ConvertKitForm.js

import React, { PureComponent } from 'react';

const IS_BROWSER = typeof window !== "undefined";

export default class Form extends PureComponent {
  componentWillMount() {
    const { formId, version = 6 } = this.props;
    if (IS_BROWSER) {
      this.script = document.createElement("script");
      this.script.id = `_ck_${formId}`;
      this.script.async = true;
      this.script.src = `https://forms.convertkit.com/${formId}?v=${version}`;
    }
  }
  componentDidMount() {
    if (IS_BROWSER) {
      // Make sure the convertKit inserts form elements again when the component is unmounted & re-mounted. 
      // This is useful to embed the form in for example a modal. Without this line, the sign up form will
      // only appear the first time when the modal is open.
      window._CK_processed = undefined;
      this.div.appendChild(this.script);
    }
  }
  saveDiv = div => (this.div = div);
  render() {
    return <div ref={this.saveDiv} />;
  }
}