zone.js

What is zone.js?

The zone.js package is a library that implements Zones for JavaScript. Zones are execution contexts that allow you to intercept and keep track of asynchronous operations in JavaScript. This is particularly useful for debugging, performance tracking, and managing multiple tasks in complex applications such as Angular.

What are zone.js's main functionalities?

Error Handling

Intercepts errors within a specific zone and allows custom error handling.

Zone.current.fork({
  name: 'errorHandlingZone',
  onHandleError: (parentZoneDelegate, currentZone, targetZone, error) => {
    console.error('Error intercepted in zone:', error);
    return false;
  }
}).run(() => {
  throw new Error('Test Error');
});

Execution Context Tracking

Tracks the scheduling and execution of asynchronous tasks, providing insights into the application's asynchronous flow.

Zone.current.fork({
  name: 'trackingZone',
  onScheduleTask: (delegate, currentZone, targetZone, task) => {
    console.log('Task scheduled:', task.source);
    return delegate.scheduleTask(targetZone, task);
  }
}).run(() => {
  setTimeout(() => {
    console.log('Timeout callback executed.');
  }, 1000);
});

Performance Monitoring

Measures the time taken to execute asynchronous tasks, which can be used for performance analysis.

Zone.current.fork({
  name: 'performanceMonitoringZone',
  onInvokeTask: (delegate, currentZone, targetZone, task, applyThis, applyArgs) => {
    const start = performance.now();
    delegate.invokeTask(targetZone, task, applyThis, applyArgs);
    const duration = performance.now() - start;
    console.log('Task took:', duration, 'ms');
  }
}).run(() => {
  setTimeout(() => {
    console.log('Timeout callback executed.');
  }, 1000);
});

Other packages similar to zone.js

async_hooks

async_hooks is a core Node.js module that provides an API to track asynchronous resources. Unlike zone.js, which works in both browser and Node.js environments, async_hooks is specific to Node.js. It offers a lower-level API compared to zone.js and requires more manual handling.

cls-hooked

cls-hooked is a Node.js package that uses async_hooks to provide continuation-local storage (CLS). It allows you to set and get context across async operations, similar to how zones work. However, cls-hooked focuses on context propagation rather than the broader range of interception capabilities that zone.js offers.

continuation-local-storage

continuation-local-storage is another Node.js package that provides CLS functionality. It predates cls-hooked and async_hooks, and it uses a different mechanism to track context. It is less performant than cls-hooked and has been largely superseded by it, but it serves a similar purpose to zone.js in terms of context management.

README

Zone.js

Implements Zones for JavaScript, inspired by Dart.

If you're using zone.js via unpkg please provide a query param ?main=browser
https://unpkg.com/zone.js?main=browser

NEW Zone.js POST-v0.6.0

See the new API here.

DEPRECATED Zone.js PRE-v0.5.15

What's a Zone?

A Zone is an execution context that persists across async tasks. You can think of it as thread-local storage for JavaScript VMs.

See this video from ng-conf 2014 for a detailed explanation:

Running Within a Zone

You can run code within a zone with zone.run. Tasks scheduled (with setTimeout, setInterval, or event listeners) stay within that zone.

Zone.current.fork({}).run(function () {
    Zone.current.inTheZone = true;

    setTimeout(function () {
        console.log('in the zone: ' + !!Zone.current.inTheZone);
    }, 0);
});

console.log('in the zone: ' + !!Zone.current.inTheZone);

The above will log:

'in the zone: false'
'in the zone: true'

Note that the function delayed by setTimeout stays inside the zone.

Forking a Zone

Zones have a set of hooks that allow you to change the behavior of code running within that zone. To change a zone, you fork it to get a new one.

zone.fork({
  beforeTask: function () {
    console.log('hi');
  }
}).run(function () {
  // do stuff
});

Hooks that you don't override when forking a zone are inherited from the existing one.

See the API docs below for more.

Usage

To start using Zones, you need to include the zone.js script in this package onto your page. This script should appear in the <head> of your HTML file before any other scripts, including shims/polyfills.

Examples

There are two kinds of examples:

1. The kind you have to run
2. Illustrative code snippets in this README

Running the ones that you have to run

For fully working examples:

1. Spawn a webserver in the root of the directory in which this repo lives. (I like to use python -m SimpleHTTPServer 3000).
2. Open http://localhost:3000/example in your browser

Below are the aforementioned snippets.

