Imagine you're in a restaurant. It's busy, so when you go to order, you don't just wait for the cook to complete your order before doing anything else. Instead, you tell the waiter your order, they give it to the cook, and then they go take other orders. Meanwhile, you're free to do other things, like chatting with friends. The kitchen completes orders as they can, and eventually, your food is ready and brought to your table. The Event Loop in JavaScript works the same way. You can give jobs to JavaScript (like ordering food), and it will handle them when it's able to. This is convenient since you don't have to wait for a large job to be finished before doing other tasks.
How does it work?
The Event Loop has a simple job: look at the stack and the task queue. If the stack is empty, it takes the first thing on the queue and pushes it onto the stack, which effectively runs it. This process is a loop, hence the name 'Event Loop'. Simply put, it ensures that JavaScript executes all tasks efficiently, whether they're initiated by user actions, like clicks, or defined by timed functions, such as setTimeout.
The Stack and the Queue
There are two main parts of Event Loop: the 'stack' and the 'task queue' (also known as 'message queue'). The stack is where the JavaScript engine keeps track of what function is currently running (think of it like a to-do list, where you're checking off things one at a time from the top). The task queue is where events from the browser are lined up to be executed (think of it like the cook's list of orders in the restaurant).
Code Example: Event Loop
A code example can demonstrate how event loops work. If you set two functions, one that's relatively lengthy and another that's short, JavaScript doesn't necessarily execute the functions in the order they're set. This is because JavaScript uses event loops to complete tasks efficiently.
console.log('Start');
setTimeout(function cb() {
console.log('Callback');
}, 5000);
console.log('End');
```
In this example, 'Start' and 'End' are logged to the console immediately, while 'Callback' - even though it appears earlier in the code bundle - won't be logged until 5 seconds later.To summarize, the Event Loop in JavaScript is like a restaurant's service flow. The tasks (orders) are executed when the JavaScript engine (cook) can handle them, while you (the customer) can continue with other jobs without getting paused. It is a fundamental concept that helps JavaScript handle tasks efficiently.
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JavaScript is a special language that computers understand. With it, we can tell a computer what we want it to do. We can use it to make websites interactive, like when you click a button and something happens.
What is the Event Loop?
Imagine you're in a restaurant. You're the chef (JavaScript), and there are lots of people (events) waiting to get their food. You can only cook one meal at a time, so you have a system to help with that. You take all the orders (tasks) and put them on a big board (the event queue) in the order they are received. Then, when you finish one order, you look at the board and take the next one. This is very similar to how Event Loop works in JavaScript.
Why is Event Loop important?
Our chef can't cook all meals at once, right? Event Loop helps JavaScript do things one after the other. Without Event Loop, JavaScript can get confused with too many orders. But with Event Loop, even if there are many orders, JavaScript can handle them smoothly.
Coding example
Look at this as orders in our restaurant. The chef (JavaScript) first handles 'First', then it sees a delayed task 'With a delay', but it doesn’t wait – it goes on to the next task, 'Last'. After a while, when the delay expired, 'With a delay' comes from the board and gets handled.
console.log('First');
setTimeout(function() {
console.log('With a delay');
}, 5000);
console.log('Last');
So, the Event Loop lets JavaScript cook our meals one by one, keeping everything in order and not letting anything burn!
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If you want to understand how JavaScript handles tasks in a more detailed manner, you might want to research about Promises in JavaScript
About author
Roman Y.
Senior Software Engineer at Nike
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