How JavaScript Works?

Last Updated : 15 Apr, 2025

JavaScript is a dynamically typed, cross-platform threaded scripting and programming language, used to put functionality and interactivity at the client side as well as to write logic on the server side of a website. It can display content updates, interactive maps, control multimedia, interactive forms, and many more.

JavaScript, in short JS, was created in 1995 by Brendan Eich, who was working at Netscape Communications. In the beginning, it was designed to add interactivity to websites. Currently, JavaScript can support both client-side and server-side development. It plays a very important role in modern web apps by helping developers manipulate the Document Object Model (DOM), handle user events, and communicate with servers asynchronously.

These are the following topics that we are going to discuss:

Table of Content

Evolution of JavaScript

JavaScript has undergone significant evolution to meet the demands of web development:

ECMAScript Standards: JavaScript is based on ECMAScript standards, which define the language syntax, semantics, and libraries. ECMAScript 6 (ES6), released in 2015, it introduced major enhancements such as arrow functions, classes, modules, and promises.
Browser Compatibility: Early versions of JavaScript varied between web browsers, which caused compatibility issues. Today, modern browsers and JavaScript engines conform closely to ECMAScript standards, which makes sure the behavior is consistent across different platforms.

JavaScript is an old programming language. Initially, it was quite basic, but as web development needs grew, so did JavaScript. Here are some key updates:

ECMAScript 3 (1999): Added useful features like regular expressions for searching text and try/catch for handling errors.
ECMAScript 5 (2009): Introduced strict mode to help catch errors and support for JSON (JavaScript Object Notation).
ECMAScript 6 (2015): Also known as ES6, it brought major improvements like let and const for variable declaration and arrow functions for cleaner syntax.

There are so many more updates of JavaScript, currently, it is on the 15th edition which is ECMAScript 2024.

JavaScript Engines

JavaScript engines are responsible for executing JavaScript code. The two most important JavaScript engines are V8 (used in Chrome and Node.js) and SpiderMonkey (used in Firefox). These engines follow a similar process to interpret and execute JavaScript:

How JavaScript Engines Interpret and Execute Code?

Parsing: When we load a webpage or execute a script, the JavaScript engine first parses the source code to understand its structure. It converts the code into an Abstract Syntax Tree (AST) which is a hierarchical representation of the script.
Compilation: Now in compilation phase, the engine translates the AST into machine-readable bytecode using JIT (Just-In-Time) compilation. JIT compilation optimizes performance by compiling frequently executed code segments at runtime.
Execution: Finally, the bytecode or machine code is executed line by line, which produces the output or behavior as defined by the JavaScript code.

Example: Here, the engine parses the calculateSum function, compiles it into bytecode or machine code, executes it with arguments 3 and 4, and logs 7 to the console.

JavaScript

function calculateSum(a, b) { return a + b; } let result = calculateSum(3, 4); console.log(result); // Output: 7

Output

Execution Contexts in JavaScript

JavaScript operates within execution contexts, which define the environment in which code is executed. Whenever we run a JavaScript code, a new execution context is created and if there is any proper function call (proper function means not arrow function or variable directly defining and calling a function), the execution context for that function is created inside the execution context of global execution context. If the function returns , the function's execution context is deleted and if there is no more code left, the global execution context is also deleted.

There are two main types of execution contexts:

Global Execution Context in JavaScript :The global execution context is the default context in which JavaScript code runs. It includes global variables and functions accessible throughout the script.
Function Execution Context in JavaScript :Every time a function is invoked, a new function execution context is created. This context manages local variables, function arguments, and the function's return value.

Call Stack and Management of Function Calls in JavaScript

The call stack is a data structure that tracks function calls in JavaScript. When a function is invoked, its context is pushed onto the call stack. Once the function completes execution, its context is popped off the stack.

The call is stack keeps track the order of execution of execution context.

The call stack contains its first function as the global execution context then if there is a function call , the execution context of that function is pushed inside the stack and if it returns, the call stack pops out the execution context of that function. After completing the whole code execution, the global execution context at bottom of the stack is also popped out which implies the code has been executed.

