Added strongly typed options interfaces so that your return value and default parameters match the type specified in the options.

src/index.ts

@@ -1,7 +1,7 @@
 import dotenv from 'dotenv'
-dotenv.config()
+import { ArrayOpts, BaseInputValue, BooleanOpts, InputValue, IOpts, IParsedOpts, NumberOpts, StringOpts } from './types'
 
-import { IOpts, IParsedOpts, InputValue } from './types'
+dotenv.config()
 
 const VALID_TYPES = [ 'string', 'array', 'boolean', 'number' ]
 
@@ -59,7 +59,16 @@ const parseValue = (val: string, type: string): InputValue => {
 	return val.trim()
 }
 
-export const getInput = (key: string | Array<string> | IOpts, opts: IOpts): InputValue => {
+// eslint-disable-next-line no-unused-vars
+export function getInput(key: string | Array<string> | IOpts, opts: BooleanOpts): boolean;
+// eslint-disable-next-line no-unused-vars,no-redeclare
+export function getInput(key: string | Array<string> | IOpts, opts: StringOpts): string;
+// eslint-disable-next-line no-unused-vars,no-redeclare
+export function getInput(key: string | Array<string> | IOpts, opts: NumberOpts): number;
+// eslint-disable-next-line no-unused-vars,no-redeclare
+export function getInput(key: string | Array<string> | IOpts, opts: ArrayOpts): BaseInputValue[];
+// eslint-disable-next-line no-redeclare
+export function getInput(key: string | Array<string> | IOpts, opts: IOpts): InputValue {
 	let parsedOptions: IOpts
 	if (typeof key === 'string' || Array.isArray(key)) {
 		parsedOptions = {

src/types.ts

@@ -1,22 +1,42 @@
 /* eslint-disable no-unused-vars */
 type ModifierFunction = (val: InputValue) => InputValue
 
-export type InputValue = undefined | string | boolean | number | Array<string | boolean | number>
-
-export interface IOpts {
-    key?: string | Array<string>
-    type?: string
-    required?: boolean,
-    disableable?: boolean
-    default?: InputValue
-    modifier?: ModifierFunction
+export type BaseInputValue = string | boolean | number;
+
+export type InputValue = undefined | BaseInputValue | Array<BaseInputValue>
+
+export type IOpts = BooleanOpts | StringOpts | NumberOpts | ArrayOpts | BaseOpts<InputValue>;
+
+export interface BooleanOpts extends BaseOpts<boolean> {
+	type: 'boolean';
+}
+
+export interface StringOpts extends BaseOpts<string> {
+	type: 'string';
+}
+
+export interface NumberOpts extends BaseOpts<number> {
+	type: 'number';
+}
+
+export interface ArrayOpts extends BaseOpts<BaseInputValue[]> {
+	type: 'array';
+}
+
+export interface BaseOpts<T extends InputValue> {
+	key?: string | Array<string>
+	type?: string
+	required?: boolean,
+	disableable?: boolean
+	default?: T
+	modifier?: ModifierFunction
 }
 
 export interface IParsedOpts {
-    key: string | Array<string>
-    type: string
-    required: boolean,
-    disableable: boolean
-    default?: InputValue
-    modifier?: ModifierFunction
+	key: string | Array<string>
+	type: string
+	required: boolean,
+	disableable: boolean
+	default?: InputValue
+	modifier?: ModifierFunction
 }