JavaScript Concurrency: Event Loop Concurrency Model Quiz and Flashcards

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Questions and Answers

Why is an event loop necessary in JavaScript?

To prioritize tasks in a multi-threaded environment

To handle I/O operations synchronously

To enable simultaneous execution of multiple threads

To allow non-blocking code execution alongside blocking code (correct)

What characteristic of JavaScript can lead to blocking code?

Event-driven I/O handling

Single-threaded nature (correct)

Asynchronous execution

Concurrent thread execution

What happens when a for loop is executing in JavaScript?

Only I/O operations can be handled in parallel

The code execution is paused until the loop finishes (correct)

The event loop is paused until the loop finishes

Other tasks can be executed simultaneously

How are I/O operations handled in JavaScript?

Asynchronously using events and callbacks Signup and view all the answers

What is the outcome of running the code example provided in the text?

The second console.log statement is delayed by the for loop's execution Signup and view all the answers

What is the main benefit of using the event loop in JavaScript?

Enablement of non-blocking code execution Signup and view all the answers

What is the first context added to the call stack when a program is initiated?

Global execution context Signup and view all the answers

What happens to a function's frame when it runs to completion?

It is cleared from the call stack Signup and view all the answers

What is the order in which messages are processed in the event queue?

First in, first out (FIFO) Signup and view all the answers

What is the role of the event loop in the event loop concept?

To add messages from the event queue to the call stack Signup and view all the answers

What happens when the call stack is empty and there are messages in the event queue?

The event loop adds messages from the event queue to the call stack Signup and view all the answers

What is the purpose of the setTimeout function in the example code?

To pass a function to be executed by a web API Signup and view all the answers

What is the role of the heap in the event loop concept?

To interact with Node and Web APIs Signup and view all the answers

What is the characteristic of JavaScript due to the event loop?

Single-threaded, event-driven language Signup and view all the answers

What is the sequence of events when the code console.log("This is the first line of code in app.js."); is executed?

Added to the call stack, executes, then pops off Signup and view all the answers

What is the purpose of the event queue in the event loop concept?

To hold functions that are waiting to be added back into the stack Signup and view all the answers

What is the main difference between the synchronous and non-blocking examples in the text?

The blocking or non-blocking nature of the code Signup and view all the answers

What is the primary purpose of the event loop in JavaScript?

To manage code execution and maintain order Signup and view all the answers

What is the term for executing multiple procedures at the same time on the same shared resources?

Concurrency Signup and view all the answers

What is the heap in the JavaScript engine?

A block of memory where objects are stored in an unordered manner Signup and view all the answers

What happens to a function's frame in the call stack when it finishes executing?

It is removed from the stack Signup and view all the answers

What is the order in which frames are added to the call stack?

Last in, first out (LIFO) Signup and view all the answers

What is the purpose of the event queue in the event loop?

To pass messages from Node or Web APIs to the call stack Signup and view all the answers

What is the main difference between the heap and the call stack?

The heap stores objects, while the call stack tracks function calls Signup and view all the answers

What is the role of the Node or Web APIs in the event loop?

To pass messages to the event queue Signup and view all the answers

What is the main benefit of using the event loop in JavaScript?

To allow for concurrency and non-blocking code Signup and view all the answers

Study Notes

Concurrency Model and Event Loop in JavaScript

JavaScript is a single-threaded language, which means two statements can't be executed simultaneously.
The Event Loop is necessary to emulate concurrency and handle non-blocking code.
I/O is handled with events and callbacks, allowing code execution to continue.

Blocking and Non-Blocking Code

Blocking code example: a for loop that takes a while to process will finish executing before the rest of the code runs.
Non-blocking code example: using setTimeout() to run a function after a delay, allowing other code to execute in the meantime.

The Event Loop Components

Heap: a block of memory where objects are stored in an unordered manner.
Call Stack: tracks what function is currently being run in the code, with frames added and removed in a Last-In-First-Out (LIFO) order.
Event Queue: a list of messages corresponding to functions waiting to be processed, with messages added in a First-In-First-Out (FIFO) order.
Node or Web APIs: enable concurrency and pass asynchronous messages back to the stack via the event queue.
Event Loop: manages the interaction between the event queue and the call stack, maintaining the order of code execution.

How the Event Loop Works

The event loop checks if the call stack is empty, and if so, adds the first waiting message from the event queue to the stack.
The event loop repeats the process until the stack is cleared.
The event loop enables concurrency in JavaScript by allowing non-blocking code to run asynchronously.

Example of the Event Loop in Action

setTimeout() is called with a callback function, which is added to the event queue after a delay.
The event loop checks the call stack and adds the callback function to the stack when it is empty.
The callback function is executed, and then popped off the stack.

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Description

Learn how JavaScript emulates concurrency using its event loop, enabling non-blocking code and multitasking. Understand the behind-the-scenes mechanics of asynchronous programming.