Q: What is a FormArray?

**Target Audience:** Developers who know FormGroup and FormControl and now need a form whose number of fields changes at runtime. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The values in sections 5 and 6 were printed on Angular 22.2.3: the runnable example compiled with ngc in strict mode, booted in jsdom and clicked through by a driver, and a script calling every Fo

Question 1

How do you create a custom validator in reactive forms?

Accepted Answer

**Target Audience:** Developers comfortable with basic reactive forms who need validation rules Angular does not ship. **Difficulty:** Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The statuses and error objects in sections 5 and 6 were printed by the runnable example (compiled with ngc in strict mode, booted in jsdom and typed into by a driver) and by small scripts run against @angular/forms 22.2.3. ## 1. Why

Question 2

How do you handle dynamic validation where one form field's validator depends on another field's value?

Accepted Answer

**Target Audience:** Developers who can write a basic [custom validator](/interview-question/how-do-you-create-a-custom-validator-in-reactive-forms) and now need rules that involve more than one field. **Difficulty:** Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The statuses in sections 5 and 6 were printed by running the code on Angular 22.2.3: the runnable example compiled with ngc in strict mode and driven in jsdom, and

Question 3

What is the difference between reactive and template-driven forms?

Accepted Answer

**Target Audience:** Anyone preparing for an Angular interview — assumes basic TypeScript and Angular components, no prior forms knowledge. **Difficulty:** Easy > **How to read this doc:** Plain language first, then 📌 Interview term: callouts with the vocabulary interviewers listen for. The log lines in section 5 were produced by compiling the example with ngc (strict templates) on Angular 22.2.3, booting it in jsdom and typing into both forms with a test driver. ##

Question 4

What is the purpose of updateOn in Reactive Forms?

Accepted Answer

**Target Audience:** Developers using reactive forms who want to control how often validation and valueChanges run. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The timings in section 5 were produced by compiling the example with ngc on Angular 22.2.3, booting it in jsdom, and firing input, blur and submit events from a driver, then reading each con

Question 5

What is a FormArray?

Accepted Answer

**Target Audience:** Developers who know FormGroup and FormControl and now need a form whose number of fields changes at runtime. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The values in sections 5 and 6 were printed on Angular 22.2.3: the runnable example compiled with ngc in strict mode, booted in jsdom and clicked through by a driver, and a script calling every Fo

ANGULARDECODED.

Medium

How do you create a custom validator in reactive forms?

How do you handle dynamic validation where one form field's validator depends on another field's value?

What is the difference between reactive and template-driven forms?

What is the purpose of updateOn in Reactive Forms?

What is a FormArray?