Project init

src/env/client.mjs

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+// @ts-check
+import { clientEnv, clientSchema } from "./schema.mjs";
+
+const _clientEnv = clientSchema.safeParse(clientEnv);
+
+export const formatErrors = (
+  /** @type {import('zod').ZodFormattedError<Map<string,string>,string>} */
+  errors,
+) =>
+  Object.entries(errors)
+    .map(([name, value]) => {
+      if (value && "_errors" in value)
+        return `${name}: ${value._errors.join(", ")}\n`;
+    })
+    .filter(Boolean);
+
+if (!_clientEnv.success) {
+  console.error(
+    "❌ Invalid environment variables:\n",
+    ...formatErrors(_clientEnv.error.format()),
+  );
+  throw new Error("Invalid environment variables");
+}
+
+for (let key of Object.keys(_clientEnv.data)) {
+  if (!key.startsWith("NEXT_PUBLIC_")) {
+    console.warn(
+      `❌ Invalid public environment variable name: ${key}. It must begin with 'NEXT_PUBLIC_'`,
+    );
+
+    throw new Error("Invalid public environment variable name");
+  }
+}
+
+export const env = _clientEnv.data;

src/env/schema.mjs

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+// @ts-check
+import { z } from "zod";
+
+/**
+ * Specify your server-side environment variables schema here.
+ * This way you can ensure the app isn't built with invalid env vars.
+ */
+export const serverSchema = z.object({
+  NODE_ENV: z.enum(["development", "test", "production"]),
+});
+
+/**
+ * You can't destruct `process.env` as a regular object in the Next.js
+ * middleware, so you have to do it manually here.
+ * @type {{ [k in keyof z.infer<typeof serverSchema>]: z.infer<typeof serverSchema>[k] | undefined }}
+ */
+export const serverEnv = {
+  NODE_ENV: process.env.NODE_ENV,
+};
+
+/**
+ * Specify your client-side environment variables schema here.
+ * This way you can ensure the app isn't built with invalid env vars.
+ * To expose them to the client, prefix them with `NEXT_PUBLIC_`.
+ */
+export const clientSchema = z.object({
+  // NEXT_PUBLIC_CLIENTVAR: z.string(),
+});
+
+/**
+ * You can't destruct `process.env` as a regular object, so you have to do
+ * it manually here. This is because Next.js evaluates this at build time,
+ * and only used environment variables are included in the build.
+ * @type {{ [k in keyof z.infer<typeof clientSchema>]: z.infer<typeof clientSchema>[k] | undefined }}
+ */
+export const clientEnv = {
+  // NEXT_PUBLIC_CLIENTVAR: process.env.NEXT_PUBLIC_CLIENTVAR,
+};

src/env/server.mjs

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+// @ts-check
+/**
+ * This file is included in `/next.config.mjs` which ensures the app isn't built with invalid env vars.
+ * It has to be a `.mjs`-file to be imported there.
+ */
+import { serverSchema, serverEnv } from "./schema.mjs";
+import { env as clientEnv, formatErrors } from "./client.mjs";
+
+const _serverEnv = serverSchema.safeParse(serverEnv);
+
+if (!_serverEnv.success) {
+  console.error(
+    "❌ Invalid environment variables:\n",
+    ...formatErrors(_serverEnv.error.format()),
+  );
+  throw new Error("Invalid environment variables");
+}
+
+for (let key of Object.keys(_serverEnv.data)) {
+  if (key.startsWith("NEXT_PUBLIC_")) {
+    console.warn("❌ You are exposing a server-side env-variable:", key);
+
+    throw new Error("You are exposing a server-side env-variable");
+  }
+}
+
+export const env = { ..._serverEnv.data, ...clientEnv };