Datatypes
Primitive Data Types
Number
let age = 25;
String
let name = "John";
Boolean
let isStudent = true;
Undefined:
let address;
Null
let salary = null;
Symbol
let sym = Symbol("id");
BigInt
let bigIntNumber = 1234567890123456789012345678901234567890n;
Not-a-Number (NaN)
NaN stands for "Not-a-Number" and represents a value that is not a legal number
console.log(0 / 0); // NaN
console.log(parseInt("abc")); // NaN
How to check datatype?
console.log(typeof a);
Class
1) Class can only have one constructor
class gfg {
constructor(name, estd, rank) {
this.n = name;
this.e = estd;
this.r = rank;
}
decreaserank() {
this.r -= 1;
}
}
const test = new gfg("tom", 2009, 43);
test.decreaserank();
console.log(test.r);
console.log(test);
Inheritance
class car {
constructor(brand) {
this.carname = brand;
}
present() {
return 'I have a ' + this.carname;
}
}
class Model extends car {
constructor(brand, model) {
super(brand);
super.present();
this.model = model;
}
show() {
return this.present() + ', it is a ' + this.model;
}
}
Get and Set
class car {
constructor(brand) {
this.carname = brand;
}
// Getter method
get cnam() {
return "It is " + this.carname; // Return a value
}
// Setter method
set cnam(x) {
this.carname = x;
}
}
const mycar = new car("Ford");
console.log(mycar.cnam);
Immediately Invoked Function Expression (IIFE)
An IIFE is a function that runs as soon as it is defined
(function() {
console.log("IIFE executed!");
})();
Higher Order Functions
Higher-order functions are functions that take other functions as arguments or return functions as their result
function higherOrderFunction(callback) {
return callback();
}
function sayHello() {
return "Hello!";
}
console.log(higherOrderFunction(sayHello)); // "Hello!"
Variable Shadowning
Variable Shadowing occurs when a local variable has the same name as a variable in an outer scope.
The local variable overrides or hides the outer variable within its own scope.
The outer variable remains intact and can be accessed outside of the local scope.
var name = "John";
function sayName() {
console.log(name);
var name = "Jane";
}
sayName();
Accessing HTML Elements in JavaScript
There are several ways to access HTML elements in JavaScript:
Select element by ID:
document.getElementById("elementId");
Select element by Classname:
document.getElementsByClassName("className");
Select element by Tagname:
document.getElementsByTagName("tagName");
Css selector:
document.querySelector(".className");
document.querySelectorAll(".className");
Pass by value
function changeValue(x) {
x = 10;
console.log("Inside function:", x)
}
let num = 5;
changeValue(num);
Pass by reference
function changeProperty(obj) {
obj.name = "Alice";
console.log("Inside function:", obj.name); // Output: Inside function: Alice
}
let person = { name: "Bob" };
changeProperty(person);
console.log("Outside function:", person.name); // Output: Outside function: Alice
use strict
It switches the JavaScript engine to strict mode, which catches common coding mistakes and throws more exceptions.
"use strict";
x = 10; // Throws an error because x is not declared
Spread operator
It allows an iterable such as an array or string to be expanded in places where zero or more arguments or elements are expected
function sum(a, b, c) {
return a + b + c;
}
const numbers = [1, 2, 3];
console.log(sum(...numbers)); // Output: 6
InstanceOf
The operator checks whether an object is an instance of a specific class or constructor function.
class Animal {
constructor(name) {
this.name = name;
}
}
class Dog extends Animal {
constructor(name, breed) {
super(name);
this.breed = breed;
}
}
const myDog = new Dog('Buddy', 'Golden Retriever');
console.log(myDog instanceof Dog); // true
console.log(myDog instanceof Animal); // true
console.log(myDog instanceof Object); // true
console.log(myDog instanceof Array); // false
Find
This method is used to find a particular element
let present = books.find(each => each.id == id);
Findindex
This method is used to find a particular element by index
var index = books.findIndex(each => each.id == param_id);
books[index].title = title;
books[index].author = author;
books[index].year = year;
Splice
array.splice(start, deleteCount, item1, item2, ...);
books.splice(index, 1); // deletion
let arr = [1, 2, 3, 4, 5];
arr.splice(1, 2); // Removes elements starting at index 1, count of 2
Filter
This method creates a new array with all elements that pass the test implemented by the provided function.
const numbers = [1, 2, 3, 4, 5, 6];
const evenNumbers = numbers.filter(num => num % 2 === 0);
console.log(evenNumbers); // [2, 4, 6]
Reduce
This method executes a reducer function on each element of the array, resulting in a single output value.
