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# The JS Event Loop

# Introducción al Event Loop

All JavaScript environments use an event loop

As long as there’s something left to do, JSs event loop will keep spinning. Whenever an event occurs, JS invokes any callbacks (event handlers) that are listening for that event.

The general algorithm of the JS Engine is like this:

executeSequentially(mainTask);
while (thereArePendingTasks()) do
  let oldestFinishedTask = waitUntilSomeTaskIsReady();
  executeSequentially(oldestFinishedTask);
end do
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There’s an endless loop, when JavaScript engine waits for tasks, executes them and then sleeps waiting for more tasks

When JS runs in the browser:

  • Rendering never happens while the engine executes a task. Doesn’t matter if the task takes a long time. Changes to DOM are painted only after the task is complete.

  • If a task takes too long, the browser can’t do other tasks, process user events, so after a time it raises an alert like Page Unresponsive suggesting to kill the task with the whole page. That happens when there are a lot of complex calculations or a programming error leading to infinite loop.

  • There is in fact a Render queue, which is another queue in the event loop system which basically takes care of all tasks to be done before every screen update or repaints.

  • Your JavaScript code runs single threaded. There is just one thing happening at a time.

    • Pay attention to how you write your code and avoid anything that could block the thread, like synchronous network calls or long loops.
    • In most browsers there is an event loop for every browser tab, to avoid a web page with heavy processing to block your entire browser.
    • Web Workers run in their own event loop as well

Quote from MDN (opens new window):

Each message is processed completely before any other message is processed [...] A downside of this model is that if a message takes too long to complete, the web application is unable to process user interactions like click or scroll. The browser mitigates this with the "a script is taking too long to run" dialog. A good practice to follow is to make message processing short and if possible cut down one message into several messages.

MDN utiliza la terminología cola de mensajes para la cola de callbacks:

A JavaScript runtime uses a message queue, which is a list of messages to be processed. Each message has an associated function which gets called in order to handle the message.

# Unas Preguntas

# La pila de LLamadas

console.trace muestra la stack trace (opens new window) en la consola. la stack trace es el camino de las llamadas hasta el punto: { nombre-de-rutina, fichero-del-programa, nº de línea, nº de columna}

¿Cual es la salida de las llamadas a console.trace?

    1	function multiply(x,y) {
    2	  // console.trace imprime una traza de la pila
    3	  console.trace("-----------At multiply-----------");
    4	  return x * y;
    5	}
    6
    7	function squared(n) {
    8	  console.trace("-----------At squared-----------");
    9	  return multiply(n,n)
    10	}
    11
    12	function printSquare(n) {
    13	   return squared(n)
    14	}
    15
    16	let numberSquared = printSquare(5);
    17	console.log(numberSquared);
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Output from execution

# Orden de Ejecución

¿En que orden ocurren las salidas?

(function() {

  console.log('this is the start');

  setTimeout(function cb() {
    console.log('Callback 1: this is a msg from call back');
  }); // has a default time value of 0

  console.log('this is just a message');

  setTimeout(function cb1() {
    console.log('Callback 2: this is a msg from call back');
  }, 0);

  console.log('this is the end');

})();
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  • El método setTimeout(funcion, retraso) establece un temporizador que ejecuta funcion después de que transcurre un tiempo retraso en milisegundos.
  • Si se omite este parámetro se usa el valor 0.
  • El valor retornado identifica el temporizador creado con la llamada a setTimeout(); este valor puede pasarse a clearTimeout() para cancelar el temporizador.

# Hoist: Cual es la salida?

console.log(hoist); 

var hoist = 'The variable has been hoisted.';
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¿Cual sería la salida de esta variante?

console.log(hoist);

hoist = 'The variable has been hoisted.'
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Para responder a esta pregunta es conveniente tener claro el concepto de Hoisting (opens new window) en JS. Hoisting is a JavaScript mechanism where variables, function and class declarations are moved to the top of their scope before code execution. Hoisting only moves the declaration. The assignments are left in place.

The scope of a variable declared with the keyword var is its current execution context. This is either the enclosing function or for variables declared outside any function, global.

