The Angular ahead-of-time (AOT) compiler converts your Angular HTML and TypeScript code
into efficient JavaScript code during the build phase before the browser downloads
and runs that code.
This is the best compilation mode for production environments, with decreased load time and increased performance.
By compiling your application using the `ngc` command-line tool, you can bootstrap directly to a module factory, meaning you don't need to include the Angular compiler in your JavaScript bundle.
In Angular, an app's root NgModule (`AppModule`) has a `bootstrap` property that identifies the app's top-level [components](guide/glossary#component).
During the bootstrap process, Angular creates and inserts these components into the `index.html` host web page.
You can bootstrap multiple apps in the same `index.html`, each app with its own components.
Angular uses capitalization conventions to distinguish the names of various types, as described in the [Style Guide "Naming" section](guide/styleguide#02-01).
- UPPER_UNDERSCORE_CASE (or UPPER_SNAKE_CASE): traditional for constants (acceptable, but prefer camelCase)
{@a class-decorator}
## Class decorator
A [decorator](guide/glossary#decorator) statement immediately before a class definition that declares the class to be of the given type, and provides metadata suitable to the type.
A [decorator](guide/glossary#decorator) statement immediately before a field in a class definition that declares the type of that field. Some examples are `@Input` and `@Output`.
The [Angular CLI](https://cli.angular.io/) is a command-line tool that can create a project, add files, and perform a variety of ongoing development tasks such as testing, bundling, and deployment.
A component is a special type of [directive](guide/glossary#directive) that represents a [view](guide/glossary#view).The `@Component` decorator extends the `@Directive` decorator with template-oriented features.
An Angular component class is responsible for exposing data and handling most of the view's display and user-interaction logic through [data binding](guide/glossary#data-binding).
A Web Platform feature, currently supported by most browsers, and available in other browsers through polyfills (see [Browser Support](guide/browser-support)).
The custom element feature extends HTML by allowing you to define a tag whose content is created and controlled by JavaScript code. A custom element (also called a *web component*) is recognized by a browser when it is added to the [CustomElementRegistry](https://developer.mozilla.org/en-US/docs/Web/API/CustomElementRegistry).
You can use the API to transform an Angular component so that it can be registered with the browser and used in any HTML that you add directly to the DOM within an Angular app. The custom element tag inserts the component's view, with change-detection and data-binding functionality, into content that would otherwise be displayed without Angular processing.
Angular defines decorators that attach metadata to classes or properties so that it knows what those classes or properties mean and how they should work.
In Angular, dependencies are typically services, but can also be values, such as strings or functions. An [injector](guide/glossary#injector) for an app (created automatically during bootstrap) creates dependencies when needed, using a registered [provider](guide/glossary#provider) of the service or value. Different providers can provide different implementations of the same service.
A class with the `@Directive` [decorator](guide/glossary#decorator) that can modify the structure of the DOM, or modify attributes in the DOM and component data model.
A directive class is usually associated with an HTML element or attribute, and that element or attribute is often referred to as the directive itself.
When Angular finds a directive in an HTML [template](guide/glossary#template), it creates the matching directive class instance and gives the instance control over that portion of the browser DOM.
Angular supplies a number of built-in directives that begin with the `ng` prefix. You can also create new directives to implement your own functionality.
You associate a _selector_ (an HTML tag such as `<my-directive>`) with a custom directive, thereby extending the [template syntax](guide/template-syntax) that you can use in your apps.
Angular extends TypeScript with domain-specific languages for a number of domains relevant to Angular apps, defined in ngModules such as [animations](guide/animations), [forms](guide/forms), and [routing and navigation](guide/router).
A technique for adding a component to the DOM at run time, which requires that you exclude the component from compilation, then connect it to Angular's change-detection and event handling framework when you add it to the DOM.
Not all browsers support the latest ECMAScript standard, but you can use a [transpiler](guide/glossary#transpile) (like [TypeScript](guide/glossary#typescript)) to write code using the latest features, which will then be transpiled to code that runs on versions that are supported by browsers.
To learn more, see the [Browser Support](guide/browser-support) page.
Angular defines an `ElementRef` class to wrap render-specific native UI elements. This allows you use Angular templates and data-binding to access DOM elements without reference to the native element in most cases.
