Angular Form, Template, and Validations | Angular 9, 8, 7, 6, 5, 4, 2 - Interview Questions Angular, JavaScript, Java, PHP, SQL, C#, Vue, NodeJs, ReactJs

What are the Validator functions?

There are two types of validator functions which are the following -

1. Async validators

2. Sync validators

Async validator functions that take a control instance and return an observable that later emits a set of validation errors or null.

Sync validator functions that take a control instance and return a set of validation errors or null.

Angular runs only Async validators due to some performance issues.

What Is a Template Reference variable?

A template reference variable is a way of capturing a reference to a specific element, component, directive, and pipe so that it can be used someplace in the same template HTML.

You should declare a reference variable using the hash symbol (#).

The Angular components and directives only match selectors for classes that are declared in the Angular module.

Template Reference Variable Syntax –

You can use a template reference variable by two ways.

1. Using hash symbol (#)

2. Using reference symbol (ref-)

The following examples of specifying a template reference variable using Input Text Box –

I have declared a reference variable “cellnumber” using the hash symbol (#) and reference symbol (ref-).

<input type="text" ref-cellnumber> //cellnumber will be a template reference variable.

And

<input #cellnumber placeholder="Cell number"> //cellnumber will be a template reference variable.

I have created a reference to the input element that can be used later on in my template and the scope for “cellnumber” variable is the entire HTML template in which the reference is defined.

Here is how I could use that reference to get the value of the input for instance –

//cellnumber refers to the input element

<button (click)="show(cellnumber)">click to see</button>

In the below line of code, the variable “cellnumber” refer to the HTMLElement object instance for the input -

show(cellnumber: HTMLInputElement){

console.log(cellnumber.value);

}

You can use the ViewChild decorator to reference it inside your component.

import {ViewChild, ElementRef} from '@angular/core';

// Reference cellnumber variable inside Component

@ViewChild('cellnumber') cellInputRef: ElementRef;

And finally, you can use this.nameInputRef anywhere inside your component class.

show(){

this.contactNumber = this.cellInputRef.nativeElement.value

}

Template Reference Variable with NgForm –

Here we will discuss about how to access NgForm directive using template reference variable.

<button type="submit" [disabled]="!empForm.form.valid">Submit</button>

</form>

In the above NgForm example contains a ngSubmit event and form directive.

The ngSubmit – The ngSubmit directive specifies a function to run when the form is submitted. Here on form submit onSubmitEmployee component method will be called.

The NgForm - It is a nestable alias of form directive. The main purpose of ngForm is to group the controls, but not a replacement of <form> tag.

As you know, the HTML does not allow nesting of form elements. It is very useful to nest forms.

How to bind to user input events to component event handlers?

Most of the DOM events are triggered by user input and bind to these events provides a way to get inputs from a user.

The following example shows a click event binding – [on-click.component.ts]

import { Component, OnInit } from '@angular/core';

@Component({

selector: 'app-on-click',

templateUrl: './on-click.component.html',

styleUrls: ['./on-click.component.css']

})

export class OnClickComponent implements OnInit {

welcomeMsg = '';

constructor() { }

ngOnInit() { }

onClick() {

this.welcomeMsg = 'Welcome you, Anil!';

}

And on-click.component.html -

<button (click)="onClick()">Click Me!</button>

<p>

</p>

</div>

<button on-click="onClick($event)">Click Me!</button>

<p>

</p>

</div>

When the user clicks the button, Angular calls the onClick method from OnClickComponent.

How to get user input from the $event object?

The DOM events carry all information that is useful to the component.

The following example shows to get user input from the $event – key-up.component.ts

import { Component, OnInit } from '@angular/core';

@Component({

selector: 'app-key-up',

templateUrl: './key-up.component.html',

styleUrls: ['./key-up.component.css']

})

export class KeyUpComponent implements OnInit {

values = '';

constructor() { }

ngOnInit() { }

//KeyUp events.

onKeyUp(event: any) {

this.values += event.target.value + ' : ';

}

And key-up.component.html –

<button (click)="onKeyUp($event)">KeyUp Event!</button>

<p>

</p>

</div>

How to get user input from a template reference variable?

This is the other way to get the user data. It is also called #var.

“A template reference variable is mostly a reference to a DOM element within a template. It can also be a reference to Angular components or directives and others.”

It looks like this.

The following example shows to get user input from a template reference variable - template-reference.component.ts

import { Component, OnInit } from '@angular/core';

@Component({

selector: 'app-template-reference',

templateUrl: './template-reference.component.html',

styleUrls: ['./template-reference.component.css']

})

export class TemplateReferenceComponent implements OnInit {

constructor() { }

ngOnInit() {

}

And template-reference.component.html –

<p>

</p>

</div>

How to create a custom validator for both model driven and template driven forms?

