Yaswanth K N

Posted on May 30, 2023 • Updated on Jun 1, 2023

JavaScript

#javascript #beginners

Loops:

`for`:

It is a basic for loop used to iterate through iterable

example:

let arr = [1,2,3,3,4,5,6,7];
for(let i = 0; i < arr.length; i++ ){
    console.log(arr[i]);
    }
// Output: prints all the elements of array we can use same loop for strings also

`forEach`:

It is a higher order function in javascript which takes in another function as a reference

example:

let arr = [1,2,3,4,5,6];
arr.forEach((num)=> num * 2);
console.log(arr);
//Output: Array of elements which are doubled

`for .. in`:

It is a looping technique majorly used to iterate over object

example:


let person = {
    fname : "raj",
    lname : "kailash",
    age : 23
    location : "banglore";
}

for(let key in person){
    console.log(`${key} : ${person[key]}`);
}

`for .. of`:

This will return the each element of a array

example:

let arr = [1,2,3,3,4,5,6,7];
for(let ele of arr){
    console.log(ele)
}
//Output: prints all the elements of array

`while` :

It is one of the most basic technique to loop through elements

example:


let arr = [1,2,3,3,4,5,6,7];
let index = 0;
while(index < arr.length){
    console.log(arr[index]);
}

//Output: prints all the elements of array

Mutable vs Immutable

Mutable are those whose value can be changed without creating a entire new object

example: Objects , arrays.

Immutable: Value can't be changed unless you create a new object example: Strings, Primitive datatypes etc.

Pass by Reference and Pass by Value

Pass by Value :

System will create it's own copy and value of variable will not change outside of block

example:


let num = 10;
function multiply(num , n){ 
    num = num*n;
    return num;
} 
 const res = multiply(num,2);
 console.log(num); // 10
 console.log(res); // 20

In the above example num value have not changed even though we pass the same variable to the function, because system will create seperate copy of num and modify that

Pass by Reference:

All the objects in javascript are passed by reference
Even if you change it's value inside a function it will change the object

let  person  = {
fname :  "raj",
lname :  "kailash",
age :  23,
location :  "banglore"
}

function  locationUpdate(person){
person["location"] =  'Vizag';
}
locationUpdate(person);
console.log(person);
// You will see location changed to vizag

Here the person object is passed by refernce hence it is changing it's property value inside function

Array Methods

`Array.pop()`:

removes the last element of a array and returns it's value

let arr = [1,2,3,3,4,5,6,7];
console.log(arr.pop()); // 7

`Array.push()`:

Appends the elements at the end of array

let arr = [1,2,3,3,4,5,6,7];
arr.push(10);
console.log(arr) // 10 is added at the end

`Array.concat()`:

Addes two arrays and returns the new array

let arr1 = [1,2,3,3,4,5,6,7];
let arr2 = [1,2,3,3,4,5,6,7];
const new_arr = arr1.concat(arr2); 
console.log(new_arr) // [1,2,3,4,5,6,7,1,2,3,4,5,6,7]

`Array.slice()`

It takes two values start,end indexes
returns new array with values between specified indexes.

let arr = [1,2,3,3,4,5,6,7];
const new_arr = arr.slice(0,3);
console.log(new_arr); // [1,2,3]

`Array.splice()`

It will remove elements from the given array
Takes two values index and number of elements you want to delete


let arr = [1,2,3,4,5,6,7];
arr.splice(0,3);
console.log(arr); // [4,5,6,7]

`Array.join()`

returns a string joining each element by given argument


let arr = [1,2,3,4,5,6,7];
const str = arr.join(" ");
console.log(str) // 1 2 3 4 5 6 7

`Array.flat()`:

It will unbox all the elements inside a array which are nested elements

const arr = [1, 2, [3, [4, 5, 6], 7], 8];
const new_arr = myArr.flat();
console.log(new_arr) // [1,2,3,4,5,6,7,8]

Finding:

`Arrays.find()`:

This will take boolean function reference as a argument and returns the first value in it's array which passes the function
If no element is found it will return undefined

let arr = [1,2,3,4,5,6,7];

const res  = arr.find((ele)=>{
    return ele > 5; 
});
console.log(res); // 6 since it is the first element which is greater than 5