An Angular class or other definition that provides a dependency using the [dependency injection](guide/glossary#di) mechanism. An injectable class is marked by the `@Injectable` [decorator](guide/glossary#decorator).
Both a [service](guide/glossary#service) and a [component](guide/glossary#component) that depends on that service must be marked as injectable. Other items, such as constant values, can be injectable.
with a registered [provider](guide/glossary#provider). Injectors are created for NgModules automatically as part of the bootstrap process, and inherited through the component hierarchy.
The Angular Just-in-Time (JIT) compiler converts your Angular HTML and TypeScript code into efficient JavaScript code at run time, as part of bootstrapping.
JIT compilation is the default when you run Angular's `ng build` and `ng serve` CLI commands, and is a good choice during development. JIT mode is strongly discouraged for production use because it results in large application payloads that hinder the bootstrap performance.
Lazy loading speeds up application load time by splitting the application into multiple bundles and loading them on demand.
For example, dependencies can be lazy-loaded as needed&emdash;as opposed to "eager-loaded" modules that are required by the root module, and are thus loaded on launch.
Similarly, the [router](guide/glossary#router) can load child views only when the parent view is activated, and you can build custom elements that can be loaded into an Angular app when needed.
An interface that allows you to tap into the lifecycle of [directives](guide/glossary#directive) and [components](guide/glossary#component) as they are created, updated, and destroyed.
In general, a module collects a block of code dedicated to a single purpose. Angular uses standard JavaScript modules, and also defines an Angular module, `NgModule`.
In JavaScript (ECMAScript), each file is a module and all objects defined in the file belong to that module. Objects can exported, making them public, and public objects can be imported for use by other modules.
Angular ships as a collection of JavaScript modules, or libraries. Each Angular library name begins with the `@angular` prefix. Install them with the NPM package manager and import parts of them with JavaScript `import` declarations.
A class definition with an `@NgModule` [decorator](guide/glossary#decorator), that declares and serves as a manifest for a block of code dedicated to an application domain, a workflow, or a closely related set of capabilities.
Like a [JavaScript module](guide/glossary#module), an NgModule can export functionality for use by other NgModules, and import public functionality from other NgModules.
The metadata for an NgModule class collects components, directives, and pipes that the application uses along with the list of imports and exports. See also [Declarable](guide/glossary#declarable).
NgModules are typically named after the file in which the exported thing is defined; for example, the Angular [DatePipe](api/common/DatePipe) class belongs to a feature module named `date_pipe` in the file `date_pipe.ts`. You import them from an Angular [scoped package](guide/glossary#scoped-package) such as `@angular/core`.
A producer of multiple values, which it pushes to [subscribers](guide/glossary#subscriber). Used for asynchronous event handling throughout Angular. You execute an observable by subscribing to it with its `subscribe()` method, passing callbacks for notifications of new values, errors, or completion.
Observables can deliver single or multiple values of any type to subscribers, either synchronously (as a function delivers a value to its caller), or on a schedule. A subscriber receives notification of new values as they are produced, and of either error or normal completion.
An object passed to the `subscribe()` method for an [observable](guide/glossary#observable) that defines the callbacks for the [subscriber](guide/glossary#subscriber).
When defining a [directive](guide/glossary#directive), the `@Output` decorator on a directive property makes that property available as a *target* of [event binding](guide/template-syntax#event-binding).
A class with the `@Pipe` decorator which defines a function that transforms input values to output values for display in a [view](guide/glossary#view).
An [NPM package](guide/npm-packages) that plugs gaps in a browser's JavaScript implementation. See the [Browser Support](guide/browser-support) guide for polyfills that support particular functionality for particular platforms.
A provider of an injectable service—specifically, a code recipe associated with a [DI token](guide/glossary#token), which an [injector](guide/glossary#injector) uses to create a new instance of a dependency for a class that requires it.
Angular registers its own providers with every injector, for services that Angular defines. You can register your own providers for services that your app needs.
The Router module is an [NgModule](guide/glossary#ngmodule) that provides the necessary service providers and directives for navigating through application views. A [routing component](guide/glossary#routing-component) is one that imports the Router module and whose template contains a `RouterOutlet` element where it can display views produced by the router.
