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Delve into the fascinating world of the Javascript Event Loop with this comprehensive guide. As an essential aspect of JavaScript's asynchronous behaviour, the ability to grasp the intricacies of the Event Loop is key to excelling in JavaScript programming. This article will walk you through understanding what the Event Loop is, its basic functions, and how it works. Further, it provides real-world examples showcasing the Event Loop in action, uncovers its relationship with the Call Stack, and explores its role in asynchronous programming. Altogether, this promises to be a must-read for anybody looking to enhance their JavaScript knowledge.
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Jetzt kostenlos anmeldenDelve into the fascinating world of the Javascript Event Loop with this comprehensive guide. As an essential aspect of JavaScript's asynchronous behaviour, the ability to grasp the intricacies of the Event Loop is key to excelling in JavaScript programming. This article will walk you through understanding what the Event Loop is, its basic functions, and how it works. Further, it provides real-world examples showcasing the Event Loop in action, uncovers its relationship with the Call Stack, and explores its role in asynchronous programming. Altogether, this promises to be a must-read for anybody looking to enhance their JavaScript knowledge.
The JavaScript Event Loop, at its core, is a mechanism that handles the execution of multiple threads, allowing JavaScript to appear multi-threaded at the machine level even though in reality, JavaScript isn't.
Think of it like a busy restaurant. The kitchen (javascript's engine) can only handle one order (function/task) at a time, while the remaining orders get queued. The Event Loop is the restaurant's system that checks if the kitchen is ready to take another order.
The JavaScript runtime contains a message queue, which is a list of messages to be processed. Each message has an associated function which gets called to handle the message. At some point during the event loop, the runtime starts handling the messages on the queue, starting with the oldest one. To handle the message, the associated function is called with the message as an input parameter. As soon as the stack is again empty, a check is made to see if there is any pending timed-out delay function and if there is, that function (such as a delayed function from setTimeout) is executed, and this starts a new message execution. So, even though JavaScript is single-threaded, it can still do concurrency thanks to the event loop and the browser's or Node.js' APIs.
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function main() {
console.log('Hi');
setTimeout(function cb1() {
console.log('cb1');
}, 5000);
console.log('Bye');
}
main();
// Output
// "Hi"
// "Bye"
// "cb1" after 5 seconds
In the above code example, we first call the main function. It has three steps - first, it logs "Hi" to the console, then it sets a timeout for 5 seconds, after which it will log "cb1", and finally, it logs "Bye". Because JavaScript only has a single call stack, it handles these operations one after another. So, you'll first see "Hi" logged, then "Bye", and finally, after five seconds have elapsed, "cb1". Even though the setTimeout function was called second, it doesn't stop the rest of the code from executing - this non-blocking behaviour is thanks to the event loop.
setTimeout(function() {
console.log("Hello after 3 seconds");
}, 3000);
When this piece of code runs, the console remains empty and waits for 3 seconds before "Hello after 3 seconds"', is printed out. This delay occurs because of the nature of the JavaScript Event Loop mechanism. Here's how JavaScript accomplishes this:
setTimeout(function() {
console.log("Hello");
}, 0);
setTimeout(function() {
console.log("Hello after 1 second");
}, 1000);
In this scenario, despite the first setTimeout function having a delay time of zero milliseconds, it will not print immediately. Instead, it gets moved into the Web API environment, and after the delay is complete, it will get pushed into the Callback Queue. In the meantime, the JavaScript engine sees the second setTimeout function and moves on to that. Once both completed setTimeout functions return from web APIs, they are sent to the Callback Queue. The function execution order in the Callback Queue is determined by their order of completion.
Finally, the Event Loop continuously monitors the call stack and the Callback Queue. When the call stack is empty and the Callback Queue has executable functions, it moves them into the Stack for execution. Therefore, at first, "Hello after 1 second" will be printed, and then "Hello" will get printed.
Throughout this process, the Event Loop prioritizes ensuring that the call stack gets cleared as quickly as possible before it can receive any incoming messages from the message queue. That's why, even when set to zero seconds, the first setTimeout function's callback doesn't execute immediately. It waits upon the completion of the entire script, demonstrating how the Event Loop gives higher priority to the call stack over the queued messages.
This interplay between the Call Stack, Web API, Callback Queue, and Event Loop is the quintessential functioning of the JavaScript runtime environment and is essential in understanding how asynchronous code works in JavaScript.
function functionOne() {
console.log('Function One');
function functionTwo() {
console.log('Function Two');
}
functionTwo();
}
functionOne();
// Output:
// Function One
// Function Two
Both functionOne and functionTwo are added to the Call Stack as they're called, and removed after they're completed. Because functionTwo was called within functionOne, it was added to the top of the stack and became the currently executing function.
for (let i = 0; i < 1000000000; i++) {
console.log('Blocking Code');
}
setTimeout(function() {
console.log("Non-blocking Code");
}, 0);
In the above example, the loop may block the call stack as it executes. The message "Non-blocking Code" won't be printed until after the for loop finishes, even though the setTimeout delay is set to 0 seconds. This shows how the call stack's 'busy' state affects the Event Loop.
