The fieldArray structure and methods on useForm()

• The useFieldArray() methods

• The fieldArray inputs and values rules on useForm() render

• A fieldArray form with nested useForm() components.

The useFieldArray hook allows us to render and update inputs in a dynamic form, improving user experience and performance.

Its properties are:

• name: The fieldArray name, it's used to set the fieldArray's defaultValues in useForm().

• control: Its useForm() control property.

• shouldUnregister: boolean; it indicates whether the elements of the fieldArray should be unregistered after unmounting. False by default.

• rules: Its validation rules apply to the number of objects in the field array, rather than to the individual input values.

The useFieldArray() hook returns an array of input fields objects, each with a unique ID. In the DOM, we map() the fieldsArray, using its item.id as the container key attribute, and its properties to render the inputs of the current index item.

Static FieldArray input render

We render index input errors with a ternary operator on the formState.errors or directly rendering its message by using optional chaining (?.).

This approach avoids the 'Cannot read properties of undefined' error that returns on an empty errors object.//The rules errors can block submit and are found in at errors._arrayName_.root

//The rendered inputs names need to correspond to the defaultValues(if set)
const {register, control, formState: {errors}} = useForm({
  defaultValues: {
    basic: "primo", 
    domandad: [{primo: "valore", second: "self select"}]
  }
})

const {fields, append} = useFieldArray({
  control: control, name: "domandad",
  rules: { minLength: { value: 4, message: "Its too short" } }
})

<form>
  <input {...register("basic")} />

  <ul>
  {fields.map((item, index)=>{
    return(
      <li key={item.id}>
        <input {...register(`domandad.${index}.primo`,{required: "missing"})}/>
        <input {...register(`domandad.${index}.second`, 
          {minLength: {value: 10, message: "too shorty"}}) }
        />

        <p> { errors?.domandad?.[index]?.primo?.message } </p>
	<p> { errors.domandad&& errors.domandad[index]?.name.message } </p>
      </li>
    )
  })
  }
  </ul>

  <p> { errors.domandad?.root.message } </p>
</form>

The arrayFields errors property will be an array of error objects, with each error corresponding to its input based on its index.

{domandad: Array(3)}
0: name: {type: 'required', message: 'Absent here', ref: input},
2: name: {type: 'required', message: 'Absent here', ref: input}

Any useFieldsArray method object will have to match the number and names of the fields.map() inputs.

<button onClick={()=>{
  append({ primo: "new inputs values", second: "With same key names" })
}}>
  Added
</button>

Dynamic FieldArray input render

The fieldArray inputs can be rendered using different components.

//Remember the field._index_.name for the input name.
<Controller
  name={`test.${index}.lastName`}
  control={control}
  rules={{ required: "Name is required" }}
  render={({ field }) => (
    <input {...field} placeholder="Enter name" />
  )}
/>

The useFieldArray() methods

The fieldArray returns include the methods to edit the rendered field object:

useFieldArray methods

• append: object. It adds a new input object at the end of the fieldArray.

• remove: index. It removes the fieldArray object with the index.

• insert: index, object. It adds the input object to the set fieldsArray index.

• prepend: object. It places the input object at the start of the fieldArray.

• swap: 2 index. It swaps the position of the 2 pre-existent input objects.

• move: 2 index. It moves the index input position to the target position, moving the inbetween inputs by 1.

• update: index, object. It updates the value of its specific index input object.

• replace: object/array. It will replace the entire fieldArray, use an array for multiple input objects.

//The methods will apply to the named fieldArray
const { append, prepend, insert, ... } = useFieldArray({
  control, name: "test"
});

append( {trina: "catri"}, {})
remove(3)

//insert will fill the first index avaiable if no pre existent length
insert(2, {paguro: "newo"}, {})
prepend({ last: "foreva", every: "body" }, {})

//swap will create blank input objects for indexes exceeding the fields length.
swap(0, 5)
//If move uses non pre-existent indexes, it will create blank inputs to fill it.
move(0, 2)
//The update triggers a re render with its unmount and remount of the input object
update(1, {master: "duel", coco: "cola", sec: "sepa"})
replace( [{party: "basic", locro: "mais"}, {giravolta: "salti"}] )

All useFieldArray() methods focus the input indexes they apply to. The append, prepend, and insert methods include optional properties, shouldFocus and focusIndex, which determine whether to focus its object index or a specific input object in the fieldArray.

