Link Two Form Fields Together | TanStack Form React Docs

You may find yourself needing to link two fields together, such that one is validated when another's value has changed. One such use case is when you have both a password and a confirm_password field. Here, you want the confirm_password field to error out if its value doesn't match that of the password field, regardless of which field triggered the value change.

Imagine the following user flow:

User updates the confirm_password field.
User updates the password field.

In this example, the form will still have errors present because the confirm_password field's validation has not been re-run to mark the field as valid.

To solve this, you need to make sure that the confirm_password field's validation is re-run when the password field is updated. To do this, you can add an onChangeListenTo prop to the confirm_password field.

tsx

function App() {
  const form = useForm({
    defaultValues: {
      password: '',
      confirm_password: '',
    },
    // ...
  })

  return (
    <div>
      <form.Field name="password">
        {(field) => (
          <label>
            <div>Password</div>
            <input
              value={field.state.value}
              onChange={(e) => field.handleChange(e.target.value)}
            />
          </label>
        )}
      </form.Field>
      <form.Field
        name="confirm_password"
        validators={{
          onChangeListenTo: ['password'],
          onChange: ({ value, fieldApi }) => {
            if (value !== fieldApi.form.getFieldValue('password')) {
              return 'Passwords do not match'
            }
            return undefined
          },
        }}
      >
        {(field) => (
          <div>
            <label>
              <div>Confirm Password</div>
              <input
                value={field.state.value}
                onChange={(e) => field.handleChange(e.target.value)}
              />
            </label>
            {field.state.meta.errors.map((err) => (
              <div key={err}>{err}</div>
            ))}
          </div>
        )}
      </form.Field>
    </div>
  )
}

function App() {
  const form = useForm({
    defaultValues: {
      password: '',
      confirm_password: '',
    },
    // ...
  })

  return (
    <div>
      <form.Field name="password">
        {(field) => (
          <label>
            <div>Password</div>
            <input
              value={field.state.value}
              onChange={(e) => field.handleChange(e.target.value)}
            />
          </label>
        )}
      </form.Field>
      <form.Field
        name="confirm_password"
        validators={{
          onChangeListenTo: ['password'],
          onChange: ({ value, fieldApi }) => {
            if (value !== fieldApi.form.getFieldValue('password')) {
              return 'Passwords do not match'
            }
            return undefined
          },
        }}
      >
        {(field) => (
          <div>
            <label>
              <div>Confirm Password</div>
              <input
                value={field.state.value}
                onChange={(e) => field.handleChange(e.target.value)}
              />
            </label>
            {field.state.meta.errors.map((err) => (
              <div key={err}>{err}</div>
            ))}
          </div>
        )}
      </form.Field>
    </div>
  )
}

This is similar to the onBlurListenTo prop, which re-runs the validation when the linked field is blurred.