```
# Angular Forms
## Template driven forms
Form logic (validation, properties) are kept in template
**sample html**
```html
```
**sample component**
```ts
@ViewChild("f") form: any;
firstName: string = "";
langs: string[] = ["English", "French", "German"];
onSubmit() {
if (this.form.valid) {
console.log("Form Submitted!");
this.form.reset();
}
}
```
## Reactive forms
Form logic (validation, properties) are kept in component
**sample html**
```html
```
**sample component**
```ts
registerForm: FormGroup;
submitted = false;
constructor(private formBuilder: FormBuilder) { }
ngOnInit() {
this.registerForm = this.formBuilder.group({
firstName: [{{here default value}}, Validators.required],
lastName: ['', Validators.required],
email: ['', [Validators.required, Validators.email]],
password: ['', [Validators.required, Validators.minLength(6)]]
});
}
// convenience getter for easy access to form fields
get f() { return this.registerForm.controls; }
onSubmit() {
this.submitted = true;
// stop here if form is invalid
if (this.registerForm.invalid) {
return;
}
alert('SUCCESS!! :-)')
}
```
## Custom Validator for Reactive forms
**Function**
```ts
validateUrl(control: AbstractControl) {
if (!control.value || control.value.includes('.png') || control.value.includes('.jpg')) {
return null;
}
return { validUrl: true };
}
```
**Usage**
```ts
this.secondFormGroup = this._formBuilder.group({
imageCtrl: ['', [Validators.required, this.validateUrl]]
});
```
**Multi-field validation**
```ts
validateNameShire(group: FormGroup) {
if (group) {
if (group.get('isShireCtrl').value && !group.get('nameCtrl').value.toString().toLowerCase().includes('shire')) {
return { nameShire : true };
}
}
return null;
}
```
**Multi-field validation usage***
```ts
this.firstFormGroup.setValidators(this.validateNameShire);
```
**Error handling**
```html
Name is too long
Shire dogs should have "shire" in name
```
## Custom Validator Directive for Template driven forms
To register our custom validation directive to NG_VALIDATORS service we have to do it like this:
(thanks to multi parameter we won't override NG_VALIDATORS but just add CustomValidator to NG_VALIDATORS)
```ts
@Directive({
selector: '[CustomValidator]',
providers: [{provide: NG_VALIDATORS, useExisting: CustomValidator, multi:true}]
})
```
Example:
```ts
@Directive({
selector: '[customValidation]',
providers: [{provide: NG_VALIDATORS, useExisting: EmailValidationDirective, multi: true}]
})
export class CustomValidation implements Validator {
constructor() { }
validate(control: AbstractControl): ValidationErrors {
return (control.value && control.value.length <= 300) ?
{myValue : true } : null;
}
}
```
For multiple fields:
```ts
validate(formGroup: FormGroup): ValidationErrors {
const passwordControl = formGroup.controls["password"];
const emailControl = formGroup.controls["login"];
if (!passwordControl || !emailControl || !passwordControl.value || !emailControl.value) {
return null;
}
if (passwordControl.value.length > emailControl.value.length) {
passwordControl.setErrors({ tooLong: true });
} else {
passwordControl.setErrors(null);
}
return formGroup;
}
```
## ngModel in custom component
1. Add to module:
```ts
providers: [
{
provide: NG_VALUE_ACCESSOR,
useExisting: forwardRef(() => TextAreaComponent),
multi: true
}
]
```
2. Implement ControlValueAccessor interface
```ts
interface ControlValueAccessor {
writeValue(obj: any): void
registerOnChange(fn: any): void
registerOnTouched(fn: any): void
setDisabledState(isDisabled: boolean)?: void
}
```
| Function | Description |
| ------------- | ------------- |
| registerOnChange | Register a function to tell Angular when the value of the input changes |
| registerOnTouched | Register a function to tell Angular when the value was touched |
| writeValue | tell Angular how to Write a value to the input |
Sample implementation:
```ts
@Component({
selector: 'app-text-area',
templateUrl: './text-area.component.html',
styleUrls: ['./text-area.component.less'],
providers: [
{
