objectenvy / WithPrefix

Type Alias: WithPrefix<T, Prefix>

type WithPrefix<T, Prefix> = { [K in keyof T as K extends string ? `${Prefix}_${K}` : never]: T[K] };

Defined in: typeUtils.ts:237

Add a Prefix_ to all keys in an env record type.

Type Parameters

Type Parameter	Description
T	The source env record type.
Prefix extends string	The string prefix to prepend (without trailing underscore).

Remarks

Maps every string key K in T to `${Prefix}_${K}`, preserving value types. Non-string keys are dropped (never). Useful for namespacing env types when combining multiple services or packages that each export their own env shapes.

Use When

• You're composing env types from multiple packages and need to namespace them with a prefix.
• You want to generate the prefixed env shape for documentation or .env.example scaffolding.

Avoid When

• You need to add the prefix at runtime — this is a type-level only utility; use string interpolation in your env object builder.

Pitfalls

• NEVER use a Prefix that already ends with _ — BECAUSE the type concatenates ${Prefix}_${K}, resulting in double underscores (APP__PORT).

Example

import type { WithPrefix } from 'objectenvy';
type Env = { PORT: string; DEBUG: string };
type PrefixedEnv = WithPrefix<Env, 'APP'>;
// { APP_PORT: string; APP_DEBUG: string }

See

WithoutPrefix for the inverse operation