Skip to main content

Angular Form, Template, and Validations | Angular 9, 8, 7, 6, 5, 4, 2

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.
<form (ngSubmit)="onSubmitEmployee(empForm)" #empForm="ngForm">
  <label>F-Name </label><input name="f-name" required [(ngModel)]="employee.fname">
  <label>L-Name </label><input name="l-name" required [(ngModel)]="employee.lname">
  <label>Age    </label><input name="age" required [(ngModel)]="employee.age">
 
  <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 -
<div class="msg">
  <button (click)="onClick()">Click Me!</button>
  <p>
    {{welcomeMsg}}
  </p>
</div>
OR
<!-- Canonical form, the (on-) prefix alternative -->
<div class="msg">
  <button on-click="onClick($event)">Click Me!</button>
  <p>
    {{welcomeMsg}}
  </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 –
<div class="event">
  <button (click)="onKeyUp($event)">KeyUp Event!</button>
  <p>
    {{values}}
  </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.
<input #name placeholder="Enter Name">

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 –
<div class="event">
  <button #keydownVal (keydown)="0"></button>
  <p>
    {{keydownVal.value}}
  </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
<div class="container">
  <h1>Employee Form</h1>
  <form #empForm="ngForm" (ngSubmit)="onSubmit()">
      <div class="form-group">
          <label for="name">Email</label>
          <input type="text" class="form-control" id="email” required [(ngModel)]="employee.email" name="email">
        </div>
    <div class="form-group">
      <label for="name">Name</label>
      <input type="text" class="form-control" id="name” required [(ngModel)]="employee.name" name="name">
    </div>

    <div class="form-group">
      <label for="Dep">Department</label>
      <input type="text" class="form-control" id="Dep” required [(ngModel)]="employee.Dep" name="Dep">
    </div>

    <div class="form-group">
      <label for="Desc">Desc</label>
      <input type="text" class="form-control" id="Desc” required [(ngModel)]="employee.Desc" name="Desc">
    </div>

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

  <div [hidden]="!submitted">
    <h4 style="color:green;">Record Added Successfully!</h4>
  </div>
</div>

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

Labels

Labels:
By Anil Singh | Rating of this article (*****)

Popular posts from this blog

nullinjectorerror no provider for httpclient angular 17

In Angular 17 where the standalone true option is set by default, the app.config.ts file is generated in src/app/ and provideHttpClient(). We can be added to the list of providers in app.config.ts Step 1:   To provide HttpClient in a standalone app we could do this in the app.config.ts file, app.config.ts: import { ApplicationConfig } from '@angular/core'; import { provideRouter } from '@angular/router'; import { routes } from './app.routes'; import { provideClientHydration } from '@angular/platform-browser'; //This (provideHttpClient) will help us to resolve the issue  import {provideHttpClient} from '@angular/common/http'; export const appConfig: ApplicationConfig = {   providers: [ provideRouter(routes),  provideClientHydration(), provideHttpClient ()      ] }; The appConfig const is used in the main.ts file, see the code, main.ts : import { bootstrapApplication } from '@angular/platform-browser'; import { appConfig } from '
Read more

39 Best Object Oriented JavaScript Interview Questions and Answers

Most Popular 37 Key Questions for JavaScript Interviews. What is Object in JavaScript? What is the Prototype object in JavaScript and how it is used? What is "this"? What is its value? Explain why "self" is needed instead of "this". What is a Closure and why are they so useful to us? Explain how to write class methods vs. instance methods. Can you explain the difference between == and ===? Can you explain the difference between call and apply? Explain why Asynchronous code is important in JavaScript? Can you please tell me a story about JavaScript performance problems? Tell me your JavaScript Naming Convention? How do you define a class and its constructor? What is Hoisted in JavaScript? What is function overloadin
Read more

25 Best Vue.js 2 Interview Questions and Answers

What Is Vue.js? The Vue.js is a progressive JavaScript framework and used to building the interactive user interfaces and also it’s focused on the view layer only (front end). The Vue.js is easy to integrate with other libraries and others existing projects. Vue.js is very popular for Single Page Applications developments. The Vue.js is lighter, smaller in size and so faster. It also supports the MVVM ( Model-View-ViewModel ) pattern. The Vue.js is supporting to multiple Components and libraries like - ü   Tables and data grids ü   Notifications ü   Loader ü   Calendar ü   Display time, date and age ü   Progress Bar ü   Tooltip ü   Overlay ü   Icons ü   Menu ü   Charts ü   Map ü   Pdf viewer ü   And so on The Vue.js was developed by “ Evan You ”, an Ex Google software engineer. The latest version is Vue.js 2. The Vue.js 2 is very similar to Angular because Evan You was inspired by Angular and the Vue.js 2 components looks like -
Read more

55 Best TypeScript Interview Questions and Answers - JavaScript!

What Is TypeScript? By definition, "TypeScript is a typed superset of JavaScript that compiles to plain JavaScript." TypeScript is a superset of JavaScript which provides optional static typing, classes and interfaces. => The TypeScript was first made public in the year 2012. => Typescript is a modern age JavaScript development language. => TypeScript is a strongly typed, object oriented, compiled language. => TypeScript was designed by Anders Hejlsberg (designer of C#) at Microsoft. => TypeScript is both a language and a set of tools. As an Example of TypeScript, class Hello {     msg : string ;      constructor ( message : string ) {          this . msg = message ;      }       getMsg () {          return "Hello, " + this . msg ;      } } TypeScript introduced a great deal of syntax taken from object-oriented programming, including but not limited to: 1) Interfaces 2) Classes 3) Enumerated t
Read more

.NET Core MVC Interview Questions and Answers

» OOPs Interview Questions Object Oriented Programming (OOP) is a technique to think a real-world in terms of objects. This is essentially a design philosophy that uses a different set of programming languages such as C#... Posted In .NET » .Net Constructor Interview Questions A class constructor is a special member function of a class that is executed whenever we create new objects of that class. When a class or struct is created, its constructor is called. A constructor has exactly the same name as that of class and it does not have any return type… Posted In .NET » .NET Delegates Interview Questions Delegates are used to define callback methods and implement event handling, and they are declared using the "delegate" keyword. A delegate in C# is similar to function pointers of C++, but C# delegates are type safe… Posted In .NET » ASP.Net C# Interview Questions C# was developed by Microsoft and is used in essentially all of their products. It is mainly used for
Read more