Co-written with Varun Vachhar.

Any chainsaw that can cut down a tree can also take off a leg if used carelessly. Equally, any framework as powerful as Angular will inevitably contain traps for the unwary. This article looks at ways in which Angular is sometimes mis-used and what developers should do instead.

1. Don't Ignore a Component's Host Element

When creating Angular components we should use the host element as a wrapper instead of creating our own wrapper elements. For example, the card component below creates a heading and a paragraph. We need a wrapper element in order to control their layout; it's tempting to wrap them in a <div> , but that is not required because Angular will render the combination with a <my-card> wrapper element. This is called a host element.

import { Component } from '@angular/core' ; @ Component ( { selector : 'my-card' , template : ` <h1 class="card-title">{{ title }}</h1> <p class="card-body"> <ng-content></ng-content> </p> ` } ) export class CardComponent { title = 'Title of card' ; }

We can style the :host element by targeting it with the :host selector in the component's CSS file. Alternatively, we can attach CSS classes to it using host metadata.

@ Component ( { selector : 'my-card' , host : { class : 'db ma0 lh-title' } , ... } ) export class CardComponent { title = 'Title of card' ; }

Host metadata allows us to bind to all properties and events of the host element, but so do the @HostBinding and @HostListener decorators. However, there are some subtle differences between the two. Consider the following scenario:

@ Component ( { selector : 'my-foo' , host : { 'class' : 'navy' } , } ) export class FooComponent { } @ Component ( { selector : 'my-bar' , } ) export class BarComponent { @ HostBinding ( 'class' ) className = 'navy' ; }

Somewhere later in our application, we use them like this:

< my - foo class = "underline" > < / my - foo > < my - bar class = "underline" > < / my - bar >

<my-foo> will render with class="underline navy" , but <my-bar> will render with class="navy" alone. This happens because @HostBinding only supports binding of values, while host metadata supports both binding and static values. Since we are using a static value in this case, Angular merges what we have defined in the metadata with what is defined on the component. If we had bound the class to a component property instead, the two examples would behave the same way:

@ Component ( { selector : 'my-foo' , host : { '[class]' : 'className' } , } ) export class FooComponent { className = 'navy' ; }

2. Don't Avoid Observables

Angular exposes HTTP requests, form events, route parameters, and many other things as observables because they are a flexible, maintainable way to manage interactions between different pieces of code. When working with Angular, we should use observables as much as possible, both because of their intrinsic merits and because architectural consistency makes code easier to understand.

We can leverage observable operators to compute data based on application state. This allows us to reduce the amount of state we need to maintain, and since they are pure functions, they make our software easier to test. In particular, these operators can be used to select slices out of state data, and can easily be memoized to improve performance.

Observables and data slicing can be adopted regardless of whether we use ngrx, angular-redux or bare services to manage state. When working with Angular services, for example, we can replicate a Redux store-style API using BehaviorSubject. The example below shows how to do this for a TodoStore service that exposes the entire data store as a single state$ observables. To display a list of filtered items we simply map over the state$ observable instead of maintaining a separate list:

enum Filter { ALL , ACTIVE , COMPLETED } interface ITodoStore { filter : Filter ; todos : ITodo [ ] ; } @ Injectable ( ) export class TodoStore { private stateSubject$ : BehaviorSubject < ITodoStore > = new BehaviorSubject ( { filter : Filter . ALL , todos : [ ] , } ) ; public readonly state$ : Observable < ITodoStore > = this . stateSubject$ . asObservable ( ) ; constructor ( private apiService : ApiService ) { this . initialize ( ) ; } initialize ( ) { this . apiService . get ( '/todos' ) . subscribe ( users => { this . stateSubject$ . next ( Object . assign ( { } , this . stateSubject$ . getValue ( ) , { users } ) ) ; } ) ; } filterTodoByType ( todo , filterType ) { if ( filterType === Filter . ALL ) { return true ; } else if ( filterType === Filter . COMPLETED ) { return todo . completed ; } else { return ! todo . completed ; } } filteredTodos$ ( ) { return this . state$ . map ( ( { filter , todos } ) => todos . filter ( todo => this . filterTodoByType ( todo , filter ) ) ; } }

Observables also allow us to merge multiple streams to implement complex workflows such as a debounced search which ignores stale requests. Again, by making these operations purely functional instead of maintaining separate state, we can make our application much easier to test.

