What is JavaScript

JavaScript ekta high-level, interpreted, single-threaded, dynamically typed programming language.

Table of Contents

1. Basic Syntax & Variables
   How to write statements, comments, and declare variables with var, let, and const.

2. Data Types
   Primitive types (number, string, boolean, null, undefined, symbol, bigint) and reference types (objects, arrays, functions).

3. Operators
   Arithmetic (+ - * / %), comparison (==, ===, >, <), logical (&&, ||, !), and the ternary operator.

4. Conditionals
   Decision making using if, else if, else, and switch to run different code based on conditions.

5. Loops
   Repeating code with for, while, do...while, and controlling flow using break and continue.

6. Functions
   Reusable blocks of code: function declarations, function expressions, arrow functions, parameters, and return values.

7. Arrays
   Ordered lists of values, indexing, looping, and useful methods like push, pop, map, filter, reduce.

8. Objects
   Key–value structures to group related data and behavior, object methods, and destructuring.

9. Scope, Hoisting, var / let / const
   How variable visibility works (global, function, block scope), what hoisting is, and how var, let, const differ.

10. Error Handling
    Handling runtime errors with try, catch, finally, and creating custom errors with throw.

11. Asynchronous Basics
    Non-blocking code using setTimeout, callbacks, Promises, and async/await.

12. DOM Basics
    Interacting with web pages: selecting elements, updating content/styles, and handling events like clicks.

Primitive data types in JavaScript:

• number – Any numeric value (integers or decimals), e.g. 10, 3.14.
• string – Text inside quotes, e.g. "Hello", 'Rahim'.
• boolean – Only true or false, mainly for conditions.
• null – An intentional “empty” value, meaning “nothing here”.
• undefined – A declared variable that has not been given any value yet.
• symbol – A special, unique, and immutable identifier, used in advanced cases.
• bigint – Very large integers beyond normal number range, e.g. 12345678901234567890n.

JavaScript Core Concepts Checklist

1. Basics: syntax, variables, data types

• Statements, semicolons, comments
• Variables: var, let, const
• Primitive types: number, string, boolean, null, undefined, symbol, bigint
• Reference types: objects, arrays, functions

Practice:

// 1) Create variables for a student:
const studentName = "Afjal";
let studentAge = 23;
let isPassed = true;

// 2) Log a sentence using them:
console.log(studentName + " is " + studentAge + " years old.");

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2. Operators & control flow

• Arithmetic: + - * / % **
• Comparison: == === != !== > < >= <=
• Logical: && || !
• Ternary operator
• if / else if / else, switch

Practice:

const marks = 72;
let grade;

if (marks >= 80) {
  grade = "A+";
} else if (marks >= 70) {
  grade = "A";
} else if (marks >= 60) {
  grade = "A-";
} else {
  grade = "Below A-";
}
console.log("Grade:", grade);

// Ternary for pass/fail:
const result = marks >= 40 ? "Pass" : "Fail";
console.log("Result:", result);

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3. Loops

• for loop
• while loop
• do...while loop
• break, continue

Practice:

// Print 1 to 10 but skip 5
for (let i = 1; i <= 10; i++) {
  if (i === 5) continue;
  console.log(i);
}

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4. Functions & scope

• Function declaration
• Function expression
• Arrow function
• Parameters, return
• Global vs function vs block scope

Practice:

function add(a, b) {
  return a + b;
}

const multiply = function (a, b) {
  return a * b;
};

const square = (x) => x * x;

console.log(add(2, 3));
console.log(multiply(2, 3));
console.log(square(5));

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5. Arrays

• Create & access items
• push, pop, shift, unshift
• Loop through arrays (for, for...of)
• map, filter, reduce basic idea

Practice:

const scores = [40, 55, 70, 90];

const passed = scores.filter(score => score >= 50);
const doubled = scores.map(score => score * 2);
const total = scores.reduce((sum, score) => sum + score, 0);

console.log("Passed:", passed);
console.log("Doubled:", doubled);
console.log("Total:", total);

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6. Objects

• Create objects
• Access: dot vs bracket
• Methods (this)
• Destructuring

Practice:

const user = {
  name: "Rahim",
  age: 20,
  isStudent: true,
  greet() {
    console.log("Hi, I'm " + this.name);
  },
};

user.greet();

const { name, age } = user;
console.log(name, age);

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7. Scope, hoisting, var / let / const

• var vs let vs const difference
• Block scope vs function scope
• Hoisting concept

Practice (just observe output):

console.log(a); // ?
var a = 5;

if (true) {
  let b = 10;
}
// console.log(b); // Uncomment to see error

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8. Asynchronous basics

• setTimeout, setInterval
• Promise ki
• async/await basic usage

Practice:

function wait(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

async function run() {
  console.log("Start");
  await wait(1000);
  console.log("After 1 second");
}

run();

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9. Error handling

• try / catch / finally
• throw diye custom error

Practice:

function divide(a, b) {
  if (b === 0) {
    throw new Error("Cannot divide by zero");
  }
  return a / b;
}

try {
  console.log(divide(10, 2));
  console.log(divide(10, 0));
} catch (err) {
  console.log("Caught error:", err.message);
} finally {
  console.log("Done");
}

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10. DOM basics (browser)

• Element select (getElementById, querySelector)
• Text/style change
• Event listener

Practice (HTML + JS):

<button id="btn">Click me</button>
<p id="text">Hello</p>

<script>
  const btn = document.getElementById("btn");
  const text = document.getElementById("text");

  btn.addEventListener("click", () => {
    text.textContent = "Button clicked!";
  });
</script>

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