Example: Here, First () is called and added to the call stack. It again calls second(), which is then added to the stack.Now, second () completes execution and is removed from the stack. First () completes execution and is removed from the stack.

JavaScript

function first() {     console.log("First function");     second(); }  function second() {     console.log("Second function"); }  first();

Output

First function Second function

Asynchronous Tasks and Event Loop in JavaScript

JavaScript is single-threaded, meaning it can only execute one task at a time. It also employs asynchronous programming techniques to handle multiple tasks concurrently without blocking the main thread.

Event Loop, Callback Queue, and Microtask Queue

The event loop

Event loop is a mechanism that continuously checks the call stack and manages asynchronous tasks in JavaScript. It ensures that tasks are executed in the correct order and which prevents the main thread from being blocked.

Callback queue and microtask queue

It hold tasks which are ready to be executed once the call stack is empty. Promises uses the microtask queue for handling asynchronous operations with higher priority.

Example: The code logs "Start" and "End" immediately since they are synchronous; the promise resolves next due to its microtask priority, logging "Promise," and finally, the setTimeout callback is executed, logging "Timeout," even though it's set to 0ms, as it is placed in the callback queue, which runs after microtasks.

JavaScript

console.log("Start");  setTimeout(() => {     console.log("Timeout"); }, 0);  Promise.resolve().then(() => {     console.log("Promise"); });  console.log("End");

Output

Start End Promise Timeout

Memory Management in JavaScript

JavaScript manages memory allocation in dynamic way to store variables, objects, and functions during runtime.

Allocation of Memory

Primitive Values: Stored directly in the stack memory.
Objects and Functions: Stored in the heap memory, and references are stored in the stack.

Garbage Collection

JavaScript has automatic garbage collection to get memory occupied by objects that are no longer reachable or referenced by the program. This process helps us to efficiently manage memory and prevent memory leaks.

Example: This example demonstrates the garbage Collection.

JavaScript

let obj = { name: "John" }; obj = null; // Object is no longer referenced and eligible for garbage collection

Call Stack vs. Heap (for function execution)

The call stack manages function calls and their contexts, while the heap is used for dynamic memory allocation during runtime.

Call Stack (for function execution)

Function Calls: Manages the execution context of function calls in JavaScript.
Last In, First Out (LIFO): Operates on a LIFO principle where the most recently called function is processed first.
Context Management: Tracks where the execution is in the program with the current function's context.
Single Threaded: Executes code sequentially, allowing only one function to be processed at a time.
Example: When a function is called (functionA()), its context is pushed onto the call stack. When it completes, its context is popped off, allowing the next function (functionB()) to be processed.

Heap (for function execution)

Memory Allocation: Used for dynamic memory allocation during runtime.
Objects and References: Stores objects and variables that are accessed globally or referenced from the call stack.
Garbage Collection: Managed by the JavaScript engine to reclaim memory occupied by objects that are no longer in use.
Example: Objects like arrays or complex data structures (let obj = { key: value }) are stored in the heap. Variables referencing these objects are stored in the call stack.

Execution Phases in JavaScript

JavaScript code execution can be broadly divided into two phases: the Compilation Phase and the Execution Phase.

Compilation Phase:

Syntax Analysis: The JavaScript engine parses the source code to understand its structure.
Memory Allocation: Allocates memory for variables and functions (hoisting).
Example: During compilation, function and variable declarations are processed first, allowing them to be accessed before they are defined in the code (console.log(greet());).

Execution Phase:

Code Execution: Executes the compiled code line by line.
Evaluation of Expressions: Processes expressions, function calls, and operations defined in the code.
Example: After compilation, the engine starts executing from top to bottom, evaluating expressions and performing operations (function greet() { return "Hello"; }).

Hoisting in JavaScript

Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their scope during the compilation phase. Function declarations '(function greet() { ... })' are fully hoisted, meaning they can be called before they are defined in the code. Variable declarations '(let, const, var)' are hoisted but not their assignments '(let x = 10; hoists let x; but not x = 10;)'.

Example: This demonstrates the hoisting in Javascript.