const numbers = [1, 2, 3, 4, 5];
const sum = numbers.reduce((sum, value) => sum + value, 0);
// sum = 0 initially
console.log(sum); // 15
Rest
This parameter syntax allows a function to accept an indefinite number of arguments as an array.
function sum(...numbers) {
return numbers.reduce((sum, value) => sum + value, 0);
}
console.log(sum(1, 2, 3)); // 6
console.log(sum(5, 10, 15, 20)); // 50
Types of Declarations
Implicit Global variable
An implicit global variable is a variable that is created automatically in the global scope when it is assigned a value without being explicitly declared with a keyword like var, let, or const. But this throws error if it is in Strict mode
function myFunction() {
x = 10; // no error
}
const
It declares a constant variable that cannot be reassigned.
const PI = 3.14;
let
It declares a block-scoped variable.
It cannot be re-intialized with same name
let c=1;
let c=3;// throws error
let count = 0;
if (true) {
let count = 1;
console.log(count); // Output: 1
}
console.log(count); // Output: 0
var
It declares a function-scoped or globally-scoped variable. It encourages hoisting and reassignment.
var name = 'John';
if (true) {
var name = 'Doe';
console.log(name); // Output: Doe
}
console.log(name); // Output: Doe
console.log(a)
var a=2 // prints undefined
Synthetic Event
Synthetic Events: React provides a SyntheticEvent wrapper around the native browser events. This wrapper normalizes the event properties and behavior across different browsers, ensuring that your event handling code works the same way regardless of the browser.
import React from 'react';
class MyComponent extends React.Component {
handleClick = (event) => {
// `event` is a SyntheticEvent
console.log(event.type); // Always 'click' regardless of browser
console.log(event.target); // Consistent target property
}
render() {
return <button onClick={this.handleClick}>Click Me</button>;
}
}
Hoisting in JavaScript
Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compile phase, allowing them to be used before they are declared in the code. However, only the declarations are hoisted, not the initializations.
console.log(x); // Output: undefined
var x = 5;
console.log(x); // Output: 5
// Function hoisting
hoistedFunction(); // Output: "Hoisted!"
function hoistedFunction() {
console.log("Hoisted!");
}
// Function expressions are not hoisted
notHoisted(); // Error: notHoisted is not a function
var notHoisted = function() {
console.log("Not hoisted");
};
Type coercion
It is the automatic conversion of values from one data type to another. There are two types of coercion: implicit and explicit.
Implicit Coercion
Ex.
let result = 5 + "10"; // "510"
let result = "5" * 2; // 10
let result = "5" - 2; // 3
let result = "5" / 2; // 2.5
Explicit Coercion
It happens when we manually convert a value from one type to another using built-in functions.
let num = 5;
let str = String(num); // "5"
let str2 = num.toString(); // "5"
let str3 = `${num}`; // "5"
Truthy Values
Non-zero numbers (positive and negative)
Non-empty strings
Objects (including arrays and functions)
Symbol
BigInt values (other than 0n)
Falsy Values
0 (zero)
-0 (negative zero)
0n (BigInt zero)
"" (empty string)
null
undefined
NaN (Not-a-Number)
Props (Properties)
To pass data from a parent component to a child component. It is immutable (read-only) within the child component.
// Parent Component
function Parent() {
const data = "Hello from Parent!";
return <Child message={data} />;
}
// Child Component
function Child(props) {
return <div>{props.message}</div>;
}
State
To manage data that can change over time within a component. It is mutable within the component.
// Function Component using useState
import { useState } from 'react';
function Counter() {
const [count, setCount] = useState(0);
return (
<div>
<p>Count: {count}</p>
<button onClick={() => setCount(count + 1)}>Increment</button>
</div>
);
}
Closure
A closure in JavaScript is a feature where an inner function has access to the outer (enclosing) function's variables and scope chain even after the outer function has finished executing.
function outerFunction(outerVariable) {
return function innerFunction(innerVariable) {
console.log('Outer Variable:', outerVariable);
console.log('Inner Variable:', innerVariable);
};
}
const newFunction = outerFunction('outside');
newFunction('inside');
Currying
Currying is a technique of transforming a function that takes multiple arguments into a sequence of functions that each take a single argument.
function add(a) {
return function(b) {
return a + b;
};
}
const add5 = add(5);
console.log(add5(3)); // Output: 8
console.log(add(2)(3)); // Output: 5
Generators
Generators are special functions that can be paused and resumed, allowing you to generate a sequence of values over time.
function* generateSequence() {
yield 1;
yield 2;
yield 3;
}
const generator = generateSequence();
console.log(generator.next()); // { value: 1, done: false }
console.log(generator.next()); // { value: 2, done: false }
console.log(generator.next()); // { value: 3, done: false }
console.log(generator.next()); // { value: undefined, done: true }
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