How to get environment variables on client side with Next.js?

Question

I want to read environment variable into Next.js, how to pass data into datadogRum.init?

// _app.tsx

import React from "react";
import { useEffect } from "react";
import type { AppProps } from "next/app";
import { datadogRum } from "@datadog/browser-rum";

export default function App({ Component, pageProps }: AppProps) {
  useEffect(() => {
    datadogRum.init({
      applicationId: applicationID, // can't use process.env.NEXT_PUBLIC_DATADOG_APPLICATION_ID directly here
      ...
    });

    datadogRum.startSessionReplayRecording();
  });

  return (
    ...
  );
}

App.getInitialProps = async () => {
  return {
    applicationID: process.env.NEXT_PUBLIC_DATADOG_APPLICATION_ID,
  };
};

This way can't get the applicationID value.

Answer 1

You don't need to pass the environment variable from getInitialProps. You can access it directly inside the useEffect, you just need to make sure process.env.NEXT_PUBLIC_DATADOG_APPLICATION_ID is defined before initialising datadogRum.init to avoid typings conflicts.

export default function App({ Component, pageProps }: AppProps) {
    useEffect(() => {
        if (typeof process.env.NEXT_PUBLIC_DATADOG_APPLICATION_ID !== "undefined")
            datadogRum.init({
                applicationId: process.env.NEXT_PUBLIC_DATADOG_APPLICATION_ID
                ...
            });

            datadogRum.startSessionReplayRecording();
        }
    });

    return (
        ...
    );
}