JavaScript

console.log(greet()); // Output: Hello function greet() { return "Hello"; }

Output:

Hello

Node.js Runtime in JavaScript

Node.js extends JavaScript's capabilities beyond the browser, which enables server-side scripting and development of scalable network applications. It provides built-in modules for file system operations, networking, and HTTP server creation.
Example: This code reads text file in node.js in a file named example.txt and displays in console.

JavaScript

const fs = require('fs');  fs.readFile('example.txt', 'utf8', (err, data) => {     if (err) throw err;     console.log(data); });

Concurrency Model in JavaScript

JavaScript uses an event-driven, non-blocking concurrency model to manage multiple tasks concurrently without blocking the main thread.

How JavaScript is Single-Threaded?

JavaScript is single-threaded, meaning it can only execute one piece of code at a time in a single sequence. This single-threaded nature simplifies code execution but poses challenges when dealing with asynchronous tasks like network requests or file handling.

Call Stack: JavaScript uses a call stack to keep track of function calls. If a function is currently being executed, it blocks the execution of other functions until it is complete.
Event Loop: To manage asynchronous operations, JavaScript uses the event loop. It ensures that even though JavaScript can only handle one task at a time, it doesn't get stuck waiting for long-running tasks like HTTP requests.

Example: Despite the setTimeout being called first, the message is logged last because setTimeout is non-blocking. The call stack clears console.log("Start") and console.log("End") first, then moves to the delayed task.

JavaScript

console.log("Start"); setTimeout(() => { console.log("Delayed Message"); }, 2000); console.log("End");

Output:

Start
End
Delayed Message

Promises, Async/Await, and Handling Concurrency in JavaScript

Promises and async/await are used to handle asynchronous tasks more effectively, avoiding "callback hell" and making the code more readable.

Promises: Promises represent a value that may be available now, or in the future, or never. They have three states: pending, fulfilled, and rejected.

Example: This example demonstrates the Promises in javascript.

JavaScript

let promise = new Promise((resolve, reject) => {   setTimeout(() => {     resolve("Promise Resolved!");   }, 1000); });  promise.then((message) => {   console.log(message); });

Output:

Promise Resolved!

Async/Await: async functions allow you to write asynchronous code that looks synchronous. The await keyword pauses the execution until the promise resolves.

Example: This example demonstrates the usuage of Async/await function.

JavaScript

async function fetchData() {   let data = await fetch("https://api.example.com/data");   console.log("Data fetched:", data); }  fetchData();

Event-Driven Architecture in JavaScript

JavaScript is inherently event-driven, meaning actions are taken in response to events such as user interactions, timers, or network responses.

JavaScript's Event-Driven Nature in Web Development

Events: Events are actions or occurrences that happen in the system you are programming, such as a user clicking a button or a webpage loading.
Event Listeners: Functions that are executed in response to certain events.

Example: When the button with the ID myButton is clicked, the event listener executes the function, logging “Button clicked!”.

JavaScript

document.getElementById("myButton").addEventListener("click", function() { console.log("Button clicked!"); });

Event Delegation and Handling User Interactions

Event delegation is a technique to handle events efficiently by using a single event listener to manage all events of a particular type.

Example: Instead of attaching an event listener to each li element, we attach it to the parent and use event delegation to identify which li was clicked.

JavaScript

document.getElementById("parent").addEventListener("click", function (event) {     if (event.target && event.target.matches("li.item")) {         console.log("List item clicked!");     } });

JavaScript’s Prototypal Inheritance

Prototypal inheritance allows objects to inherit properties and methods from other objects, using prototypes.

prototypincalInheritanceInJs — JavaScript’s Prototypal Inheritance

How Objects Inherit Properties and Methods in JavaScript

Prototype Chain: Each object in JavaScript has a prototype object, which acts as a template from which it inherits properties and methods.

Example: This JavaScript code defines a constructor function Person to create person objects with name and age properties, and adds a greet method to the Person prototype to print a greeting message.

