useWatch

useWatch: ({ control?: Control, name?: string, defaultValue?: any }) => object

Behaves similarly to the watch API, however, this will isolate re-rendering at the component level and potentially result in better performance for your application.

Props

Name	Type	Description
`name`	`string \| string[]`	Name of the field.
`control`	`Object`	`control` object provided by `useForm`. It's optional if you are using FormContext.
`defaultValue`	`any`	default value for `useWatch` to return before the initial render. Note: the first render will always return `defaultValue` when it's supplied.
`disable`	`boolean = false`	Option to disable the subscription.

Rules

The initial return value from useWatch will always return what's inside of defaultValue or defaultValues from useForm.
The only different between useWatch and watch is at root level or component level update.

useWatch execution order matters, which means if you are update form value before the subscription is in place, then the value update will be ignored.

setValue('test', 'data');
useWatch('test'); // ❌ subscription is happened after value update, no update received

useWatch('example'); // ✅ input value update will be received and trigger re-render
setValue('example', 'data');

useWatch result is optimised for render phase instead of useEffect's deps, to detect value update you may want to use an external custom hook for value comparison.

CodeSandbox

import React from "react";
import { useForm, useWatch } from "react-hook-form";

function IsolateReRender({ control }) {
  const firstName = useWatch({
    control,
    name: 'firstName', // without supply name will watch the entire form, or ['firstName', 'lastName'] to watch both
    defaultValue: 'default' // default value before the render
  });

  return <div>{firstName}</div>; // only re-render at the component level, when firstName changes
}

function App() {
  const { register, control, handleSubmit } = useForm();
  
  return (
    <form onSubmit={handleSubmit(data => console.log("data", data))}>
      <input {...register("firstName")} />
      <input {...register("last")} />
      <IsolateReRender control={control} />
      
      <input type="submit" />
    </form>
  );
}
import React from "react";
import { useForm, useWatch } from "react-hook-form";

interface FormInputs {
  firstName: string
}

function FirstNameWatched({ control }: { control: Control<FormInputs> }) {
  const firstName = useWatch({
    control,
    name: "firstName", // without supply name will watch the entire form, or ['firstName', 'lastName'] to watch both
    defaultValue: "default" // default value before the render
  });

  return <p>Watch: {firstName}</p>; // only re-render at the component level, when firstName changes
}

function App() {
  const { register, control, handleSubmit } = useForm<FormInputs>();

  const onSubmit = (data: FormInputs) => {
    console.log(data)
  };

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <label>First Name:</label>
      <input {...register("firstName")} />
      <input type="submit" />
      <FirstNameWatched control={control} />
    </form>
  );
}