fix: small tweaks to types and formatting

src/deepmerge.ts

@@ -25,7 +25,7 @@ export default function deepmerge<T1 extends object, T2 extends object, O extend
  * @param options Deep merge options.
  */
 export function deepmergeAll<
-	Ts extends readonly [object, ...Array<object>],
+	Ts extends readonly [object, ...ReadonlyArray<object>],
 	O extends Options = object
 >(objects: [...Ts], options?: O): DeepMergeAll<Ts, ExplicitOptions<O>>
 
@@ -38,7 +38,7 @@ export function deepmergeAll<
 export function deepmergeAll(objects: ReadonlyArray<object>, options?: Options): object
 export function deepmergeAll(objects: ReadonlyArray<object>, options?: Options): object {
 	if (!Array.isArray(objects)) {
-		throw new Error(`first argument should be an array`)
+		throw new TypeError(`first argument should be an array`)
 	}
 
 	return objects.reduce((prev, next) => deepmergeImpl(prev, next, getFullOptions(options)), {})

src/impl.ts

@@ -7,7 +7,7 @@ function emptyTarget(value: unknown) {
 
 export function cloneUnlessOtherwiseSpecified<T>(value: T, options: FullOptions): T {
 	return options.clone !== false && options.isMergeable(value)
-		? (deepmergeImpl(emptyTarget(value), value, options) as T)
+		? deepmergeImpl(emptyTarget(value), value, options) as T
 		: value
 }
 
@@ -30,7 +30,7 @@ function getKeys(target: object): Array<string> {
 function propertyIsOnObject(object: object, property: Property): boolean {
 	try {
 		return property in object
-	} catch (_) {
+	} catch {
 		return false
 	}
 }
@@ -101,17 +101,17 @@ export function deepmergeImpl<T1 extends any, T2 extends any, O extends Options>
 
 	if (!sourceAndTargetTypesMatch) {
 		return cloneUnlessOtherwiseSpecified(source, options) as DeepMerge<T1, T2, ExplicitOptions<O>>
-	} else if (sourceIsArray) {
+	}
+	if (sourceIsArray) {
 		return options.arrayMerge(
 			target as Array<unknown>,
 			source as Array<unknown>,
 			options,
 		) as DeepMerge<T1, T2, ExplicitOptions<O>>
-	} else {
-		return mergeObject(
+	}
+	return mergeObject(
 			target as Record<Property, unknown>,
 			source as Record<Property, unknown>,
 			options,
-		) as DeepMerge<T1, T2, ExplicitOptions<O>>
-	}
+	) as DeepMerge<T1, T2, ExplicitOptions<O>>
 }

src/options.ts

@@ -7,32 +7,32 @@ import type { FlattenAlias, Property } from "./types"
  * Deep merge options.
  */
 export type Options = Partial<{
-	arrayMerge?: ArrayMerge
-	clone?: boolean
-	customMerge?: ObjectMerge
-	isMergeable?: IsMergeable
+	readonly arrayMerge?: ArrayMerge
+	readonly clone?: boolean
+	readonly customMerge?: ObjectMerge
+	readonly isMergeable?: IsMergeable
 }>
 
 /**
  * Deep merge options with explicit keys.
  */
 export type ExplicitOptions<O extends Options = Options> = {
-	[K in keyof Options]-?: undefined extends O[K] ? never : O[K]
+	readonly [K in keyof Options]-?: undefined extends O[K] ? never : O[K]
 }
 
 /**
  * Deep merge options with defaults applied.
  */
 export type FullOptions<O extends Options = Options> = FlattenAlias<{
-	arrayMerge: O[`arrayMerge`] extends undefined
+	readonly arrayMerge: O[`arrayMerge`] extends undefined
 		? typeof defaultArrayMerge
 		: NonNullable<O[`arrayMerge`]>
-	clone: O[`clone`] extends undefined ? true : NonNullable<O[`clone`]>
-	customMerge?: O[`customMerge`]
-	isMergeable: O[`isMergeable`] extends undefined
+	readonly clone: O[`clone`] extends undefined ? true : NonNullable<O[`clone`]>
+	readonly customMerge?: O[`customMerge`]
+	readonly isMergeable: O[`isMergeable`] extends undefined
 		? typeof defaultIsMergeable
 		: NonNullable<O[`isMergeable`]>
-	cloneUnlessOtherwiseSpecified: <T>(value: T, options: FullOptions) => T
+	readonly cloneUnlessOtherwiseSpecified: <T>(value: T, options: FullOptions) => T
 }>
 
 /**

src/types.ts

@@ -4,9 +4,9 @@ import type { Options } from "./options"
  * Deep merge 1 or more types given in an array.
  */
 export type DeepMergeAll<
-	Ts extends readonly [any, ...Array<any>],
+	Ts extends readonly [any, ...ReadonlyArray<any>],
 	O extends Options
-> = Ts extends readonly [infer T1, ...Array<any>]
+> = Ts extends readonly [infer T1, ...ReadonlyArray<any>]
 	? Ts extends readonly [T1, infer T2, ...infer TRest]
 		? TRest extends ReadonlyArray<never>
 			? DeepMerge<T1, T2, O>
@@ -81,6 +81,7 @@ type DeepMergeObjectPropsCustom<T1, T2, O extends Options> = ReturnType<
  *
  * Cannot get return type from arrayMerge passing generics.
  * TypeScript does not yet support higher order types.
+ *
  * @see https://github.com/Microsoft/TypeScript/issues/1213
  */
 type DeepMergeArrays<T1, T2, O extends Options> = IsUndefinedOrNever<O[`arrayMerge`]> extends true

test/merge-all.ts

@@ -4,7 +4,7 @@ import test from "tape"
 test(`throw error if first argument is not an array`, (t) => {
 	// eslint-disable-next-line @typescript-eslint/ban-ts-comment, @typescript-eslint/prefer-ts-expect-error -- FIXME: Can't use @ts-expect-error due to it failing in tests.
 	// @ts-ignore Expect a type error when calling the function incorrectly.
-	t.throws(deepmergeAll.bind(null, { example: true }, { another: `2` }), Error)
+	t.throws(deepmergeAll.bind(null, { example: true }, { another: `2` }), TypeError)
 	t.end()
 })