provide: NG_VALUE_ACCESSOR,
useExisting: forwardRef(() => TextAreaComponent),
multi: true
}
]
})
export class TextAreaComponent implements ControlValueAccessor, OnInit {
@Input() value: string;
private _onChange = (data: any) => { console.log('changed: ' + data); };
private _onTouched = (data?: any) => {console.log('touched: ' + data); };
ngOnInit(): void {
const self = this;
}
constructor() {}
writeValue(obj: any): void {
this.value = obj;
}
registerOnChange(fn) {
this._onChange = fn;
}
registerOnTouched(fn: any): void {
this._onTouched = fn;
}
}
```
# Tests
## Unit tests
**Service**
```ts
describe('MyService', () => {
let service: MyService;
beforeEach(() => service = new MyService();
it('#fetch should update data', () => {
service.fetchData();
expect(service.data.length).toBe(4);
expect(service.data[0].id).toBe(1);
});
});
```
For async functions
```ts
it('#fetch should update data', (done: DoneFn) => {
// some code
done(); // we need 'done' to avoid test finishing before date was received
// some code
});
```
example async test:
```ts
it('http client works', (done: DoneFn) => {
service.getUser().subscribe((data) => {
expect(data).toBe('test');
done();
});
});
```
**Spy and stub**
Spy:
```ts
// create spied object by copy getDataAsync from HttpService
const valueServiceSpy =
jasmine.createSpyObj('HttpService', ['getDataAsync']);
```
Stub:
```ts
const stubValue = of('StubValue');
valueServiceSpy.getDataAsync.and.returnValue(stubValue);
```
**TestBed**
Mock whole module/environment for unit tests
```ts
beforeEach(() => {
let httpClientMock = TestBed.configureTestingModule({ providers: [{ provide: MyService, useValue: new MyService(httpClientMock)}] });
});
```
Then use tested object (for example service) like this:
```ts
service = TestBed.get(MyService);
```
we can add schemas: [NO_ERRORS_SCHEMA]. This means that we don’t have to mock children component dependencies of this component as Angular won’t yell at us anymore for our lack of doing so.
# Others
## Http interceptor
Intercepts and handles an HttpRequest or HttpResponse.
Class:
```ts
@Injectable()
export class MyInterceptor implements HttpInterceptor {
constructor() { }
intercept(request: HttpRequest
, next: HttpHandler): Observable> {
// do sth (like check and throw error)
return next.handle(request); //if want continue
}
}
```
Module:
```ts
{ provide: HTTP_INTERCEPTORS, useClass: TokenInterceptor, multi: true },
```
## host
Refer to host element/component
| Value | Description |
| ------------- | ------------- |
| :host(selector) { ... } | to match attributes, classes on the host element and add styling to it |
| :host-context(selector) { ... } | to match elements, classes on parent components and add styling to it |
| :host ::ng-deep | styling will be applied also to all child components |
# Interview questions
**When would you use the useFactory provider method?**
With useFactory we can use a factory at runtime to decide which kind of service we want to return if it got requested by any other class in our application or you need to parameterize the construction of a service
```ts
export function sampleFactory() {
return new Service1();
}
export class Service1 {}
@Injectable({
providedIn: 'root',
useFactory: xyzFactory
})
export class Service2 {}
```
Service1 will be injected into another class
**What is router-outlet**
Acts as a placeholder that Angular dynamically fills based on the current router state. Generally in place of your main app component will be generated
**How to declare global value?**
Use InjectionToken
**Which decorator lets you inject a service registered with an Injection Token?**
@Inject
for example
```ts
@Inject(CONTROLS_GLOBAL_CONFIG) globalVlues: ControlsConfig
```
**How to mimick environment for components/services in tests?**
Use TestBed. See [Unit tests](https://github.com/delprzemo/angular-cheatsheet#unit-tests "Unit tests")
**What is Resolve interface?**
Interface that classes can implement to be a data provider for component while routing.
