What is Angular Reactive Forms in depth?

Angular Reactive Forms give explicit, programmatic control over form structure and validation. Core classes: FormControl: individual form field: new FormControl("initialValue", [Validators.required, Validators.email]). Properties: value, valid, invalid, dirty (user changed), touched (user blurred), errors. FormGroup: group of related controls: new FormGroup({ email: new FormControl("", Validators.required), password: new FormControl("") }). Access control: form.get("email")?.value. FormArray: ordered list of controls (dynamic fields): new FormArray([new FormControl(""), new FormControl("")]); array.push(new FormControl(""));. FormBuilder: shorthand for creating form structures: this.fb.group({ email: ["", [Validators.required, Validators.email]], tags: this.fb.array([]) }). Built-in validators: Validators.required, Validators.email, Validators.minLength(8), Validators.maxLength(50), Validators.pattern(/regex/), Validators.min(0), Validators.max(100). Custom validators: synchronous: (control: AbstractControl): ValidationErrors|null => control.value === "forbidden" ? { forbidden: true } : null; asynchronous: returns Observable<ValidationErrors|null> (for server-side validation — unique email check). Cross-field validation: apply a validator to the group: this.fb.group({ password: "", confirm: "" }, { validators: passwordMatchValidator }). updateOn: control when validation fires: new FormControl("", { updateOn: "blur" }) — validates on blur, not every keystroke. Value changes: form.get("email")!.valueChanges.pipe(debounceTime(300)).subscribe(v => this.checkEmail(v)).