Angular form validation: compare two fields

Question

In an Angular 4 application, how can I validate two fields of a form doing a comparison?

For example, let's suppose that my form has a startDate and an endDate date fields and I want to make sure that the endDate must be bigger than the startDate.

Answer 1

When you want to implement validations containing one or more sibling (form)controls, you have to define the validator function on a level up/above that of the sibling controls. For ex:

ngOnInit() {
    this.form = this.formbuilder.group({
        'startDate': ['', [<control-specific - validations >]],
        'endDate': ['', [<control-specific - validations >]]
    }, { validator: checkIfEndDateAfterStartDate });
}

Then outside the component class's definition (in the same file), define the function checkIfEndDateAfterStartDate as well.

export function checkIfEndDateAfterStartDate (c: AbstractControl) {
    //safety check
    if (!c.get('startDate').value || !c.get('endDate').value) { return null }
    // carry out the actual date checks here for is-endDate-after-startDate
    // if valid, return null,
    // if invalid, return an error object (any arbitrary name), like, return { invalidEndDate: true }
    // make sure it always returns a 'null' for valid or non-relevant cases, and a 'non-null' object for when an error should be raised on the formGroup
}

This validation will make the FormGroup invalid by adding the error-flag (here invalidEndDate) to true to the errors object for that FormGroup. If you want to have specific errors to be set on any of the sibling controls instead, then you can manually set the error flags on that formControl by using something like, c.get('endDate').setErrors({ invalidEndDate: true }). If you do this, then make sure you clear them for a valid case by setting the errors to null like this, c.get('endDate').setErrors(null).

A live demo of a similar validation can be seen here.