JavaScript

function Person(name, age) {     this.name = name;     this.age = age; }  Person.prototype.greet = function () {     console.log("Hello, my name is " + this.name); };  let john = new Person("John", 30); john.greet();

Output

Hello, my name is John

Role of Prototypes in JavaScript

Inheritance: Prototypes enable inheritance in JavaScript. An object can use another object's methods and properties through its prototype chain.
Shared Methods: Methods defined on the prototype are shared across all instances, saving memory.

JIT Compilation (Just-in-Time Compilation) in JavaScript

JIT compilation is an optimization technique used by modern JavaScript engines to improve performance by compiling code at runtime.

How Modern JavaScript Engines Use JIT for Performance Optimization

Parsing and Compilation: Initially, JavaScript code is parsed and compiled to an intermediate bytecode, which is executed by the engine.
Dynamic Optimization: The JIT compiler optimizes frequently executed code paths, compiling them to machine code to speed up execution.

Example: The add function, called repeatedly in a loop, may be optimized by the JIT compiler to reduce execution time.

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

for (let i = 0; i < 1000000; i++) {
    add(10, 20);
}

Modules and Scope in JavaScript

Modules and scopes in JavaScript help in organizing code, preventing conflicts, and improving maintainability.

Understanding JavaScript Modules (ES6 Modules, CommonJS)

ES6 Modules: Introduced in ES6, they use import and export statements to include or share code between files.

Example:

// In utils.js
export function add(a, b) {
    return a + b;
}

// In main.js
import { add } from './utils.js';
console.log(add(10, 20)); // Output: 30

CommonJS: CommonJS modules use require and module.exports for importing and exporting code (commonly used in Node.js).

Example:

// In utils.js
module.exports = {
    add: function (a, b) {
        return a + b;
    }
};

// In main.js
const utils = require('./utils.js');
console.log(utils.add(10, 20)); //Output:30

Global, Function, and Block-Level Scopes

Global Scope: Variables declared outside of functions or blocks are in the global scope, accessible from anywhere in the code.

var globalVar = "I am global";

function showGlobal() {
    console.log(globalVar);
}

showGlobal(); //Output: I am Global

Function Scope: Variables declared within a function are only accessible within that function.

function showLocal() {
    var localVar = "I am local";
    console.log(localVar);
}

showLocal(); // Output: I am local
console.log(localVar); // Error: localVar is not defined

Block Scope: Introduced in ES6, let and const are block-scoped, meaning they are only accessible within the block they are declared in.

if (true) {
    let blockVar = "I am block scoped";
    console.log(blockVar); // Output: I am block scoped
}
console.log(blockVar); // Error: blockVar is not defined

Conclusion

Understanding these advanced concepts such as concurrency, event-driven architecture, prototypal inheritance, and scopes, helps us to write more efficient and scalable JavaScript code. It allows us to handle asynchronous operations smoothly, utilize inheritance effectively, and organize code through modules and scopes, enhancing both frontend and backend development.

History of JavaScript

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Article Tags :

How JavaScript Works?

Evolution of JavaScript

JavaScript Engines

How JavaScript Engines Interpret and Execute Code?

Execution Contexts in JavaScript

Call Stack and Management of Function Calls in JavaScript

Asynchronous Tasks and Event Loop in JavaScript

Event Loop, Callback Queue, and Microtask Queue

The event loop

Callback queue and microtask queue

Memory Management in JavaScript

Allocation of Memory

Garbage Collection

Call Stack vs. Heap (for function execution)

Call Stack (for function execution)

Heap (for function execution)

Execution Phases in JavaScript

Hoisting in JavaScript

Node.js Runtime in JavaScript

Concurrency Model in JavaScript

How JavaScript is Single-Threaded?

Promises, Async/Await, and Handling Concurrency in JavaScript

Event-Driven Architecture in JavaScript

Event Delegation and Handling User Interactions

JavaScript’s Prototypal Inheritance

How Objects Inherit Properties and Methods in JavaScript

Role of Prototypes in JavaScript

JIT Compilation (Just-in-Time Compilation) in JavaScript

How Modern JavaScript Engines Use JIT for Performance Optimization

Modules and Scope in JavaScript

Understanding JavaScript Modules (ES6 Modules, CommonJS)

Global, Function, and Block-Level Scopes

Conclusion

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