declare type LogFn = (msg: string) => void;
declare type Logger = {
    debug?: LogFn;
    info: LogFn;
    warn: LogFn;
    error: LogFn;
};
/**
 * Creates a new config object, described by the {@code ConfigSpec} instance,
 * after populating/validating it against the {@code env} environment.
 *
 * The {@code ConfigSpec} should be an object literal that looks like:
 *
 * ```typescript
 * const AppEnv = {
 *   prop_a: string(),
 * }
 * ```
 *
 * And calling `newConfig(AppEnv, env)` will return a new object literal
 * that has the same type as `MyEnv`, but with `prop_a` resolved to the
 * `PROP_A` (or as appropriate) env value.
 */
export interface ConfigOptions {
    ignoreErrors?: boolean;
    doNotLogErrors?: boolean;
    /** The NODE_ENV e.g. production or development; used for properties that use `notNeededIn`. */
    nodeEnv?: string;
    /**
     * Pass a custom logger (e.g., a Pino instance).
     *
     * @default - console
     */
    logger?: Logger;
}
export declare function newConfig<S>(spec: S, env: Environment, options?: ConfigOptions): S;
/**
 * The environment to pull setting names out of.
 *
 * This doesn't have to be Node's `process.env`, but that is the general
 * assumption and primary use case.
 */
export interface Environment {
    [name: string]: string | undefined;
}
export declare class ConfigError extends Error {
}
/** The settings that can be defined for each configuration option. */
export interface ConfigOptionSettings<V> {
    /** The environment variable name. */
    env?: string;
    /** The default value to use if the environment variable is not set. */
    default?: V;
    /** If this is optional, in which case the type should be {@code V | undefined}. */
    optional?: boolean;
    /** A list of NODE_ENVs where this setting is not needed in. */
    notNeededIn?: string | string[];
}
/** Construct a config option that is a number. */
export declare function number(options: ConfigOptionSettings<number> & {
    optional: true;
}): number | undefined;
export declare function number(options?: ConfigOptionSettings<number>): number;
/** Construct a config option that is a string. */
export declare function string(options: ConfigOptionSettings<string> & {
    optional: true;
}): string | undefined;
export declare function string(options?: ConfigOptionSettings<string>): string;
/** Construct a config option that is a boolean, only the exact string value 'true' is treated as true, everything else is false. */
export declare function boolean(options: ConfigOptionSettings<boolean> & {
    optional: true;
}): boolean | undefined;
export declare function boolean(options?: ConfigOptionSettings<boolean>): boolean;
/** Construct a generic config option. */
export declare function option<V>(options: ConfigOptionSettings<V>, parser: (s: string) => V): V;
export {};