
1.Primitive data types
2.Non primitive data
1.Number
2.Boolean
3.Undefined
4.Bigint
5.String
-> undefined means variable declared but not initialized.null means variable is initiallized with null.
-> typeof udefined is =>undefined
-> typeof null is object
# Array
# Object
# Function
typeof operator is used to show the datatype of the variable
It checks for syntax and semantics. it breaking the code into tokens in order to understand their meanings and these tokens get converted into Abstract Syntax tree(AST).
# it is a heirarchical tree like structure of program representation which allows interpreter to understand the program.
# This AST is initially goes to the interpreter.
It lets the AST to get converted into bytecode.
it will check for the repeating code that can be optimized. As soon as, it gets the repeating code, it basically moves the code into compiler.
It produced the most optimized code.
Chrome ==> v8 engine
edge ==> Chakra
firefox==> Spider monkey
* function is a set of code, used for doing some particular task.
* It is used for code reusability.
* Whenever we are creating function, if we want to execute, we have to call the function.
1.<!--! Named Function -->
the function having some name, is called named function.
<!-- ? syntax -->
function function_name(){
code......
}
//? count the digit of the numbers
let count=function(a){ let i=1; while(a>=10){ a=a/10 i++ } console.log(i);
} count(10) 2. the function that does not have any name, is called as anonymous function. function(){
}
# In nested function inner function can access all the properties of the outer function but the outer function can not access the properties of inner function ,this is called lexial scopping
Function that takes another function as parameter, is called higher order function.
The function that is send to the higher order function as an argument is called a Callback function.
<!--? Method of String -->
1. ToUpperCase():
this method id used to convert one string into uppercase and it will return on new String original string it will not change
2.toLowerCase():
this method id used to convert one string into uppercase and it will return on new String original string it will not change
3. trim():
It is used to remove the space from both the sides of the String
4.indexOf():
# this method is used to return the index of the given character
# it will tae the last appearance of the given character
5. lastIndexOf():
this method is used to return the index of the given character
6.charAt():
this method is used to know the character, present in the given position.
7.includes():
* it is used to know the given String is present or not
* if it present it will return true else it will return false
<!--? To declare an array in js -->
let arr=[30,"html",true, undefined];
<!--? how to find the length of the array -->
console.log(arr.length);
<!--? Methods in Array -->
<!-- ! 1. push() -->
This method is used to add any element in an array
It will returns length of an array
<!--! pop() -->
It is used to remove the last element from the Array
it will returns the removed element
<!--! 3.unshift -->
this methods will add the elemnt at the start of an array
It will return the length of the modified array length
<!--! 4. shift() -->
This methods will remove the first element present in an Array
It will returns the value of an removed element
<!--! 5. indexOf -->
it is used to know the index of an array present in Array
if the element is not pesent it will return -1
<!-- ! 6.concat -->
It is used to combine two or more arrays and it will return one new Array
<!-- ! 7.includes -->
It return true if the element present in array else it will return false
<!-- ! 8. Map -->
* It is a higher order function and it will returns the a new Array.
* It will takes the three parameter.
* It is used to traverse the array, if we want to perform any operation in all the element in a array we should use map method.
syntax: arr.map((ele, index, array)=>{
})
for-in loop:
# for in loop is used to traverse and it willreturn the index.