How to Create Custom Validators in Angular 9/8?

Step 1: Install Angular App

Here, in this step you need to create new ng app for this demo. if you have already created then don’t create new angular 8 app.

ng new my-custom-val-app

Step 2: Import FormsModule

If you want to create form in angular app then you need to import FormsModule from @angular/forms library. so let’s add following code to app.module.ts file.

src/app/app.module.ts

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { FormsModule, ReactiveFormsModule } from '@angular/forms';
import { AppComponent } from './app.component';
@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule,
    FormsModule,
    ReactiveFormsModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

Step 3: Form with ngModel

In this step, we will write code of html form with ngModel. so add following code to app.component.html file.

I used bootstrap class on this form. if you want to add than then follow this link too: Install Boorstrap 4 to Angular 8.

src/app/app.component.html

<h1>How to Create Custom Validators in Angular 8 - ItSolutionStuff.com</h1>
<form [formGroup]="form" (ngSubmit)="submit()">
    <div >
        <label for="username">Username</label>
        <input 
            formControlName="username"
            id="username" 
            type="text" 
            >
        <div *ngIf="f.username.touched && f.username.invalid" >
            <div *ngIf="f.username.errors.required">Username is required.</div>
            <div *ngIf="f.username.errors.minlength">Username should be 3 character.</div>
            <div *ngIf="f.username.errors.cannotContainSpace">Username can not contain space.</div>
        </div>
    </div>
    <div >
        <label for="password">Password</label>
        <input 
            formControlName="password"
            id="password" 
            type="password" 
            >
        <div *ngIf="f.password.touched && f.password.invalid" >
            <div *ngIf="f.password.errors.required">Password is required.</div>
        </div>
    </div>
    <button  type="submit">Submit</button>
</form>

Step 4: updated Ts File

In ts file. we will write submit() and get all input fields values. so let’s add following code to app.component.ts file.

src/app/app.component.ts

import { Component } from '@angular/core';
import { FormGroup, FormControl, Validators} from '@angular/forms';
import { UsernameValidator } from './username.validator';
@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  form = new FormGroup({
    username: new FormControl('', [Validators.required, Validators.minLength(3), UsernameValidator.cannotContainSpace]),
    password: new FormControl('', Validators.required)
  });
  get f(){
    return this.form.controls;
  }
  submit(){
    console.log(this.form.value);
  }
}

Step 5: Create Custom Validation File

You need to create custom ‘username.validator.ts’ file and write code as bellow:

src/app/username.validator.ts

import { AbstractControl, ValidationErrors } from '@angular/forms';
export class UsernameValidator {
    static cannotContainSpace(control: AbstractControl) : ValidationErrors | null {
        if((control.value as string).indexOf(' ') >= 0){
            return {cannotContainSpace: true}
        }
        return null;
    }
}

Now you can run your application using following command:

ng serve

Now you can see layout as like bellow screen shot:

I hope it can help you…