Angular 4 patchValue based on index in FormArray

Use FormArray#at + AbstractControl#patchValue:

this.items.at(index).patchValue(...);

DEMO

You can do this simply by creating a new FormControl if you want to update the hole array structure:

this.form.setControl('items',  new FormControl(arrayItemsValue));

Or by removing all items before updating them :

const items = (<FormArray>this.form.get('items'));
 for (let i = 0; i < items.length; i++) {
     items.removeAt(i);
 }
 this.form.get('items').setValue(arrayItemsValue);