Enable ESLint explicit-function-return-type rule (#206)

package.json

@@ -86,7 +86,7 @@
 		"rules": {
 			"no-useless-return": "off",
 			"ava/no-ignored-test-files": "off",
-			"@typescript-eslint/explicit-function-return-type": "off",
+			"@typescript-eslint/explicit-function-return-type": "error",
 			"@typescript-eslint/ban-types": "off",
 			"@typescript-eslint/no-explicit-any": "off",
 			"@typescript-eslint/no-empty-function": "off",

source/argument-error.ts

@@ -1,6 +1,6 @@
 import {generateStackTrace} from './utils/generate-stack';
 
-const wrapStackTrace = (error: ArgumentError, stack: string) => `${error.name}: ${error.message}\n${stack}`;
+const wrapStackTrace = (error: ArgumentError, stack: string): string => `${error.name}: ${error.message}\n${stack}`;
 
 /**
 @hidden

source/index.ts

@@ -60,7 +60,7 @@ export interface ReusableValidator<T> {
 	(value: T, label?: string): void;
 }
 
-const ow = <T>(value: T, labelOrPredicate: unknown, predicate?: BasePredicate<T>) => {
+const ow = <T>(value: T, labelOrPredicate: unknown, predicate?: BasePredicate<T>): void => {
 	if (!isPredicate(labelOrPredicate) && typeof labelOrPredicate !== 'string') {
 		throw new TypeError(`Expected second argument to be a predicate or a string, got \`${typeof labelOrPredicate}\``);
 	}
@@ -79,7 +79,7 @@ const ow = <T>(value: T, labelOrPredicate: unknown, predicate?: BasePredicate<T>
 
 Object.defineProperties(ow, {
 	isValid: {
-		value: <T>(value: T, predicate: BasePredicate<T>) => {
+		value: <T>(value: T, predicate: BasePredicate<T>): boolean => {
 			try {
 				ow(value, predicate);
 				return true;
@@ -89,11 +89,11 @@ Object.defineProperties(ow, {
 		}
 	},
 	create: {
-		value: <T>(labelOrPredicate: BasePredicate<T> | string | undefined, predicate?: BasePredicate<T>) => (value: T, label?: string) => {
+		value: <T>(labelOrPredicate: BasePredicate<T> | string | undefined, predicate?: BasePredicate<T>) => (value: T, label?: string): void => {
 			if (isPredicate(labelOrPredicate)) {
 				const stackFrames = callsites();
 
-				test(value, label ?? (() => inferLabel(stackFrames)), labelOrPredicate);
+				test(value, label ?? ((): string | void => inferLabel(stackFrames)), labelOrPredicate);
 
 				return;
 			}

source/modifiers.ts

@@ -17,7 +17,7 @@ export interface Modifiers {
 export default <T>(object: T): T & Modifiers => {
 	Object.defineProperties(object, {
 		optional: {
-			get: () => predicates({}, {optional: true})
+			get: (): Predicates => predicates({}, {optional: true})
 		}
 	});
 

