Angular 2 Multiple validators

Is it possible to have multiple validators on a form field? I tried this, but it resulted in some strange errors (field was never valid, even if requirements were met)

this.username = new Control('', Validators.minLength(5), Validators.required);

How can I use multiple validators?

You can combine validators using Validators.compose()

this.username = new Control('', 
    Validators.compose(
        [Validators.minLength(5), Validators.required]));

for async validators use

this.username = new Control('', null,
    Validators.composeAsync(
        [someAsyncValidator, otherAsyncValidator]));

There are open issues with async validators, especially sync validators combined with async validators don't work

• https://github.com/angular/angular/issues/8923
• https://github.com/angular/angular/issues/1068

To make sync validators work with async validators, wrap the sync validators in promises and compose them as async valdiators like

this.username = new Control('', null,
    Validators.composeAsync([
        (control:Control) => Promise.resolve(Validators.minLength(5)(control)), 
        (control:Control) => Promise.resolve(Validators.required(control)),
        someAsyncValidator, otherAsyncValidator
    ]));

this problem has been addressed

you can make an array of validators

this.username = new FormControl('', [ Validators.minLength(5), Validators.required ]); 

I suggest utilizing the Validators.compose() method for combining all non-async validators and separately passing in the Validators.composeAsync() for any async calls.

Basically the constructor args for FormControl are as follows:

1. formState (or simply the initial starting value)
2. validators (I suggest using Validators.compose([...]) here)
3. asyncValidators (I suggest using Validators.composeAsync([...]) here)

Example using FormBuilder (feel free to use straight up Control):

this.acctForm = this.fb.group({
            'name': [
                '',
                Validators.compose([
                    Validators.required, Validators.minLength(2), Validators.maxLength(20), Validators.pattern('[a-zA-Z]')
                ])
            ],
            'cellNumber': [
                '',
                Validators.compose([
                    Validators.required, Validators.pattern('[0-9]{10}')
                ]),
                Validators.composeAsync([
                    this.checkPhoneValid.bind(this)
                ])
            ]
        });

This helps avoid async validation until the non-async validators are valid (excl. the initial check, which can easily be handled, see further below).

Everything Combined Example (validators, asyncValidators & debouncing):

import { Component, Injectable, OnInit } from '@angular/core';
import { Http } from '@angular/http';
import { FormBuilder, FormGroup, Validators, AbstractControl } from '@angular/forms';


@Component({
    selector: 'app-sandbox',
    templateUrl: './sandbox.component.html',
    providers: []
})
export class FormControlsDemoComponent implements OnInit {
    private debouncedTimeout;
    public acctForm: FormGroup;

    constructor(private http: Http, private fb: FormBuilder) {
        // @note Http should never be directly injected into a component, for simplified demo sake...
    }

    ngOnInit() {
        this.acctForm = this.fb.group({
            // Simple Example with Multiple Validators (non-async)
            'name': [
                '',
                Validators.compose([
                    Validators.required, Validators.minLength(2), Validators.maxLength(20), Validators.pattern('[a-zA-Z]')
                ])
            ],
            // Example which utilizes both Standard Validators with an Async Validator
            'cellNumber': [
                '',
                Validators.compose([
                    Validators.required, Validators.minLength(4), Validators.maxLength(15), Validators.pattern('[0-9]{10}')
                ]),
                Validators.composeAsync([
                    this.checkPhoneValid.bind(this) // Important to bind 'this' (otherwise local member context is lost)
                    /*
                        @note if using a service method, it would look something like this...
                        @example:
                            this.myValidatorService.phoneUniq.bind(this.myValidatorService)
                    */
                ])
            ],
            // Example with both, but Async is implicitly Debounced
            'userName': [
                '',
                Validators.compose([
                    Validators.required, Validators.minLength(4), Validators.maxLength(15), Validators.pattern('[a-zA-Z0-9_-]')
                ]),
                Validators.composeAsync([
                    this.checkUserUniq.bind(this) // @see above async validator notes regarding use of bind
                ])
            ]
        });

    }

    /**
     * Demo AsyncValidator Method
     * @note - This should be in a service
     */
    private checkPhoneValid(control: AbstractControl): Promise<any> {
        // Avoids initial check against an empty string
        if (!control.value.length) {
            Promise.resolve(null);
        }

        const q = new Promise((resolve, reject) => {
            // determine result from an http response or something...
            let result = true;

            if (result) {
                resolve(null);
            } else {
                resolve({'phoneValidCheck': false});
            }
        });
        return q;
    }

    /**
     * Demo AsyncValidator Method (Debounced)
     * @note - This should be in a service
     */
    private checkUserUniq(control: AbstractControl): Promise<any> {
        // Avoids initial check against an empty string
        if (!control.value.length) {
            Promise.resolve(null);
        }

        clearTimeout(this.debouncedTimeout);

        const q = new Promise((resolve, reject) => {

            this.debouncedTimeout = setTimeout(() => {

                const req = this.http
                    .post('/some/endpoint', { check: control.value })
                    .map(res => {
                        // some handler logic...
                        return res;
                    });

                req.subscribe(isUniq => {
                    if (isUniq) {
                        resolve(null);
                    } else {
                        resolve({'usernameUnique': false });
                    }
                });

            }, 300);
        });
        return q;
    }

}

Some people like to hack in debounced async validator by binding to the control's Observable valueChanges like this:

this.someControl.debounceTime(300).subscribe(val => {
      // async call...
});

I personally don't recommend this for most cases, as it adds unnecessary complications.

NOTE: This should work, as of the latest version of Angular (2 & 4) since the writing of this post.