In conclusion, the Call Stack's behaviour influences the effectiveness of the Event Loop significantly, impacting the perceived performance of a JavaScript application. By understanding this relationship, you can write better and more effective JavaScript code.
setTimeout(() => {
console.log('Hello World');
}, 0);
The code above does not guarantee that 'Hello World' will be printed exactly after 0 milliseconds. This is because the timing doesn’t account for the time taken by the Call Stack, the Event Loop, and the Callback Queue to process other operations. When the Event Loop takes the callback function out of the Callback Queue and onto the Call Stack, it only does so when the Stack is empty. This can create a delay.
setTimeout(() => {
console.log('First');
}, 2000);
setTimeout(() => {
console.log('Second');
}, 1000);
Although 'First' appears before 'Second' in the code and hence, into the Call Stack first, 'Second' will be printed before 'First'. This is because the Event Loop will push 'Second' to the Callback Queue before 'First', as it has a smaller time delay. As a result, 'Second' gets printed first.
Understanding these timing nuances and factors can significantly enhance your JavaScript proficiency. Throughout the process, the Event Loop ensures the smooth execution of tasks, maintaining concurrency in JavaScript's single-threaded environment.
console.log('Before setTimeout');
setTimeout(() => {
console.log('Inside setTimeout');
}, 0);
console.log('After setTimeout');
In the output, you'll notice that 'Inside setTimeout' gets printed last, despite a delay of 0 milliseconds. This is due to the Event Loop. Although 'Inside setTimeout' is part of the setTimeout function, it gets passed to the Web API, freeing up the Call Stack. The Call Stack moves on to execute 'After setTimeout'. Only when the Call Stack is cleared and the timer has completed does the Event Loop move 'Inside setTimeout' from the Callback Queue back to the Call Stack for execution.
Understanding the Event Loop's role in this scenario explains why asynchronous JavaScript works the way it does. Even though the setTimeOut function appears as though it should output 'Inside setTimeout' before 'After setTimeout', it doesn't.
Let's examine another example:
console.log('Before fetch request');
fetch('https://jsonplaceholder.typicode.com/posts')
.then(() => console.log('Fetch successful'))
.catch(() => console.log('Fetch failed'));
console.log('After fetch request');
In this case, the fetch request is handled asynchronously, similar to the setTimeout function. Upon calling fetch, it's offloaded to a Web API (the HTTP requests module), letting the Call Stack move on to 'After fetch request'. The '.then()' callback only gets pushed to the Callback Queue once the fetch operation completes. Even if it takes time for the resource to be fetched from the URL, the rest of the program continues running without waiting for the fetch request to complete. The Event Loop, once again, manages this smoothly.
From these examples, the evolving role of the JavaScript Event Loop becomes clear: to keep your JavaScript code non-blocking and running smoothly by efficiently managing the Call Stack and the Callback Queue around asynchronous operations.
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Start learningWhat is the JavaScript Event Loop?
The JavaScript Event Loop is a mechanism that handles the execution of multiple threads, allowing JavaScript to appear multi-threaded even though it's a single-threaded language. It maintains a task queue, supervises the execution stack, and runs tasks as space becomes available.
How does the JavaScript Event Loop work?
The JavaScript Event Loop works in four stages: Stack Frame creation, initial code execution, asynchronous functions are sent to the callback queue, and when stack frame is empty, the event loop checks the callback queue, dequeues functions and pushes them onto the stack frame for execution.
How does the event loop handle asynchronous functions?
Asynchronous functions are sent to a Web API and pushed to the Callback Queue after they're completed. Then, when the Stack Frame is empty, the event loop checks the Callback Queue, and functions are dequeued and pushed onto the Stack Frame for execution.
What is the JavaScript Event Loop mechanism and how does it function?
The JavaScript Event Loop mechanism involves interplay between the Call Stack, Web API, Callback Queue, and Event Loop. Functions are moved from the stack to the Web APIs and then to the Callback Queue. The Event Loop continuously monitors these and moves executable functions from the queue to the stack.
Why does the console remain empty for 3 seconds in the simple Event Loop example?
The console remains empty because the setTimeout function was moved to the Timer Web API, which counts for 3000 milliseconds. After this, the function is moved to the Callback Queue, and finally to the Call Stack when it's empty.
Why is the message "Hello" printed out after "Hello after 1 second" despite a delay time of zero milliseconds in the complex Event Loop example?
Even though the delay is set to zero, the function is moved to the Web API environment. It waits even after the delay completion, for the entire script to finish before being pushed to the Callback Queue. This shows the priority of call stack clearing over queued messages.
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