`Arrays.indexOf()`:

It will return the index of first element that it find

const fruits = ["Banana", "Orange", "Apple", "Mango"];  
let index = fruits.indexOf("Apple");
console.log(index); // 2

`Array.includes()`:

returns true if the element is present else returns false
we can also optionly give another parameter as a start value which by default is zero

const fruits = ["Banana", "Orange", "Apple", "Mango"];  
let bool = fruits.includes("Apple");
console.log(bool) ;// true

`Arrays.findIndex()`:

takes the reference of a boolean returning function and returns the index of element that first passes the constion
returns -1 if no element passes the test

let arr = [1,2,3,4,5,6,7];

const res  = arr.findIndex((ele)=>{
    return ele > 4; 
});
console.log(res); // 4

Higher Order Functions

`Arrays.forEach()`:

This takes in the function as a argument and changes the existing array according to the argument function

let arr = [1,2,3,4,5,6,7];

arr.forEach((ele)=>{
ele = ele*2;
});

console.log(arr) // [2,4,6,8,10,12,14]

`Array.map()`:

This takes in the function and will not modify the array and returns new array with modified values
It will apply given function on each element of array


let arr = [1,2,3,4,5,6,7];

const new_arr = arr.map((ele)=>{
    return ele*2
});
console.log(new_arr) // [2,4,6,8,10,12,14]

`Array.filter()`:

This will take a function as a argument which returns boolean value
It will apply given function on each element of array
this will return all the elements that passes the boolean constion

let arr = [1,2,3,4,5,6,7];

const new_arr = arr.filter((ele) => {
    return ele > 3;
});
console.log(new_arr); //4,5,6,7

`Arrays.reduce()`:

This method will take two arguments one is the function and the other one is the initial value
The argument function will further take two arguments which are accumilator and current element

let arr = [1,2,3,4,5,6,7];
const sum = arr.reduce((acc,curr)=>{
    acc += curr;
    return acc;
},0)

console.log(sum); // 28

`Arrays.sort()`:

This method will take a function as a argument and sort depending on the result of that function
The given function will also take two arguments which are compared to return positive, negitive or zero value

const sorter = inventory.sort((a,b)=> {
    if (a.car_year > b.car_year){
        return 1;
    }else if (a.car_year < b.car_year){
        return -1;
    }
    return 0
});

depending on the returned value array is sorted accordingly

Array methods chaining

Using of higher order functions on output of other higher order function is called array method chaining

console.log(inventory.filter((car)=>{
if(car.car_make  ===  'Audi'  ||  car.car_make  ===  'BMW'){
    return  true;
}else  return  false;
})
.reduce((acc, curr)=>{
    acc.push({id :  curr.id , car_year :  curr['car_year']});
    return  acc;
},[]))

In the above code in inventory array first filters out the car_maker and pass the result to reduce function to output in given format

String Methods

`string.charAt()`:

returns the character at given index

let str = "abcd efg hij"
console.log(str.charAt(0)) // a

`string.concat()`:

appends two strings

let str1 = "abcd";
let str2 = "efgh";

let new_str = str1.concat(str2);
console.log(new_str);

`string.length`:

returns the length of string as integer

let str = "abcdef"
console.log(str.length); // 6

`string.toUpperCase()`:

returns the new string in upper case letters

let str = 'abcdef';
console.log(str.toUpperCase()) // ' ABCDEF'

`string.toLowerCase()`:

returns the new string in lower case letters

let str = 'ABCDEF';
console.log(str.toLowerCase()) // 'abcdef'

`string.split()`:

splits the string at given argument and returns array of strings

let arr = 'abcd efgh'.split(' ');
console.log(arr) // ["abcd", "efgh"]

`string.substring()`:

returns the substring of string with given arguments
It takes two arguments start and end(exclusive)


let str = 'abcdefg';
console.log(str.substring(1,5)) // "bcde"

