Angular Web Framework Reactive Forms & Validation 5

Question 1

What is the correct way to provide a custom async validator as a class implementing `AsyncValidator`?

Accepted Answer

Implement the `validate()` method returning Observable or Promise

Answer

Classes implementing `AsyncValidator` must define `validate()` returning an observable or promise that resolves to an error map or null.

Question 2

Which FormArray property tells you how many controls it currently contains?

Accepted Answer

length

Answer

`FormArray.length` returns the number of controls currently in the array, mirroring the standard array API.

Question 3

When using `nonNullable: true` in a FormControl, what happens when `reset()` is called?

Accepted Answer

The control resets to its initial value instead of null

Answer

A non-nullable FormControl stores its initial value and restores it on `reset()` rather than defaulting to null.

Question 4

What does `Validators.composeAsync()` do?

Accepted Answer

Merges multiple async validators into one that runs them in parallel and combines their errors

Answer

`Validators.composeAsync()` combines an array of async validators, runs them concurrently, and merges all returned errors.

Question 5

Which technique prevents an async validator from firing on every keystroke by caching the last checked value?

Accepted Answer

Return the cached result immediately if the value hasn't changed since the last check

Answer

Storing the previously validated value and returning the cached error map when the value is unchanged avoids redundant HTTP calls.

Angular Web Framework Practice Test