use Envalid to validate and expose Next.js client-side and server-side variables

env.ts

import { cleanEnv, Spec, str, ValidatorSpec } from 'envalid'

// https://github.com/af/envalid

/**
 * This module is the single source of truth for application environment configuration.
 * When imported as a TypeScript module, it provides runtime access to validated environment variables.
 * When executed as a script, it may cover the following use cases:
 *     - validate current environment
 *     - load current environment beforehand (when CLI flag --force-load-env is used)
 *     - print empty environment configuration as .env-compatible string (when CLI flag --print-env is used)
 *     - print valid environment configuration as .env-compatible string (when CLI flag --print-env is used)
 * This module detects if it is executed in a client-side environment and excludes server-only variables, see:
 * https://nextjs.org/docs/basic-features/environment-variables#exposing-environment-variables-to-the-browser
 */

const FORCE_LOAD_CLI_FLAG = '--force-load-env'
const PRINT_ENV_CLI_FLAG = '--print-env'

const printEnvCliFlagProvided = process.argv?.includes(PRINT_ENV_CLI_FLAG)
const forceLoadCliFlagProvided = process.argv?.includes(FORCE_LOAD_CLI_FLAG)
if (forceLoadCliFlagProvided) {
    console.info('Force-loading environment ...')
    const { loadEnvConfig } = require('@next/env')
    loadEnvConfig(process.cwd())
}

// ==========  environment definition and validation ==========

const isServer = typeof window === 'undefined'

const envVars = {
    NEXT_PUBLIC_CLIENT_VAR: process.env.NEXT_PUBLIC_CLIENT_VAR,
    SERVER_VAR: process.env.SERVERSIDE_VAR,
}

type EnvKey = keyof typeof envVars

const optionalVarOpts: Spec<string> = { default: undefined }
const serverVarOpts = isServer ? undefined : optionalVarOpts

const envConfig: Record<EnvKey, ValidatorSpec<string>> = {
    NEXT_PUBLIC_CLIENT_VAR: str(),
    SERVER_VAR: str(serverVarOpts),
}

if (printEnvCliFlagProvided) {
    console.info('Printing empty .env configuration ...')
    const emptyEnvConfigString = Object.keys(envConfig)
        .reduce((acc, curr) => [...acc, `${curr}=`], [])
        .join('\n')
    console.info(`${emptyEnvConfigString}\n`)
}

const env = cleanEnv(envVars, envConfig)

if (printEnvCliFlagProvided && forceLoadCliFlagProvided) {
    console.info('Printing current valid .env configuration ...')
    const validEnvConfigString = Object.entries(env)
        .reduce((acc, [key, value]) => [...acc, `${key}=${value}`], [])
        .join('\n')
    console.info(`${validEnvConfigString}\n`)
}

export default env

This is solid. Thanks.