Object Methods

Assigning:

let  person  = {
    fname :  "raj",
    lname :  "kailash",
    age :  23
}

person["location"] = "banglore"; // Assigning a new property to a object

Checking:

in key word is used to check if property is present or not

let  person  = {
    fname :  "raj",
    lname :  "kailash",
    age :  23,
    location : "banglore"
}

console.log("fname" in person) // true

`Object.keys()`:

returns the keys array iterable object

let  person  = {
    fname :  "raj",
    lname :  "kailash",
    age :  23,
    location : "banglore"
}

const keys = Object.keys(person);
for(let key of keys){
    console.log(key) // Prints all the keys of person object
}

`Object.values()`:

returns the all the values of a object as a array

let  person  = {
    fname :  "raj",
    lname :  "kailash",
    age :  23,
    location : "banglore"
}

const values = Object.values(person);
for(let value of values){
    console.log(value) // Prints all the values of person object
}

`delete object[key]`:

it will remove the entire property and it's value from object

let  person  = {
    fname :  "raj",
    lname :  "kailash",
    age :  23,
    location : "banglore"
}

delete person["location"];
console.log(person) //prson object will not have location property

`Object.entries()`:

This will return a array of key, value pairs as a array

let  person  = {
    fname :  "raj",
    lname :  "kailash",
    age :  23,
    location : "banglore"
}
const entries = Object.entries(person);

for(let pair of entries){
    console.log(pair) //[key , value]
}

Hoisting

It is special property of javscript where we can use the variables or functions without declaring first

console.log(b); //It will print undefined not error
var b

Hoisting is posible only with variables declared using var key word

Scopes

There are two types of scopes of variables or functions
Global Scope : variables which can be accessed any where in the program is called global scoped varibales
Local Scope: The access is limited to a block

let a = 10; // global scope
if(a){
    let b = 5; // Local scope or block level scope
    console.log(b) // prints 5 
    console.log(a) // prints 10 
}
console.log(a) // prints 10 
console.log(b) // Throws error

Closures

It is process where fucntion is declared and returned inside another function
It helps in improving privacy
for example if you want to increase a variable value only inside a function not on outside(global scope)

function sum() {
    let a = 0;
    function increaseSum() {
        // the value of a is increased by 1
        return a = a + 1;
    }
    return increaseSum;
}
let x = sum();
let a = 5;
console.log(x()); // 1
console.log(x()); // 2
console.log(a); // 5

Debugging

Debugging in javascript can be done in multiple ways
console.log(): Most commonly used debugging technique
console.trace(): returns the function call stack
console.time() and console.timelLog(): this gives the time taken by a program from the time() to timeLog() function.
developer tools: browser provides developer tools which very handy to debugg a javascript code
Using break points in editor etc

Loops:

for:

forEach:

for .. in:

for .. of:

while :

Mutable vs Immutable

Pass by Reference and Pass by Value

Array Methods

Array.pop():

Array.push():

Array.concat():

Array.slice()

Array.splice()

Array.join()

Array.flat():

Finding:

Arrays.find():

Arrays.indexOf():

Array.includes():

Arrays.findIndex():

Higher Order Functions

Arrays.forEach():

Array.map():

Array.filter():

Arrays.reduce():

Arrays.sort():

Array methods chaining

String Methods

string.charAt():

string.concat():

string.length:

string.toUpperCase():

string.toLowerCase():

string.split():

string.substring():

Object Methods

Assigning:

Checking:

Object.keys():

Object.values():

delete object[key]:

Object.entries():

Hoisting

Scopes

Closures

Debugging

References

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The Ultimate Node.js Cheat Sheet for Developers

ES6 Spread Operator: Unleashing the Power of Modern JavaScript

`for`:

`forEach`:

`for .. in`:

`for .. of`:

`while` :

`Array.pop()`:

`Array.push()`:

`Array.concat()`:

`Array.slice()`

`Array.splice()`

`Array.join()`

`Array.flat()`:

`Arrays.find()`:

`Arrays.indexOf()`:

`Array.includes()`:

`Arrays.findIndex()`:

`Arrays.forEach()`:

`Array.map()`:

`Array.filter()`:

`Arrays.reduce()`:

`Arrays.sort()`:

`string.charAt()`:

`string.concat()`:

`string.length`:

`string.toUpperCase()`:

`string.toLowerCase()`:

`string.split()`:

`string.substring()`:

`Object.keys()`:

`Object.values()`:

`delete object[key]`:

`Object.entries()`: