In this article, we analyse the ClientOptions interface provided for the client object in createEnv parameter, a function in T3 Env. A simple usage of t3-env is provided below:

export const env = createEnv({
 /*
 * Serverside Environment variables, not available on the client.
 * Will throw if you access these variables on the client.
 */
 server: {
 DATABASE_URL: z.string().url(),
 OPEN_AI_API_KEY: z.string().min(1),
 },
 /*
 * Environment variables available on the client (and server).
 *
 * 💡 You'll get type errors if these are not prefixed with NEXT_PUBLIC_.
 */
 client: {
 NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY: z.string().min(1),
 },
 /*
 * Due to how Next.js bundles environment variables on Edge and Client,
 * we need to manually destructure them to make sure all are included in bundle.
 *
 * 💡 You'll get type errors if not all variables from `server` & `client` are included here.
 */
 runtimeEnv: {
 DATABASE_URL: process.env.DATABASE_URL,
 OPEN_AI_API_KEY: process.env.OPEN_AI_API_KEY,
 NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY:
 process.env.NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY,
 },
});

We are interested in finding out the types/interfaces of client object.

client: {
 NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY: z.string().min(1),
 },

Straight away, I could tell we could expect this type along the lines of Record<String, ZodType>, but the way t3-env defines is different. Look at the below type picked from T3 Env source code.

export interface ClientOptions<
 TPrefix extends string | undefined,
 TClient extends Record<string, ZodType>,
> {
 /**
 * The prefix that client-side variables must have. This is enforced both at
 * a type-level and at runtime.
 */
 clientPrefix: TPrefix;
/**
 * Specify your client-side environment variables schema here. This way you can ensure the app isn't
 * built with invalid env vars.
 */
 client: Partial<{
 [TKey in keyof TClient]: TKey extends `${TPrefix}${string}`
 ? TClient[TKey]
 : ErrorMessage<`${TKey extends string
 ? TKey
 : never} is not prefixed with ${TPrefix}.`>;
 }>;
}

You will find that this uses generic type and TClient is of type Record<string, ZodType>, but client does not have this type, instead it has a check in place to ensure your key defined in client is prefixed with whatever you define in ClientPrefix.

Say, for example, you have defined your prefix as “NEXT_PUBLIC_” and you try to define some variable with a key that is not prefixed with “NEXT_PBULIC_”, you will see an error along the lines “{variable} is not prefixed with “NEXT_PBULIC_”

This is powerful in frameworks like Next.js where you don’t want to accidentally expose server side varaibles to the client side.

Check this docs — https://env.t3.gg/docs/core#create-your-schema, this talks about prefix error.

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References:

1. https://github.com/t3-oss/t3-env/blob/main/packages/core/src/index.ts#L130

2. https://env.t3.gg/docs/core#create-your-schema