source/predicates.ts

@@ -207,109 +207,109 @@ export interface Predicates {
 export default <T>(object: T, options?: PredicateOptions): T & Predicates => {
 	Object.defineProperties(object, {
 		string: {
-			get: () => new StringPredicate(options)
+			get: (): StringPredicate => new StringPredicate(options)
 		},
 		number: {
-			get: () => new NumberPredicate(options)
+			get: (): NumberPredicate => new NumberPredicate(options)
 		},
 		boolean: {
-			get: () => new BooleanPredicate(options)
+			get: (): BooleanPredicate => new BooleanPredicate(options)
 		},
 		undefined: {
-			get: () => new Predicate('undefined', options)
+			get: (): Predicate => new Predicate('undefined', options)
 		},
 		null: {
-			get: () => new Predicate('null', options)
+			get: (): Predicate => new Predicate('null', options)
 		},
 		nullOrUndefined: {
-			get: () => new Predicate('nullOrUndefined', options)
+			get: (): Predicate => new Predicate('nullOrUndefined', options)
 		},
 		nan: {
-			get: () => new Predicate('nan', options)
+			get: (): Predicate => new Predicate('nan', options)
 		},
 		symbol: {
-			get: () => new Predicate('symbol', options)
+			get: (): Predicate => new Predicate('symbol', options)
 		},
 		array: {
-			get: () => new ArrayPredicate(options)
+			get: (): ArrayPredicate => new ArrayPredicate(options)
 		},
 		object: {
-			get: () => new ObjectPredicate(options)
+			get: (): ObjectPredicate => new ObjectPredicate(options)
 		},
 		date: {
-			get: () => new DatePredicate(options)
+			get: (): DatePredicate => new DatePredicate(options)
 		},
 		error: {
-			get: () => new ErrorPredicate(options)
+			get: (): ErrorPredicate => new ErrorPredicate(options)
 		},
 		map: {
-			get: () => new MapPredicate(options)
+			get: (): MapPredicate => new MapPredicate(options)
 		},
 		weakMap: {
-			get: () => new WeakMapPredicate(options)
+			get: (): WeakMapPredicate => new WeakMapPredicate(options)
 		},
 		set: {
-			get: () => new SetPredicate(options)
+			get: (): SetPredicate => new SetPredicate(options)
 		},
 		weakSet: {
-			get: () => new WeakSetPredicate(options)
+			get: (): WeakSetPredicate => new WeakSetPredicate(options)
 		},
 		function: {
-			get: () => new Predicate('Function', options)
+			get: (): Predicate => new Predicate('Function', options)
 		},
 		buffer: {
-			get: () => new Predicate('Buffer', options)
+			get: (): Predicate => new Predicate('Buffer', options)
 		},
 		regExp: {
-			get: () => new Predicate('RegExp', options)
+			get: (): Predicate<RegExp> => new Predicate('RegExp', options)
 		},
 		promise: {
-			get: () => new Predicate('Promise', options)
+			get: (): Predicate => new Predicate('Promise', options)
 		},
 		typedArray: {
-			get: () => new TypedArrayPredicate('TypedArray', options)
+			get: (): TypedArrayPredicate<TypedArray> => new TypedArrayPredicate('TypedArray', options)
 		},
 		int8Array: {
-			get: () => new TypedArrayPredicate('Int8Array', options)
+			get: (): TypedArrayPredicate<Int8Array> => new TypedArrayPredicate('Int8Array', options)
 		},
 		uint8Array: {
-			get: () => new TypedArrayPredicate('Uint8Array', options)
+			get: (): TypedArrayPredicate<Uint8Array> => new TypedArrayPredicate('Uint8Array', options)
 		},
 		uint8ClampedArray: {
-			get: () => new TypedArrayPredicate('Uint8ClampedArray', options)
+			get: (): TypedArrayPredicate<Uint8Array> => new TypedArrayPredicate('Uint8ClampedArray', options)
 		},
 		int16Array: {
-			get: () => new TypedArrayPredicate('Int16Array', options)
+			get: (): TypedArrayPredicate<Int16Array> => new TypedArrayPredicate('Int16Array', options)
 		},
 		uint16Array: {
-			get: () => new TypedArrayPredicate('Uint16Array', options)
+			get: (): TypedArrayPredicate<Uint16Array> => new TypedArrayPredicate('Uint16Array', options)
 		},
 		int32Array: {
-			get: () => new TypedArrayPredicate('Int32Array', options)
+			get: (): TypedArrayPredicate<Int32Array> => new TypedArrayPredicate('Int32Array', options)
 		},
 		uint32Array: {
-			get: () => new TypedArrayPredicate('Uint32Array', options)
+			get: (): TypedArrayPredicate<Uint32Array> => new TypedArrayPredicate('Uint32Array', options)
 		},
 		float32Array: {
-			get: () => new TypedArrayPredicate('Float32Array', options)
+			get: (): TypedArrayPredicate<Float32Array> => new TypedArrayPredicate('Float32Array', options)
 		},
 		float64Array: {
-			get: () => new TypedArrayPredicate('Float64Array', options)
+			get: (): TypedArrayPredicate<Float64Array> => new TypedArrayPredicate('Float64Array', options)
 		},
 		arrayBuffer: {
-			get: () => new ArrayBufferPredicate('ArrayBuffer', options)
+			get: (): ArrayBufferPredicate<ArrayBuffer> => new ArrayBufferPredicate('ArrayBuffer', options)
 		},
 		sharedArrayBuffer: {
-			get: () => new ArrayBufferPredicate('SharedArrayBuffer', options)
+			get: (): ArrayBufferPredicate<SharedArrayBuffer> => new ArrayBufferPredicate('SharedArrayBuffer', options)
 		},
 		dataView: {
-			get: () => new DataViewPredicate(options)
+			get: (): DataViewPredicate => new DataViewPredicate(options)
 		},
 		iterable: {
-			get: () => new Predicate('Iterable', options)
+			get: (): Predicate => new Predicate('Iterable', options)
 		},
 		any: {
-			value: (...predicates: BasePredicate[]) => new AnyPredicate(predicates, options)
+			value: (...predicates: BasePredicate[]): AnyPredicate => new AnyPredicate(predicates, options)
 		}
 	});
 