To access a data property, we should use operators such as map or pluck to derive separate streams. This allows us to utilize the async pipe in our templates to ensure that rendering happens once data is available, which in turn removes the need for managing subscriptions in component classes. If multiple properties need to be accessed in order to render a component, subscribing to the base observable is likely a better option. However, this is often also a sign that the code should be refactored so that each component is only coupled to a small slice of the application's state. And keep in mind that just as promises beget promises, observables beget observables: any operators or transformations that occurred in a base observable are passed on to any subsequent "child" or "branch" observables.

3. Don't Use the Raw Http Service

Angular provides an Http service, but we should always create a wrapper around it instead of consuming it directly in our applications so that we can create a layered architecture for consuming RESTful APIs. The wrapper service---let's call it ApiService---acts as the base for all other operations, and is where we can put common operations like mapping each request to call res.json(). It provides a centralized location for error handling, logging, attaching authentication tokens, and other middleware operations.

The next layer up is then domain-specific services, commonly having names like UserService , PostsService , or CommentsService . These services use ApiService , and only ApiService , to make HTTP requests. This makes testing easier, since it means we only have to mock out ApiService rather than mocking HTTP responses.

These higher-level services are also the perfect place to implement data transformations. Our application will usually not directly consume the data returned by the API, but will instead normalize and reshape it into something better for managing state. normalizr is a useful tool for this task, but many others can be used.

4. Keep the Injector Tree in Mind When Lazy Loading Modules

Angular has a hierarchical dependency injection (DI) system: its tree of injectors parallels the application's component tree, and when a component needs something, Angular walks up the tree to find the injectable that mostly closely fits the request.

That's the good news. The not-so-good news is that lazy loaded modules create their own branch in the DI tree, which can have some unexpected effects:

Lazy loaded module providers (services) are not available until the module has been loaded.

These providers are not available globally, but instead are scoped to the lazy loaded module.

Together, these consequences mean that it's possible to have services that belong to a lazy loaded module, but are not accessible to the root module or any other eagerly loaded module of our application.

As a rule of thumb:

If we want to provide a service as an application-level singleton, we should use the forRoot() function.

function. If we want to provide a service to feature modules, we should always use forChild() .

5. Treat Routes as Part of Application State

It is common (or at least tempting) for code to try to manage state and router events as two separate things rather than treating state changes like "selecting an item" as a route transition to being with. This is an unnecessary complication, since Angular's router allows us to define a client-side routing structure and manages transitions between those routes, but it does not manage or mutate our application's state. This means that we are responsible for updating that state in response to navigation events.

Luckily, the router exposes routing events as an observable called Router.events . To synchronize the application's state or to initialize state based on the URL when the application is first loaded, we should subscribe to router events and take appropriate actions. Crucially, since router events are the only hooks available to us, we should ensure that all route transitions are triggered using routerLink or programmatically using router.navigate() . Doing an end-run around these facilities will almost certainly get us into trouble sooner rather than later.

6. Use Content Projection

Most programmers now prefer composition over inheritance when designing software systems. In the Angular world, the equivalent is to prefer content projection over templates with complex nested markup. When we do this, our application consists of a toolbox of small, single-responsibility components which are then composed into a "just-in-time architecture" to satisfy current business needs. This approach:

makes the component hierarchy flatter,

avoids excessive chaining of @Input/@Output to pass state down the component tree and events up,

to pass state down the component tree and events up, makes it easier to test components, since we don't have to deal with logic in views, and

allows us to leverage existing components for implementing new features.

This pattern is especially important for form elements since it allows us to connect models and event handlers at the container level instead of having to pass them down through multiple levels.

For example, by creating generic wrapper components we can use the same set of components to create:

an image card

a card with a title and caption

a card with a title, caption, and icon

and many more combinations

< Card > < Media [ source ] = "'img/rotary-phone.png'" > < / Media > < Block paddingX = "2" paddingY = "2" > < Heading size = "2" > Choosing the Right Antique Rotary Phone for You . < / Heading > < Block > < Heading size = "3" muted > 21 hours ago < / Heading > < Button style = "clear" ( handleClick ) = "addToCart($event)" > < Icon name = "add-to-cart" / > < / Button > < / Block > < / Block > < / Card >

For more on this subject see the post Flattening Deep Hierarchies of Components on the Rangle.io blog.