Véase Understanding Hoisting in JavaScript by Mabishi Wakio (opens new window)

# Hoisting y Asíncronía ¿Cual es la salida?

for(var i=0;i<=3; i++) {
    setTimeout(()=> console.log(i), 0)
}
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➜  event-loop-examples-1920 git:(master) ✗ pwd -P
/Users/casianorodriguezleon/campus-virtual/1920/sytws1920/apuntes/tema1-introduccion/practicas/p2-t1-c3-file-system/event-loop
➜  event-loop-examples-1920 git:(master) ✗ ls
README.md                              hoist.js                               splitting-cpu-hungry-task.html
animated-circle-with-promise.html      how-the-event-loop-works.png           var.js
callstack.js                           order.js
event-loop.png                         settimeout-does-not-run-inmediately.js
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# JS is single threaded

¿Cual es la salida?

const s = new Date().getSeconds();

setTimeout(function() {
  console.log("Ran after " + (new Date().getSeconds() - s) + " seconds");
}, 500);

while(true) {
  if(new Date().getSeconds() - s >= 2) {
    console.log("Good, looped for 2 seconds");
    break;
  }
}
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  • Si no proporciona argumentos, el constructor new Date() crea un objeto Date con la hora y fecha de hoy según la hora local.
  • El método getSeconds() devuelve un número entre 0 y 59 representando los segundos en la fecha especificada de acuerdo a la hora local.

# Repasando las Preguntas a la luz del Bucle de Eventos

# Ejemplo: La Pila de Llamadas

Este ejemplo es tomado del vídeo:

se le puede proporcionar a loupe:

Está en este directorio en mi laptop:

  • ~/campus-virtual/1920/sytws1920/ull-mii-sytws-1920.github.io/tema1-introduccion/practicas/p2-t1-c3-file-system/event-loop/callstack.js

Este es el código:

function multiply(x,y) {
  // console.trace imprime una traza de la pila
  console.trace("-----------At multiply-----------");
  return x * y;
}

function squared(n) {
  console.trace("-----------At squared-----------");
  return multiply(n,n)
}

function printSquare(n) {
   return squared(n)
}

let numberSquared = printSquare(5);
console.log(numberSquared);
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Output from execution

# Orden de Ejecución

Directorio en mi máquina:

tema1-introduccion/practicas/p2-t1-c3-file-system/event-loop/order.js
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Ejemplo sacado del Tutorial Concurrency model and Event Loop (opens new window) at https://developer.mozilla.org (opens new window)

(function() {

  console.log('this is the start');

  setTimeout(function cb() {
    console.log('Callback 1: this is a msg from call back');
  }); // has a default time value of 0

  console.log('this is just a message');

  setTimeout(function cb1() {
    console.log('Callback 2: this is a msg from call back');
  }, 0);

  console.log('this is the end');

})();
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# Ejemplo: JS is single threaded

En mi máquina:

tema1-introduccion/practicas/p2-t1-c3-file-system/event-loop/settimeout-does-not-run-inmediately.js
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Tomado del tutorial:

Primero, un ejemplo, para entender el funcionamiento de Datey getSeconds:

[~/.../p2-t1-c3-file-system/event-loop(master)]$ node
Welcome to Node.js v12.10.0.
Type ".help" for more information.
> d = new Date()
2020-02-16T10:07:51.682Z
> s = d.getSeconds()
51
> e = new Date()
2020-02-16T10:07:57.992Z
> e.getSeconds()-d.getSeconds()
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¿Cual es la salida?

const s = new Date().getSeconds();

setTimeout(function() {
  console.log("Ran after " + (new Date().getSeconds() - s) + " seconds");
}, 500);

while(true) {
  if(new Date().getSeconds() - s >= 2) {
    console.log("Good, looped for 2 seconds");
    break;
  }
}
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# Race Condition

See the examples:

# The Event Loop en el libro The Modern JavaScript Tutorial (opens new window)

# The section Concurrency model and the event loop at https://developer.mozilla.org/

# References

Last Updated: a month ago