//The properties are contained in the method optional object property
<button onClick={()=>{insert(2, {paguro: "newo"}, {shouldFocus: false}) }} >
  inserto
</button>

<button onClick={()=>{prepend({ last: "foreva", every: "body" }, {focusIndex: 2}) }}>
  prependa
</button>

Do not stack useFieldArray methods; instead, we useEffect() to sequentially trigger methods based on specific useState dependencies.

//We set a condition for teh first render useEffect() trigger
const [doppio, setDoppio] = useState(0)

useEffect(() => {
  if (fields.length > 1) {
    remove(0); // Remove the first item after the component mounts, min 1 index
  }
  
}, [ doppio ]);

function mettiaggiunt(){
  append({ firstName: "aggiunto elemento", lastName: "rimosso"  })
  setDoppio((x)=> x + 1)
}

The fieldArray inputs and values rules on useForm() render

The fieldArray <input> index can be used to conditionally render an input or dynamically modify the form based on the input's position.

fieldArray index conditional inputs

A useFieldArray will automatically create its array even without defaultValues when input objects are added.

We useWatch() the <input> index value as condition to render the specific <input> property. Any added input property will be returned by the onSubmit function, even if it is not rendered. The static form ensures that only one conditional input will be rendered at a time.

//A dynamic form might re-renderinputs with the same name or returned value
//We destruct and pass the index and control as object properties 
//The handlesubmit can filter the un-rendered input properties.
const ConditionalInput = ({ control, index }) => {
  const value = useWatch({ name: "test", control });

  return (
    <Controller
      control={control} name={`test.${index}.firstName`}
      render={({ field }) =>
        value?.[index]?.checkbox === "on" ? <input {...field} /> : null
      }
    />
  );
};

const { handleSubmit, control, register } = useForm();
const { fields, append, prepend } = useFieldArray({
  control, name: "test"
});

function vediamo(dati){
  let test = dati.test.filter((ogge)=>( ogge.checkbox && ogge ))
}

return(
  <div>
  <form onSubmit={handleSubmit(vediamo)}>
  <input {...register("fisso")} />

  {fields.map((field, index) => {
    const id = `test.${index}.checkbox`;
    return (
      <div key={field.id}>
        <section>
          <label htmlFor={id}>Show Input</label>
          <input {...register(id)} type="checkbox" value="on" id={id} />
          <ConditionalInput {...{ control, index }} />
        </section>
        <hr />
      </div>
    );
  })}

    <input type="submit" className="btn btn-warning" />
  </form>
  
  <button onClick={ () => append({ firstName: "" }) }>
    append
  </button>
  </div>
)

Conditional Input component with fieldArray methods

fieldArray index dynamic form methods

The fieldArray will batch updates during the component's re-render, and any functions that access it will retrieve the last rendered form object.

We create an object that merges the fieldArray with its watch() current values, allowing us to have the complete field values available.

//The formArray will be avaiable on the onSubmit() event handler 
//We map() loop and update each fieldArray input object with the watch() 
function Noinput(){
  const { register, control, watch, formState } = useForm();
  const { fields, append, remove, move } = useFieldArray({
    control, name: "fieldArray"
  });

  const watchFieldArray = watch("fieldArray");
  const controlledFields = fields.map((field, index) => {
    return {
      ...field,
      ...watchFieldArray[index]
    };
  });
  //The current fieldArray inputs and values will be avaiable
  const passo = () => console.log(controlledFields);

  return(
    <form>
      <input {...register("primo")} />

      {fields.map((field, index) => {
        return <input key={field.id} {...register(`fieldArray.${index}.name`)} />; 
      })}

      <button onClick={() => append({ primo: "yet to", secondo: "be updated" })}>
        Double
      </button>
      
      <button onClick={() => passo()}>
        Access current firldArray values
      </button>
    </form>
  )
}

A fieldArray form with nested useForm() components.

