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Milestone 1: Basics of JavaScript (INDEX)
Install JavaScript, Console - `let`, `const`, `var` - Variable declaration - Data types - Primitive: Number, String, Boolean, Undefined, Null, Symbol, BigInt - Non primitive: Objects, Arrays, Functions
Install JavaScript:
- Download nodejs via nodejs.org
- Insatll nodejs
- Run command
=> node -v ==>To check the nodejs version
=> node index.js ==>To run the index.js file
JavaScript:
- To program the behavior of website
- Invented by Brandon Eich (1995)
- ECMAScript is the official name of the language
Console:
- The console in JavaScript is a tool provided by the browser or Node.js that allows developers to interact with their code by displaying messages, errors, and debugging information.
- It provides methods for logging messages and inspecting code behavior during execution.
1. Console Methods:
- `console.log()`: Prints general information to the console
(eg) console.log("Hello, World!"); // Outputs: Hello, World!
- `console.error()`: Displays errors in the console
(eg) console.error("This is an error!"); // Outputs an error message
- `console.warn()`: Outputs warnings
(eg) console.warn("This is a warning!"); // Outputs a warning message
- `console.table()`: Displays data in a table format
(eg) const people = [{name: "John", age: 30}, {name: "Jane", age: 25}];
console.table(people);
- `console.dir()`: Displays an interactive list of the properties of an object
- `console.time() / console.timeEnd()`: Measures the time taken by a piece of code to execute
(eg) console.time("test");
// Some code execution
console.timeEnd("test"); // Outputs the time elapsed for execution
2. Usage in Browsers:
- Most modern browsers (like Chrome, Firefox, Safari, etc.) have built-in developer tools with a console tab.
- You can open it by pressing F12 or Ctrl + Shift + I, then navigating to the "Console" tab.
- You can execute JavaScript code directly in this console, which helps in testing and debugging.
3. Console in Debugging:
- Developers often use console.log() to trace the flow of their program, see the values of variables, and find bugs.
- It is one of the simplest yet most effective debugging tools.
JavaScript Variables:
- JavaScript Variables can be declared in 4 ways:
Automatically
Using var
Using let
Using const
- When to Use var, let, or const?
1. Always declare variables
2. Always use const if the value should not be changed
3. Always use const if the type should not be changed (Arrays and Objects)
4. Only use let if you can't use const
5. Only use var if you MUST support old browsers.
6. You cannot re-declare a variable declared with let or const.
var:
- Print the output in console
var firstName="UserName"
console.log(firstName); // Output => UserName
- We can redecalre same variable using var keyword
var age=56
var age=78
console.log(age); // Output => 78
- by using Var keyword , we can able to redeclare and reassign an variable.
let:
- To print the value in cosole
let name = "yourName"
console.log(name) // Output => yourName
- We can't redeclare the same variable twice using let keyword
let name ="javascript";
let name = "werty";
console.log(name) // Output => Syntaxerror
- We can edit the already decalred variable using let
let user='username'
user="name"
console.log(user) // Output => name
- by using let , we cannot redeclare an varibale at twice,but we can able to change/edit the value
const:
- We can't reassign, reedit, redeclare value to the variable which use the const keyword
const userNAme="javascript";
userNAme='werty';
console.log(userNAme); // Output => TypeError
- by using const , we cannot redeclare or reassign an value to teh variable.
variable decleration principle:
1. variable name must begin with lowercase , uppercase , _ ,
2. number can be used in inbetween not at inital state
3 expect underscore, dolor, astrik, other special chacter are not allowed
eg.
let name // valid variable
let 0name // invalid variable format
let name0 // valid variable format
DataTypes:
- JavaScript has 8 Datatypes under 2 variations.(Primitive , Non-Primitive).
1. Primitive Data Types (Immutable):
- These types hold simple, single values and are immutable (cannot be changed directly).
String: Represents text data.
eg. let name = "John"; // Example of a string
Number: Represents numeric values, both integers and floats.
eg. let age = 30; // Example of a number
BigInt: Used for very large integers beyond the safe limit of Number.
eg. let bigNumber = 12345678901234567890n; // Example of a BigInt
Boolean: Represents logical values, either true or false.
eg. let isActive = true; // Example of a boolean
Undefined: Represents a variable that has been declared but not assigned a value.
eg. let x; // Example of undefined
Null: Represents the intentional absence of any object value.
eg. let y = null; // Example of null
Symbol: Represents a unique and immutable value used as object property keys.
eg. let id = Symbol("id"); // Example of a symbol
2. Non-Primitive (Reference) Data Types:
- These types store references to objects, and their contents can be modified.
Object: Used to store collections of data and more complex entities.
eg. let person = {name: "John", age: 30}; // Example of an object
- The object data type can contain both built-in objects, and user defined objects:
Built-in object types can be:
objects, arrays, dates, maps, sets, intarrays, floatarrays, promises, and more.
- Primitive Data Types:
Number, String, Boolean, Undefined, Null, Symbol, BigInt
- Non-Primitive Data Types:
Objects (including Arrays, Functions), Arrays, Dates