Why should I use Validators.compose()?

Question

I have a field I want to validate with multiple validators.
Using the Module Driven approach the code looks likes this:

this.exampleForm = this.fb.group({
    date_start : ['', Validators.compose([
                          Validators.required, 
                          Validators.pattern("[0-9]{2}-[0-9]{2}-[0-9]{4}")
                      ])
                 ]
})

But I can also write this withouth Validators.compose() like:

this.exampleForm = this.fb.group({
    date_start : ['', [
                          Validators.required, 
                          Validators.pattern("[0-9]{2}-[0-9]{2}-[0-9]{4}")
                      ]
                 ]
})

And it works just fine. Personally I prefer the 2nd version (without compose), less code and better readability. And this begs the question, why should I use Validators.compose()?

Answer 1

When we create new FormControl/FormGroup/FormArray(AbstractControl) - coerceToValidator is called.

function coerceToValidator(
    validatorOrOpts?: ValidatorFn | ValidatorFn[] | AbstractControlOptions | null): ValidatorFn|
    null {
  const validator =
      (isOptionsObj(validatorOrOpts) ? (validatorOrOpts as AbstractControlOptions).validators :
                                       validatorOrOpts) as ValidatorFn |
      ValidatorFn[] | null;

  return Array.isArray(validator) ? composeValidators(validator) : validator || null;
}

export function composeValidators(validators: Array<Validator|Function>): ValidatorFn|null {
  return validators != null ? Validators.compose(validators.map(normalizeValidator)) : null;
}

So there is no need to compose validators before we pass it to an AbstractControl.

6/13/16 was added feat(forms): compose validator fns automatically if arrays from now on, Validators.compose is there for backward compatibility.