Overriding A Zone's Hook

var someZone = zone.fork({
  afterTask: function () {
    console.log('goodbye');
  }
});

someZone.fork({
  afterTask: function () {
    console.log('cya l8r');
  }
}).run(function () {
  // do stuff
});

// logs: cya l8r

Augmenting A Zone's Hook

When you fork a zone, you'll often want to control how the parent zone's hook gets called.

Prefixing a hook with $ means that the hook will be passed the parent zone's hook, and the hook will be expected to return the function to be invoked rather than be the function itself.

var someZone = zone.fork({
  afterTask: function () {
    console.log('goodbye');
  }
});

someZone.fork({
  $afterTask: function (parentOnLeave) {
    // return the hook
    return function afterTask() {
      parentOnLeave();
      console.log('cya l8r');
    };
  }
}).run(function () {
  // do stuff
});

// logs: goodbye
//       cya l8r

+ and - Sugar

Most of the time, you'll want to run a hook before or after the parent's implementation. You can prefix a hook with - for running before, and + for running after.

The above can be written like this:

var someZone = zone.fork({
  afterTask: function () {
    console.log('goodbye');
  }
});

someZone.fork({
  '+afterTask': function () {
    console.log('cya l8r');
  }
}).run(function () {
  // do stuff
});

// logs: goodbye
//       cya l8r

This frees you from writing boilerplate to compose a new hook.

API

Zone.js exports a single object: window.zone.

zone.run

Runs a given function within the zone. Explained above.

zone.bind

Transforms a function to run within the given zone.

zone.fork

var myZone = zone.fork({
  onZoneCreated: function () {},
  beforeTask: function () {},
  afterTask: function () {},
  onError: function () {},
  enqueueTask: function() {},
  dequeueTask: function() {},
  setTimeout: function () {},
  setInterval: function () {},
  alert: function () {},
  prompt: function () {},
});
myZone.run(function () {
  // woo!
});

Below describes the behavior of each of these hooks.

zone.onZoneCreated

Runs when a zone is forked.

zone.beforeTask

Before a function invoked with zone.run, this hook runs. If zone.beforeTask throws, the function passed to run will not be invoked.

zone.afterTask

After a function in a zone runs, the afterTask hook runs. This hook will run even if the function passed to run throws.

zone.onError

This hook is called when the function passed to run or the beforeTask hook throws.

zone.enqueueTask

This hook is called when a function is registered with the VM. For instance setTimeout and addEventListener.

zone.dequeueTask

This hook is called when a function is unregistered with the VM. For instance clearTimeout and removeEventListener.

zone.setTimeout, zone.setInterval, zone.alert, zone.prompt

These hooks allow you to change the behavior of window.setTimeout, window.setInterval, etc. While in this zone, calls to window.setTimeout will redirect to zone.setTimeout.

zone.requestAnimationFrame, zone.webkitRequestAnimationFrame, zone.mozRequestAnimationFrame

These hooks allow you to change the behavior of window.requestAnimationFrame(), window.webkitRequestAnimationFrame, and window.mozRequestAnimationFrame.

By default the wrapCallback is executed in the zone where those methods have been called to avoid growing the stack size on each recursive call.

zone.addEventListener

This hook allows you to intercept calls to EventTarget#addEventListener.

var clickListenerCount = 0;

zone.fork(
  $addEventListener: function(parentAddEventListener) {
    return function (type, listener) {
      if (type === 'click') clickListenerCount++;
      return parentAddEventListener.apply(this, arguments);
    };
  }
);

zone.run(function() {
  myElement.addEventListener('click', listener);
  myOtherElement.addEventListener('click', listener);

  console.log(clickListenerCount); // 2
});

zone.removeEventListener

This hook allows you to intercept calls to EventTarget#removeEventListener.

var clickListenerCount = 0;

zone.fork(
  $removeEventListener: function(parentRemoveEventListener) {
    return function (type, listener) {
      if (type === 'click') clickListenerCount--;
      return parentRemoveEventListener.apply(this, arguments);
    };
  }
);

zone.run(function() {
  myElement.addEventListener('click', listener);
  myElement.removeEventListener('click', listener);

  console.log(clickListenerCount); // 0
});

Status

• setTimeout, setInterval, and addEventListener work in FF23, IE10, and Chrome.
• stack trace rewrite is kinda ugly and may contain extraneous calls.
• elt.onevent works in FF23, IE10, but not Chrome. There's a fix in the works though!

See also

• async-listener - a similar library for node
• Async stack traces in Chrome
• strongloop/zone
• vizone - control flow visualizer that uses zone.js

License

MIT