7. Don't Use Model-Driven Forms Straight Away

Template-driven forms require less boilerplate, setup, and mental overhead than model-driven forms. When we rely on templates, we don't need to configure a back-end model, but instead can simply compose the form in your template. Easy composition of these template-driven forms is facilitated by leveraging content projection so that form logic and state can be handled at the container level.

That said, model-driven forms are more appropriate for working with complex dynamically generated forms. When ngIf and ngFor aren't enough, or become unreadable and unmaintainable, we have probably entered territory where model-driven forms are a more appropriate solution.

8. Don't Use Redux Directly Inside Components

We can divide components into Presentational and Container Components. Presentational components are responsible for how things look: they receive data through @Input and handle events by invoking callbacks or by dispatching events through @Output . Container components are responsible for defining how things work, i.e., for fetching data and updating state.

When we are using Redux, every component should be of exactly one kind. Presentational components should be completely unaware that Redux is being used: as far as they're concerned, "data happens" and their job is to render it. Container components, on the other hand, should subscribe to data, passing relevant parts down to their children and mapping events to dispatch calls.

But even container components should avoid directly interacting with Redux since this leads to components that are tightly coupled to state management. Instead, we should use selector services to access state and action creator services to dispatch actions. This pattern may feel unnecessarily convoluted in a small application, but quickly proves its worth as the application scales.

The example below shows a common scenario in which components require authorization-related state such as isAuthenticated , currentUser , and userProfile . All of this information is available in the data store, so container components could access it directly. In order to achieve that, though, these components would need to be aware of the shape of the store. Additionally, this information will be used by multiple container components, which means the selector logic would be repeated in each one. Both factors would make our application brittle to future changes.

We can avoid this tight coupling by using selector services, so that refactoring the store or the reducers will only impact one service as opposed to multiple components. This also provides a good location to introduce memoization to improve selector performance. For example:

import { Injectable } from '@angular/core' ; import { Observable } from 'rxjs/Observable' ; import { path } from 'ramda' ; import { NgRedux } from '@angular-redux/store' ; import { AppState } from '../store' ; import { AuthState , AuthProfile , AuthToken } from './types' ; @ Injectable ( ) export class AuthSelectors { constructor ( private ngRedux : NgRedux < AppState > ) { } currentUser$ ( ) : Observable < AuthState > { return this . ngRedux . select < AuthState > ( path < AuthState > ( [ 'user' ] ) ) ; } userProfile$ ( ) : Observable < AuthProfile > { return this . ngRedux . select < AuthProfile > ( path < AuthProfile > ( [ 'user' , 'profile' ] ) ) ; } isAuthenticated$ ( ) : Observable < boolean > { return this . ngRedux . select < AuthToken > ( path < AuthToken > ( [ 'user' , 'token' ] ) ) . map < AuthToken , boolean > ( Boolean ) . distinctUntilChanged ( ) ; } error$ ( ) : Observable < Error > { return this . ngRedux . select < Error > ( path < Error > ( [ 'user' , 'error' ] ) ) ; } }

Note that in the example above, path is a utility function from Ramda that allows us to retrieve the value at a given location (or path) in our data store.

As a second example, we can build an action creator service to handle actions and action types related to a specific domain. Again, this feels like over-engineering in a twenty-line example, but avoiding tight coupling with Redux and not having to import action types into multiple components pays off quickly.

import { Injectable } from '@angular/core' ; import { dispatch } from '@angular-redux/store' ; import { Action } from '../store' ; import { LoginCredentials , AuthToken , AuthProfile } from './types' ; @ Injectable ( ) export class AuthActions { static readonly USER = { LOGIN : 'AUTH/USER/LOGIN' , LOGGED_IN : 'AUTH/USER/LOGGED_IN' , LOGOUT : 'AUTH/USER/LOGOUT' , } ; static readonly PROFILE = { LOAD : 'AUTH/PROFILE/LOAD' , LOADED : 'AUTH/PROFILE/LOADED' , } ; @ dispatch ( ) loginUser ( email : string , password : string ) : Action < LoginCredentials > { return { type : AuthActions . USER . LOGIN , payload : { email , password } , } ; } @ dispatch ( ) logoutUser ( ) : Action { return { type : AuthActions . USER . LOGOUT , } ; } userLoggedIn ( token : AuthToken ) : Action < AuthToken > { return { type : AuthActions . USER . LOGGED_IN , payload : token , } ; } loadProfile ( token : string ) : Action < string > { return { type : AuthActions . PROFILE . LOAD , payload : token , } ; } profileLoaded ( profile : AuthProfile ) : Action < AuthProfile > { return { type : AuthActions . PROFILE . LOADED , payload : profile , } ; } }

