When NOT to use Formik's FastField?

Question

In the docs of Formik's FastField it have alot of when you should use it and how it works, but is there a case where you shouldn't use?

Because if in somecases FastField is faster and other cases it doesn't make any difference, why not always use FastField? Is there a case where using Field is better than FastField?

Answer 1

FastField has a shouldComponentUpdate which is 'concerned' about changes to only its own props.

If, your use-case, requires field to re-render because of any other change then don't go for FastField. Even if some other prop change, your component won't be updated.

Also, as per the docs, performance issues due to Formik Field's re-render might surface only on cases where the form is huge (>30 Fields). They recommend using FastFields for >30 Fields only.

shouldComponentUpdate(props: FastFieldInnerProps<Values, Props>) {
    if (this.props.shouldUpdate) {
      return this.props.shouldUpdate(props, this.props);
    } else if (
      props.name !== this.props.name ||
      getIn(props.formik.values, this.props.name) !==
        getIn(this.props.formik.values, this.props.name) ||
      getIn(props.formik.errors, this.props.name) !==
        getIn(this.props.formik.errors, this.props.name) ||
      getIn(props.formik.touched, this.props.name) !==
        getIn(this.props.formik.touched, this.props.name) ||
      Object.keys(this.props).length !== Object.keys(props).length ||
      props.formik.isSubmitting !== this.props.formik.isSubmitting
    ) {
      return true;
    } else {
      return false;
    }
  }