The Router defines navigation among views on a single page, as opposed to navigation among pages. It interprets URL-like links to determine which views to create or destroy, and which components to load or unload. It allows you to take advantage of [lazy-loading](guide/glossary#lazy-load) in your Angular apps.
NgModules are delivered within *scoped packages* whose names begin with the Angular *scope name*`@angular`. For example, `@angular/core`, `@angular/common`, `@angular/http`, and `@angular/router`.
In Angular, a service is a class with the [@Injectable](guide/glossary#injectable) decorator that encapsulates non-UI logic and code that can be re-used across an application.
Angular distinguishes components from services in order to increase modularity and reusability.
The `@Injectable` metadata allows the service class to be used with the [dependency injection](guide/glossary#di) mechanism. The injectable class is instantiated by a [provider](guide/glossary#provider), and a module maintains a list of providers that can provide a particular type of service as needed by components or other services that require it.
A category of [directive](guide/glossary#directive) that is responsible for shaping or reshaping HTML layout by modifying the DOM (adding, removing, or manipulating elements and their children).
To learn more, see the [Structural Directives](guide/structural-directives) page.
{@a subscriber}
## Subscriber
A function that defines how to obtain or generate values or messages to be published. This function is executed when a consumer calls the `subscribe()` method of an [observable](guide/glossary#observable).
The act of subscribing to an observable triggers its execution, associates callbacks with it, and creates a `Subscription` object that lets you unsubscribe.
The `subscribe()` method takes a JavaScript object (called an "observer") with up to three callbacks, one for each type of notification that an observable can deliver:
- The `next` notification: sends a value such as a Number, a String, an Object, etc.
- The `error` notification sends a JavaScript Error or exception.
- The `complete` notification does not send a value, but the handler is called when the call completes. Scheduled values can continue to be returned after the call completes.
A template combines straight HTML with Angular [data-binding](guide/glossary#data-binding) syntax, [directives](guide/glossary#directive), and [template expressions](guide/glossary#template-expression) (logical constructs). The Angular elements insert or calculate values that modify the HTML elements before the page is displayed.
A template is associated with a [component](guide/glossary#component) class through `@Component` [decorator](guide/glossary#decorator). The HTML can be provided inline, as the value of the `template` property, or in a separate HTML file linked through the `templateUrl` property.
Additional templates, represented by a `TemplateRef` object, can define alternative or _embedded_ views, which can be referenced from multiple components.
* The "source of truth" is the template. The validation is defined using attributes on the individual input elements.
* [Two-way binding](guide/glossary#data-binding) with `ngModel` keeps the component model synchronized with the user's entry into the input elements.
* Behind the scenes, Angular creates a new control for each input element, provided you have set up a `name` attribute and two-way binding for each input.
* The associated Angular directives are all prefixed with `ng` such as `ngForm`, `ngModel`, and `ngModelGroup`.
An opaque identifier used for efficient table lookup. In Angular, a [DI token](guide/glossary#di-token) is used to find [providers](guide/glossary#provider) of dependencies in the [dependency injection](guide/glossary#di) system.
especially [component](guide/glossary#component) directives and their companion [templates](guide/glossary#template).
A view is specifically represented by a `ViewRef` instance associated with the component.
A view that belongs to a component is called a _host view_.
Views are typically collected into [view hierarchies](guide/glossary#view-tree).
Properties of elements in a view can change dynamically, in response to user actions; the structure (number and order) of elements in a view cannot. You can change the structure of elements by inserting, moving, or removing nested views within their view containers.
View hierarchies can be loaded and unloaded dynamically as the user navigates through the application, typically under the control of a [router](guide/glossary#router).
A tree of related views that can be acted on as a unit. The root view is a component's _host view_. A host view can be the root of a tree of _embedded views_, collected in a _view container_ (`ViewContainerRef`) attached to an anchor element in the hosting component. The view hierarchy is a key part of Angular change detection.
The view hierarchy does not imply a component hierarchy. Views that are embedded in the context of a particular hierarchy can be host views of other components. Those components can be in the same NgModule as the hosting component, or belong to other NgModules.
An execution context for a set of asynchronous tasks. Useful for debugging, profiling, and testing apps that include asynchronous operations such as event processing, promises, and calls to remote servers.
An Angular app runs in a zone where it can respond to asynchronous events by checking for data changes and updating the information it displays by resolving [data bindings](guide/glossary#data-binding).