9. Don't Manipulate the DOM Directly With ElementRef

One of Angular's strengths is that it allows us to decouple the application code from the renderer, which in turn allows us to write applications that can be executed in the browser, on the server, or even as native apps (using NativeScript) with a single codebase. In order to achieve this, though, we need to be mindful of how we interact with the DOM. In particular, mutating ElementRef.nativeElement directly makes the application aware of DOM rendering, which means we can no longer execute the application on the server or in a web worker.

We can use a few strategies to avoid this:

Use data-binding, components or directives to achieve a dynamic look and feel.

Prefer CSS for dynamic styling and layout.

Use structural directives for dynamic templates.

Use Renderer2 for lower-level DOM access.

As an example, this component uses a renderer to handle tabs:

@ Component ( { selector : 'md-tab-group, mat-tab-group' , ... } ) export class MdTabGroup { constructor ( private _renderer : Renderer2 ) { } _setTabBodyWrapperHeight ( tabHeight : number ) : void { ... this . _renderer . setStyle ( this . _tabBodyWrapper . nativeElement , 'height' , this . _tabBodyWrapperHeight + 'px' ) ; } }

While it may seem clumsier or more complicated than direct DOM manipulation, an approach like this is better suited to server-side rendering and similar use cases.

10. Don't Use TestBed Prematurely

TestBed allows us to create a testing module that can then be configured using configureTestingModule and used to instantiate components or even a full dependency injection tree. It is a very powerful utility, but it does require a fair bit of setup and maintenance. In many cases, leaner testing strategies can achieve the same goals.

Let's take a look at class instantiation with mocked dependencies. When we use services, we aim to write business logic using pure functions, i.e., to (mostly) build Presentational components without any internal state. This allows us to test these components and services by simply instantiating their classes. An example of such a class is:

@ Injectable ( ) export class QuoteService { constructor ( private apiService : ApiService ) { this . getTodos ( ) ; } getTodos ( ) { this . apiService . get ( '/todos' ) . subscribe ( todos => { this . todos = todos ; } ) ; } }

To test this QuoteService we need to provide a mock ApiService , which we must cast to the type of service being mocked. This same strategy can be used for testing services that inject Http instead of having to use MockBackend .) Here's what it looks like in practice:

describe ( 'Quote Service' , ( ) => { let mockApiService : ApiService ; let quoteService : QuoteService ; let mockResponse = [ ... ] ; beforeEach ( ( ) => { mockApiService = { get : null } as ApiService ; spyOn ( mockApiService , 'get' ) . and . returnValue ( Observable . of ( { json : ( ) => mockResponse } ) ) ; quoteService = new QuoteService ( mockApiService ) ; } ) ; it ( 'should get to dos' , ( ) => QuoteService . getTodos ( ) . subscribe ( res => { expect ( mockApiService . get ) . toHaveBeenCalledWith ( '/todos' ) ; expect ( res ) . toEqual ( mockResponse ) ; } ) ; ) ; } ) ;

While lightweight tests can be done using lightweight tools, there are some scenarios where TestBed does make it easier to write an effective test:

Testing template logic or structural directives.

Shallow rendering test for stateless components.

Integration tests where you want to test a feature as a whole.

Testing services or components that rely on router events.

In these cases, TestBed 's learning curve is amply rewarded.

Conclusion

Some application frameworks seem to be built entirely out of chainsaws. Angular isn't, but investing a little effort up front in learning how not to use it pays dividends for the life of the project. As it continues to evolve, we expect that some of these traps will be eliminated, or that developers will find and share patterns to guide their peers toward what is safest and most productive.

Interested in learning more about the latest version of Angular and migraton? Follow the link here!