Validating user input for accuracy and completeness helps in improving overall data quality. Angular and its form package turns up with a Validators class that has some beneficial validators like minLength, maxLength, required and pattern. However, occasionally if we wish to validate different fields under more complex/custom rules we can make optimum use of custom validator. Defining custom validators while using Reactive Forms in Angular comes very easy as they are more of regular functions. One can conveniently generate function for custom validators within the component file in case the validator is not supposed to be used elsewhere.

1. C r e a t i n g C u s t o m Va l i d a t o r s O n
R e a c t i v e F o r m s U s i n g A n g u l a r 6
A I M D E K T E C H N O L O G I E S I N C .
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2. Introduction
• Validating user input for accuracy and completeness helps in improving overall data quality.
• Angular and its form package turns up with a Validators class that has some beneficial validators like
minLength, maxLength, required and pattern.
• However, occasionally if we wish to validate different fields under more complex/custom rules we can make
optimum use of custom validator.
• Defining custom validators while using Reactive Forms in Angular comes very easy as they are more of regular
functions.
• One can conveniently generate function for custom validators within the component file in case the validator is
not supposed to be used elsewhere. But, herein we will be assuming the re-use and create respective
validators in separate files.
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3. Step 1: Create a project and add material library
Ng new ng6material
Cd ng6mateial
Ng add @angular/material
Ng serve
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4. Step 2: Create component called
reactiveform.component.ts
Ng generate component reactiveform
Configure route of reactiveform.component.ts so that we can navigate it at http://localhost:4200/reactive-
form
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5. Step 3: Create a simple form with a model using
Angular material and @angular/form
import { Component, OnInit } from '@angular/core';
import{FormBuilder,FormGroup} from '@angular/forms';
@Component({
selector: 'app-login',
templateUrl: './reactiveform.component.html',
styleUrls: ['./reactiveform.component.css']
})
export class ReactiveformComponent implements OnInit {
complexForm : FormGroup;
constructor(fb:FormBuilder) {
this.complexForm = fb.group({
userName:””,
email:””,
passWord:””,
})
}
ngOnInit() {}
submitForm(value: any){
console.log(value);
}
}
Reactiveform.components.ts
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6. Step 3: contd.
<div class="login-wrapper">
<mat-card>
<div style="text-align:center;">
<img src="http://www.aimdek.com/wp-
content/uploads/2016/12/height_70px.png">
</div>
<form class="example-form" [formGroup]="complexForm"
(ngSubmit)="submitForm(complexForm.value)">
<mat-form-field>
<input matInput placeholder="Username"
[formControl]="complexForm.controls['userName']" />
</mat-form-field>
<br>
<mat-form-field>
<input matInput placeholder="email"
[formControl]="complexForm.controls['email']" />
</mat-form-field>
<br>
<mat-form-field >
<input matInput placeholder="Password"
[formControl]="complexForm.controls['passWord']" />
</mat-form-field>
<br>
<button type="submit" mat-raised-button color="primary"
align="right">Login</button>
</form>
</mat-card>
</div>
Reactiveform.component.html
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7. Step 3: contd.
This will generate the simple model driven reactive form without in build and custom validation.
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8. Step 3: contd.
• Now we will add in build validation first and then will go through the custom validation.
• For validation we need to import Validators from the @angular/forms and have to add that particular
Validators in each field of model.
• We are going to use Validators.required and Valodators.email in this demo.
• Following snippet shows the usage of build in Validators.
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9. Step 4: Generate custom validator for password
import { Component, OnInit } from '@angular/core';
import{FormBuilder,FormGroup} from '@angular/forms';
@Component({
selector: 'app-login',
templateUrl: './reactiveform.component.html',
styleUrls: ['./reactiveform.component.css']
})
export class ReactiveformComponent implements OnInit {
complexForm : FormGroup;
constructor(fb:FormBuilder) {
this.complexForm = fb.group({
userName:””,
email:””,
passWord:””,
})
}
ngOnInit() {}
submitForm(value: any){
console.log(value);
}
}
Reactiveform.components.ts
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10. Step 3: contd.
<div class="login-wrapper">
<mat-card>
<div style="text-align:center;">
<img src="http://www.aimdek.com/wp-
content/uploads/2016/12/height_70px.png">
</div>
<form class="example-form" [formGroup]="complexForm"
(ngSubmit)="submitForm(complexForm.value)">
<mat-form-field>
<input matInput placeholder="Username"
[formControl]="complexForm.controls['userName']" />
</mat-form-field>
<br>
<mat-form-field>
<input matInput placeholder="email"
[formControl]="complexForm.controls['email']" />
</mat-form-field>
<br>
<mat-form-field >
<input matInput placeholder="Password"
[formControl]="complexForm.controls['passWord']" />
</mat-form-field>
<br>
<button type="submit" mat-raised-button color="primary"
align="right">Login</button>
</form>
</mat-card>
</div>
Reactiveform.component.html
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11. Step 4: Generate custom validator for password
• For this we have to create password.validator.ts
manually because angular CLI is not providing this
in it’s package.
• In this file we have to import abstractValidator
which is also part of @angular/form package.
• Following is the snnipet code of
password.validator.ts
import { AbstractControl,Validator } from '@angular/forms';
export function ValidatePassword(control: AbstractControl) {
if (!/^(?=.*[A-Za-z])(?=.*d)[A-Za-zd]{4,20}/.test(control.value)) {
return { validPassword: true };
}
return null;
}
This code will return boolean if password is valid
and return NULL if it is not
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12. Step 4: contd.
• Now import this validator in reactiveform.components.ts as following
– import { ValidatePassword } from '../validators/password.validator’;
• Use the ValidatePassword function along with Validator.required in password field of form model as following:
– passWord:[null,[Validators.required,ValidatePassword]],
• Now we have to use this in reactiveform.component.html inorder to display custom error message for password as following :-
– <div class="error"
*ngIf="complexForm.get('passWord').errors &&
complexForm.get('passWord').dirty &&
complexForm.get('passWord').errors.validPassword">
Password must contain letter and digit.
</div>
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13. Step 4: contd.
And there you have it: how to create a custom validator for Angular Reactive Forms. We at AIMDek cater
future-proof front-end development services.
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14. INDIA
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Sales: sales@aimdek.com | General: hello@aimdek.com
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Canada
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7030 Woodbine Avenue, Suite 500, Markham, Ontario, L3R 6G2, Canada
Sales: sales@aimdek.com | General: hello@aimdek.com
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