There are two types of Validators –

1. Built-in Validators

2. Custom Model Form Validators

a. Email Validator

b. Password Validator

c. Secure Site Validator

d. Credit card validator

Built-in Validators -

1. Validators .required - Requires a form control to have a non-empty value

2. Validators .minlength - Requires a form control to have a value of a min length

3. Validators .maxlength - Requires a form control to have a value of a max length

4. Validators .pattern - Requires a form control’s value to match a given regex

5. And so on

Built-in validator looks like –

this.empForm = new FormGroup({

'email': new FormControl(this.employee.email,[Validators.required, ValidationService.emailValidator]),

'name': new FormControl(this.employee.name, [Validators.required,Validators.minLength(4)]), 'Dep': new FormControl(this.employee.Dep, [Validators.required, Validators.minLength(10)]), 'Desc': new FormControl(this.employee.Desc, [Validators.required, Validators.minLength(100),Validators.minLength(500)]),

});

Custom Model Form Validators – Validators are core functions, they take as input a FormControl instance and returns either null if it’s valid or flag for errors.

You can use the custom validator to validate a specific requirement like -

1. Email Validator

2. Password Validator

3. Secure Site Validator

4. Credit card validator

5. And may more

The Following Steps involve CREATING custom validators -

Steps 1- Create validation service using the CLI command.

ng g service validation

Steps 2 - import validation service in your app NgModule –

import { BrowserModule } from '@angular/platform-browser';

import { NgModule } from '@angular/core';

import {FormsModule, FormGroup} from '@angular/forms';

import {RouterModule} from '@angular/router';

import {HttpClientModule} from "@angular/common/http";

//MY COMPONENTS

import { AppComponent } from './app.component';

import { LoginComponent } from './login/login.component';

import { RegisterComponent } from './register/register.component';

import { EmployeeComponent } from './employee/employee.component';

//My Services

import { AuthServiceService } from './auth-service.service';

import { AuthGuard } from './auth.guard';

import { EmployeeService} from './employee.service';

import { ValidationService } from './validation.service';

@NgModule({

declarations: [

AppComponent,

LoginComponent,

RegisterComponent,

EmployeeComponent

imports: [

BrowserModule,

FormsModule,

HttpClientModule,

RouterModule.forRoot([

{ path: '', component: AppComponent, pathMatch: 'full' },

{ path: 'register', component: RegisterComponent },

{ path: 'employee', component: EmployeeComponent},

{ path: 'login', component: LoginComponent}])

providers: [EmployeeService, ValidationService],

bootstrap: [AppComponent]

})

export class AppModule { }

Steps 3 - Write the customer validation method in your validation.service.ts -

import { Injectable } from '@angular/core';

@Injectable()

export class ValidationService {

constructor() { }

//Check Site contains SSL Security protocol or Not.

static secureSiteValidator(control){

if (!control.value.startsWith('https') || !control.value.includes('.in')) {

return { IsSecureSite: true };

}

return null;

}

//Email Validator

static emailValidator(control) {

if (control.value.match(/[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?/)) {

return null;

}

else {

return { 'InvalidEmail': true };

}

//Password Validator

static passwordValidator(control) {

if (control.value.match(/^(?=.*[0-9])[a-zA-Z0-9!@#$%^&*]{6,100}$/)) {

return null;

}

else {

return { 'InvalidPassword': true };

}

Steps 4 - Use of validation service in your components and its looks like –

import { Component, OnInit } from '@angular/core';

import {Employee } from '../employee'

import { Validators, FormGroup, FormControl } from '@angular/forms';

import {EmployeeService} from '../employee.service'

import { ValidationService } from '../validation.service';

@Component({

selector: 'app-employee',

templateUrl: './employee.component.html',

styleUrls: ['./employee.component.css']

})

export class EmployeeComponent implements OnInit {

constructor( public _empService: EmployeeService) { }

empForm:any;

ngOnInit() {

this.empForm = new FormGroup({

'email': new FormControl(this.employee.email,[Validators.required, ValidationService.emailValidator]),

'name': new FormControl(this.employee.name, [Validators.required,Validators.minLength(4)]),

'Dep': new FormControl(this.employee.Dep, [Validators.required, Validators.minLength(10)]),

'Desc': new FormControl(this.employee.Desc, [Validators.required, Validators.minLength(100),Validators.minLength(500)]),

});

}

employee = new Employee(0,'','','','','');

submitted = false;

//Add new Employee

onSubmit() {

this.submitted = true;

let isSuccess = this._empService.addEmployee(this.employee);

if(isSuccess){

//handle success

console.log(isSuccess);

}else{

//handle errors

}

And

<h1>Employee Form</h1>

<label for="name">Email</label>

</div>

</div>

<label for="Dep">Department</label>

</div>

</div>

<button type="submit" class="btn btn-success" [disabled]="!empForm.form.valid">Submit</button>

</form>

<h4 style="color:green;">Record Added Successfully!</h4>

</div>

For more detail kindly refer the link - https://www.code-sample.com/2018/05/angular-6-7-documentation-and-examples.html

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Anil Singh is an author, tech blogger, and software programmer. Book writing, tech blogging is something do extra and Anil love doing it. For more detail, kindly refer to this link..
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