example:
```ts
@Injectable()
class UserResolver implements Resolve {
constructor(private service: MySampleService) {}
resolve(
route: ActivatedRouteSnapshot,
state: RouterStateSnapshot
): Observable {
return this.service.fetchData(route.params.id);
}
}
@NgModule({
imports: [
RouterModule.forRoot([
{
path: 'user/:id',
component: UserComponent,
resolve: {
userData: UserResolver
}
}
])
],
providers: [UserResolver]
})
```
We can use it to pre-load data for a component before the component is displayed
**How to begin validation after the user will enter a value and pause?**
Use debounceTime, for example
```ts
this.formCtrlSub = this.firstNameControl.valueChanges
.debounceTime(1000)
.subscribe(newValue => this.firstName = newValue);
```
**What is valueChanges in form control?**
To catch value changes and implement some logic in observable result. See example above
**How to execute canActivate if any of child routes will change?**
Use canActivateChild
**What are compilation types in Angular?**
| Type | Description |
| ------------- | ------------- |
| AoT | Ahead of time - compile full application / module when application was opened. Used mainly on production |
| JiT | Just in time - compile specific element when it has been opened. Used mainly while programming |
| Ivy | Since Angular 8 - engine based on concept Incremental DOM |
**What is ng-content in Angular?**
See [ng-content](https://github.com/delprzemo/angular-cheatsheet/blob/master/README.md#content-projection "ng-content")
**How to create application with cutom prefix?**
```
ng new app-name --prefix my-cutom-prefix
```
**What is module lazy loading?**
Instead of loading all modules while app starts we can load particular module when needed.
```ts
const routes: Routes = [
{
path: 'users',
loadChildren: () => import('./users/users.module').then(m => m.UserModule)
}
```
This is usually used for big apps in order to improve performance
**Why should we consider using ngIf instead of ngClass/ngStyle for hiding element?**
ngIf won't generate element when condition result is fale, so html will be lighter. ngClass/ngStyle will just hide element but it will be still existing in DOM
**What is Done() function in tests?**
We need 'done' to avoid test finishing before date was received
See [done](https://github.com/delprzemo/angular-cheatsheet#unit-tests "done")
**What "import", "providers" and "declarations" stand for in NgModule?**
```ts
declarations: [AppComponent], // components, pipes, directives
imports: [BrowserModule, AppRoutingModule], // other modules
providers: [], // services
```
See [Sample module](https://github.com/delprzemo/angular-cheatsheet#sample-module-with-comments "Sample module")
**Explain the difference between Constructor and ngOnInit**
Constructor is a method assigned to a class, so it is called when class object was initialized.
ngOnInit is part of Component life cycle and it is dependent on the current state of view initialization.
Constructor is called before ngOnInit
**What is a difference between ElementRef and TemplateRef?**
ElementRef is reference to particular element while TemplateRef can refer to whole ng-template
```html
Test
```
```ts
export class SthComponent {
@ViewChild('msg')
private testTempRef : TemplateRef
}
```
**Point all data biding ways for element**
| Bindint type | Example |
| ------------- | ------------- |
| Property binding | [src]="this.src" |
| Event binding | (click)="this.doSth()" |
| Two way data bidning | [(ngModel)]="this.form.userName" |
**How to handle ngModel property in custom component?**
Implement ControlValueAccessor interface.
See [ngModel](https://github.com/delprzemo/angular-cheatsheet#ngmodel-in-custom-component "ngModel")
**What is differenct between default and onPush change detector?**
default change detector will check bidnings in whole application component tree after event
OnPush change detector informs Angular that our component biding is depend on input parameters. Moreover it won't check bindings in whole application but only in subtree where our component belongs.
**Why is it better to use pure pipes instead of some functions in template view?**
If we want to calculate for example user age and show it on template then we can do it with function:
```html
{{this.getUserAge(user.birthDate)}}
```
But it will be calculated every time when change detector is run, so it can affect on performance.
Instead we can use pipe for that:
```html
{{user.birthDate | calculateAge}}
```
Now age calculation won't be performed so many times.
**How to detect change for any @Input property?**
Use ngOnChanges hook
```ts
ngOnChanges(values) {
// logic here
}
```
**How to detect change for specific @Input in component?**
Use 'set' accessor
```ts
@Input() set name(value) {
// some logic here
}
```