JavaScript Fundamentals with Engine, Call Stack, Event Loop, and Memory Management

Introduction

JavaScript, a dynamic and versatile programming language, is integral to the interactivity of modern web applications. To fully leverage its capabilities, it’s essential to understand its internal workings. This exploration delves into the JavaScript engine, the call stack, the event loop, and memory management.

The JavaScript Engine

The JavaScript engine is at the heart of code execution, interpreting, and running JavaScript. Each browser has its own engine: Chrome uses V8, Firefox uses SpiderMonkey, Safari uses JavaScriptCore, and Microsoft Edge uses Chakra.

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Parsing and Compilation

The execution process starts with parsing:

Lexical Analysis (Tokenization): The source code is broken down into tokens, the smallest units such as keywords, operators, and identifiers.
Syntax Analysis (Parsing): These tokens are parsed into an Abstract Syntax Tree (AST), which represents the hierarchical structure of the code.

Next comes compilation:

Interpreter: Initially, an interpreter may quickly translate and execute the AST, but this can be slow for complex programs.
Just-In-Time (JIT) Compilation: To optimize performance, modern engines use JIT compilation, which compiles code to machine language at runtime, optimizing based on actual usage patterns.

Optimization Techniques

JavaScript engines use various optimization methods:

Hidden Classes and Inline Caching: These improve the performance of property access and method calls on objects.
Garbage Collection: This process automatically reclaims memory no longer in use, preventing memory leaks.

The Call Stack

JavaScript operates as a single-threaded language, executing one command at a time, managed through the call stack:

Call Stack: This data structure tracks function calls. When a function is called, it’s pushed onto the stack, and when it returns, it’s popped off.

Example

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

function first() {

second();

console.log("First function");

}

function second() {

console.log("Second function");

}

first();

Here’s the call stack in action:

first() is called and pushed onto the stack.
second() is called within first() and pushed onto the stack.
second() executes, logs “Second function,” and is popped off.
first() resumes, logs “First function,” and is popped off.

The Event Loop

Although JavaScript is single-threaded, it can handle asynchronous operations efficiently through the event loop. This allows for non-blocking operations, such as handling I/O, timers, and user interactions.

Components

Web APIs: The browser provides these and handle asynchronous operations like setTimeout, HTTP requests, and DOM events.
Callback Queue: Completed asynchronous operations place their callback functions in this queue.
Event Loop: Continuously checks the call stack and callback queue. If the call stack is empty, the event loop moves the first callback from the queue to the stack for execution.

Example

console.log("Start");
 
setTimeout(() => {
    console.log("Timeout");
}, 1000);
 
console.log("End");

console.log("Start");

setTimeout(() => {

console.log("Timeout");

}, 1000);

console.log("End");

Execution Flow:

“Start” is logged.
setTimeout registers a callback with a delay.
“End” is logged.
After the delay, the callback is moved to the call stack, and “Timeout” is logged.

Memory Management

JavaScript handles memory automatically through garbage collection, periodically freeing up memory used by objects no longer in use.

Garbage Collection Process

Mark-and-Sweep Algorithm: This common method traverses the object graph, marking reachable objects and sweeping away unmarked ones.
Reference Counting: Tracks the number of references to each object. When an object’s reference count drops to zero, it’s eligible for garbage collection.

let user = {
    name: "John"
};
 
let admin = user;
user = null;

let user = {

name: "John"

};

let admin = user;

user = null;

Initially, user references the object { name: “John” }. When user is set to null, the object is still referenced by admin, so it isn’t collected. If admin is also set to null, the object becomes unreachable and is collected.

Conclusion

Grasping JavaScript’s internal operations provides a deeper understanding of its capabilities and performance. From parsing and JIT compilation to the call stack’s execution model, the event loop’s management of asynchronous tasks, and efficient memory management, these elements collectively enhance JavaScript’s robustness.

This foundational knowledge aids developers in writing more efficient, optimized, and maintainable code.

Drop a query if you have any questions regarding JavaScript and we will get back to you quickly.

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FAQs

1. What is the role of the JavaScript engine?

ANS: – The JavaScript engine is responsible for interpreting and executing JavaScript code. It includes parsing the code into an Abstract Syntax Tree (AST), compiling the code into machine language using Just-In-Time (JIT) compilation, and executing the compiled code.

2. How does JavaScript handle asynchronous operations if it is single-threaded?

ANS: – JavaScript uses the event loop to manage asynchronous operations. The event loop allows JavaScript to handle non-blocking operations by delegating tasks to Web APIs, placing callbacks in the callback queue, and executing these callbacks when the call stack is empty.