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Understanding Environment Variable Pitfalls in Next.js

Updated: at 02:12 PM

Understanding Environment Variable Pitfalls in Next.js

Next.js, a popular framework for building React applications, offers robust features that streamline the development process, such as automatic static optimization and server-side rendering. However, one area where developers—regardless of their experience level—often encounter issues is in the handling of environment variables, particularly when these are used in client-side logic. In this article, we will explore why certain methods of using environment variables in Next.js can lead to unexpected behaviors, backed by clear examples and a detailed analysis.

The Nature of Environment Variables in Next.js

In Next.js, environment variables can be exposed to the browser by prefixing them with NEXT_PUBLIC_. This signals to Next.js that these variables should be included in the JavaScript bundle sent to the client. The replacement of these variables is done at build time, which is a crucial detail that affects their behavior in the application.

Example 1: Simple Arithmetic Operation

Let’s start with a simple example to illustrate the problem:

// This will be set at build time
const publicVar = process.env.NEXT_PUBLIC_NUMBER || "5";

// Attempt to perform addition at runtime
const result = publicVar + 1;

In this scenario, you might expect result to be 6 if NEXT_PUBLIC_NUMBER is not set. However, because environment variables are treated as strings, and the replacement happens at build time, publicVar becomes the string "5". Therefore, publicVar + 1 results in the string "51", not the number 6. This example underscores the need for careful type handling and awareness of when the variable values are set.

Deep Dive into the Given Example

Now, let’s analyze the provided code snippet in more detail:

export const debugFlagTrickAndFailure = envVarToBoolean(

export const debugFlag = process.env.NEXT_PUBLIC_DEBUG_FLAG === "true";

function envVarToBoolean(envVarName: string, defaultValue: boolean): boolean {
  const value = process?.env[envVarName];
  return value !== undefined ? value === "true" : defaultValue;

What Goes Wrong with envVarToBoolean:

  1. Build-Time vs. Runtime: envVarToBoolean attempts to dynamically access an environment variable. However, because NEXT_PUBLIC_ prefixed variables are replaced at build time with their literal values, any subsequent changes to these variables after the build will not be reflected. The function is essentially operating on a “static snapshot” of the environment variable taken during the build.

  2. Dynamic Evaluation Misconception: Developers might expect that envVarToBoolean evaluates the current state of an environment variable at runtime, allowing for dynamic control based on the environment where the application is running. This misunderstanding can lead to bugs when the application behaves differently than expected because the environment variable value is fixed at build time and does not change post-deployment.

Why debugFlag Works as Expected:

Best Practices and Recommendations

  1. Awareness of Build-Time Evaluation: Always remember that NEXT_PUBLIC_ prefixed variables are evaluated at build time. Design your code with this in mind to avoid unexpected behaviors.

  2. Use Type Coercion When Necessary: Explicitly convert environment variables to the required type at the point of use to avoid issues like the arithmetic example shown earlier.

  3. Centralize Environment Variable Access: Implement a utility function or custom hook for accessing environment variables, which includes type validation and conversion. This centralizes the logic and reduces the chances of repeated mistakes.

  4. Prefer Runtime Configurations for Dynamic Needs: For settings that need to change dynamically without a rebuild, consider using other strategies such as fetching configuration from an API or using feature flags managed by a third-party service.


While Next.js facilitates many complex aspects of modern web development, it also introduces specific challenges, such as those associated with the use of environment variables. By understanding these intricacies and adopting best practices, developers can leverage Next.js effectively without falling into common traps, thereby creating more robust and maintainable applications.

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