What types of validators are there?

Angular has two types of validators:

#Synchronous validators

• Checked immediately, without waiting.
• Used most often.
• Examples of built-in ones:
  • Validators.required - the field must not be empty;
  • Validators.email - checks that the e-mail is correctly formatted;
  • Validators.minLength(n) / Validators.maxLength(n) - limit the length;
  • Validators.pattern(regex) - checks against a regular expression.
  • You can write your own, returning null (if all is fine) or an error object.

#Asynchronous validators

• Used when the check takes time, for example a request to the server.

• Return a Promise or Observable.

• Example: checking whether a username is already taken:

  typescript

  Copy

  checkUsername(control: FormControl) { return this.api.checkUser(control.value).pipe( map(isTaken => (isTaken ? { usernameTaken: true } : null)) ); }

Both types can be applied to individual FormControl instances as well as to entire FormGroup instances.