angular-cn/TOOLS_JS.md

# Developer Tools for JavaScript

Here you will find a collection of tools and tips for keeping your application
perform well and contain fewer bugs.

## Angular debug tools in the dev console

Angular provides a set of debug tools that are accessible from any browser's
developer console. In Chrome the dev console can be accessed by pressing
Ctrl + Shift + j.

### Enabling debug tools

By default the debug tools are disabled. You can enable debug tools as follows:

```typescript
import {enableDebugTools} from '@angular/platform-browser';

bootstrap(Application).then((appRef) => {
  enableDebugTools(appRef);
});
```

### Using debug tools

In the browser open the developer console (Ctrl + Shift + j in Chrome). The
top level object is called `ng` and contains more specific tools inside it.

Example:

```javascript
ng.profiler.timeChangeDetection();
```

## Performance

### Change detection profiler

If your application is janky (it misses frames) or is slow according to other
metrics it is important to find the root cause of the issue. Change detection
is a phase in Angular's lifecycle that detects changes in values that are
bound to UI, and if it finds a change it performs the corresponding UI update.
However, sometimes it is hard to tell if the slowness is due to the act of
computing the changes being slow, or due to the act of applying those changes
to the UI. For your application to be performant it is important that the
process of computing changes is very fast. For best results it should be under
3 milliseconds in order to leave room for the application logic, the UI updates
and browser's rendering pipeline to fit withing the 16 millisecond frame
(assuming the 60 FPS target frame rate).

Change detection profiler repeatedly performs change detection without invoking
any user actions, such as clicking buttons or entering text in input fields. It
then computes the average amount of time it took to perform a single cycle of
change detection in milliseconds and prints it to the console. This number
depends on the current state of the UI. You will likely see different numbers
as you go from one screen in your application to another.

#### Running the profiler

Enable debug tools (see above), then in the dev console enter the following:

```javascript
ng.profiler.timeChangeDetection();
```

The results will be printed to the console.

#### Recording CPU profile

Pass `{record: true}` an argument:

```javascript
ng.profiler.timeChangeDetection({record: true});
```

Then open the "Profiles" tab. You will see the recorded profile titled
"Change Detection". In Chrome, if you record the profile repeatedly, all the
profiles will be nested under "Change Detection".

#### Interpreting the numbers

In a properly-designed application repeated attempts to detect changes without
any user actions should result in no changes to be applied on the UI. It is
also desirable to have the cost of a user action be proportional to the amount
of UI changes required. For example, popping up a menu with 5 items should be
vastly faster than rendering a table of 500 rows and 10 columns. Therefore,
change detection with no UI updates should be as fast as possible. Ideally the
number printed by the profiler should be well below the length of a single
animation frame (16ms). A good rule of thumb is to keep it under 3ms.

#### Investigating slow change detection

So you found a screen in your application on which the profiler reports a very
high number (i.e. >3ms). This is where a recorded CPU profile can help. Enable
recording while profiling:

```javascript
ng.profiler.timeChangeDetection({record: true});
```

Then look for hot spots using
[Chrome CPU profiler](https://developer.chrome.com/devtools/docs/cpu-profiling).

#### Reducing change detection cost

There are many reasons for slow change detection. To gain intuition about
possible causes it would help to understand how change detection works. Such a
discussion is outside the scope of this document (TODO link to docs), but here
are some key concepts in brief.

By default Angular uses "dirty checking" mechanism for finding model changes.
This mechanism involves evaluating every bound expression that's active on the
UI. These usually include text interpolation via `{{expression}}` and property
bindings via `[prop]="expression"`. If any of the evaluated expressions are
costly to compute they could contribute to slow change detection. A good way to
speed things up is to use plain class fields in your expressions and avoid any
kinds of computation. Example:

```typescript
@Component({
  template: '<button [enabled]="isEnabled">{{title}}</button>'
})
class FancyButton {
  // GOOD: no computation, just return the value
  isEnabled: boolean;

  // BAD: computes the final value upon request
  _title: String;
  get title(): String { return this._title.trim().toUpperCase(); }
}
```

Most cases like these could be solved by precomputing the value and storing the
final value in a field.

Angular also supports a second type of change detection - the "push" model. In
this model Angular does not poll your component for changes. Instead, the
component "tells" Angular when it changes and only then does Angular perform
the update. This model is suitable in situations when your data model uses
observable or immutable objects (also a discussion for another time).