We render a parent fieldArray useForm() with a nested useForm() component.

The nested component <Singular> will render and register the fieldArray input objects, and onSubmit() it will update the parent fieldArray, using the index and useFieldArray() update props

The useWatch() hook update on different useForm() control object properties

A custom hook (like useWatch()) will update based on the useForm() structure of its control property.

The parent useForm() will get updated based on its fieldArray structure, not its field values. The child useForm() registers the inputs, so its control will trigger the useWatch() on input change to return the current field values.

We use a ternary operator useWatch() name to specify the input index being watched, an undefined name would return the entire form, so we use a non-existent input name.

//Parent fieldArray useForm(), will update only on re-render
function Diretto(){
  ...
  return(
    <div className="mb-5">
      ...
      {fields.map((item, index)=>{
        return(
          <div key={item.id}>
            <Singular 
              control={control} update={update}
              index={index} value={item}
            />
          </div>
        ) 
      })}
    </div>
  )
}

//Child useForm(), the useWatch() nestControl will trigger on input change
//The dot notation to access the array object
const Singular = ({ control, update, index, value }) => {
  const { register, handleSubmit, control: nestControl } = useForm({
    defaultValues: value
  })

  const data = useWatch({
    control: nestControl,
    name: index == 1 ? `secondName` : "none",
  });
  console.log( "Current value", data )

  //This can also access the field value, but only on fieldArray change.
  const data1 = useWatch({
    control,
    name: "valori.1.secondName"
  })
  console.log( "Current value on updated fieldArray", data1 )
  
  return(
    <div>
      <Risulta control={control} index={index}/>

      <input {...register("firstName", {required: "true"})}/>
      <input {...register("secondName", {required: "true"})}/>
      <label> Input checked:
        <input type="checkbox" {...register("working")}/>
      </label>
    </div>
  )
}

The third child component <Risulta> will be conditionally rendered based on its index input values. It will be re-rendered each time the fieldArray is updated due to the useWatch() control property.

//On fieldArray update will return null if field value is empty
const Risulta = ({ control, index }) => {
  const data = useWatch({
    name: `valori.${index}`,control
  })

  if (!data?.firstName) return null;

  return(
    <div>
      <p> Updated: {data?.firstName} {data?.lastName} </p>
      <p>{Boolean(data?.working) && data?.working && "I am working."}</p>
    </div>
  )
}

//Any new fieldArray input object can be passed as a prop into a defaultValue.
//The update prop form useFieldArray() method updates the fieldArray
const Singular = ({ control, update, index, value }) => {
  const { register, handleSubmit, control: nestControl } = useForm({
    defaultValues: value
  })
  
  return(
    <div>
      <Risulta control={control} index={index}/>

      <input {...register("firstName", {required: "true"})}/>
      <input {...register("secondName", {required: "true"})}/>
      <label> Input checked:
        <input type="checkbox" {...register("working")}/>
      </label>

      <button type="button" onClick={handleSubmit((data)=>{update(index, data)})}>
        fieldArrayUpdate
      </button>
    </div>
  )
}

function Diretto(){
  const { register, handleSubmit, control } = useForm()
  const {fields, update, remove, append} = useFieldArray({
    name: "valori", control
  })

  const manda = (dati) => console.log( dati )

  return(
    <div>
      <form onSubmit={handleSubmit(manda)}>
        <input {...register("primo")} />

        {fields.map((item, index)=>{

          return(
            <div key={item.id}>
              <Singular 
                control={control} update={update}
                index={index} value={item}
              />

              <button type="button" onClick={()=>{ remove(index) }}> 
                delete 
              </button>
            </div>
          ) 
        })}

        <button onClick={()=>{ append({firstName: "", secondName: ""}) }}>
          primo 
        </button>

        <input type="submit" />
      </form>
    </div>
  )
}

Append and render fieldArray input object components

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