source/predicates/array-buffer.ts

@@ -6,7 +6,7 @@ export class ArrayBufferPredicate<T extends ArrayBufferLike> extends Predicate<T
 
 	@param byteLength - The byte length of the array buffer.
 	*/
-	byteLength(byteLength: number) {
+	byteLength(byteLength: number): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to have byte length of \`${byteLength}\`, got \`${value.byteLength}\``,
 			validator: value => value.byteLength === byteLength
@@ -18,7 +18,7 @@ export class ArrayBufferPredicate<T extends ArrayBufferLike> extends Predicate<T
 
 	@param byteLength - The minimum byte length of the array buffer.
 	*/
-	minByteLength(byteLength: number) {
+	minByteLength(byteLength: number): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to have a minimum byte length of \`${byteLength}\`, got \`${value.byteLength}\``,
 			validator: value => value.byteLength >= byteLength,
@@ -31,7 +31,7 @@ export class ArrayBufferPredicate<T extends ArrayBufferLike> extends Predicate<T
 
 	@param length - The minimum byte length of the array buffer.
 	*/
-	maxByteLength(byteLength: number) {
+	maxByteLength(byteLength: number): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to have a maximum byte length of \`${byteLength}\`, got \`${value.byteLength}\``,
 			validator: value => value.byteLength <= byteLength,

source/predicates/array.ts

@@ -18,7 +18,7 @@ export class ArrayPredicate<T = unknown> extends Predicate<T[]> {
 
 	@param length - The length of the array.
 	*/
-	length(length: number) {
+	length(length: number): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to have length \`${length}\`, got \`${value.length}\``,
 			validator: value => value.length === length
@@ -30,7 +30,7 @@ export class ArrayPredicate<T = unknown> extends Predicate<T[]> {
 
 	@param length - The minimum length of the array.
 	*/
-	minLength(length: number) {
+	minLength(length: number): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to have a minimum length of \`${length}\`, got \`${value.length}\``,
 			validator: value => value.length >= length,
@@ -43,7 +43,7 @@ export class ArrayPredicate<T = unknown> extends Predicate<T[]> {
 
 	@param length - The maximum length of the array.
 	*/
-	maxLength(length: number) {
+	maxLength(length: number): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to have a maximum length of \`${length}\`, got \`${value.length}\``,
 			validator: value => value.length <= length,
@@ -56,7 +56,7 @@ export class ArrayPredicate<T = unknown> extends Predicate<T[]> {
 
 	@param searchElement - The value that should be the start of the array.
 	*/
-	startsWith(searchElement: T) {
+	startsWith(searchElement: T): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to start with \`${searchElement}\`, got \`${value[0]}\``,
 			validator: value => value[0] === searchElement
@@ -68,7 +68,7 @@ export class ArrayPredicate<T = unknown> extends Predicate<T[]> {
 
 	@param searchElement - The value that should be the end of the array.
 	*/
-	endsWith(searchElement: T) {
+	endsWith(searchElement: T): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to end with \`${searchElement}\`, got \`${value[value.length - 1]}\``,
 			validator: value => value[value.length - 1] === searchElement
@@ -80,7 +80,7 @@ export class ArrayPredicate<T = unknown> extends Predicate<T[]> {
 
 	@param searchElements - The values that should be included in the array.
 	*/
-	includes(...searchElements: readonly T[]) {
+	includes(...searchElements: readonly T[]): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to include all elements of \`${JSON.stringify(searchElements)}\`, got \`${JSON.stringify(value)}\``,
 			validator: value => searchElements.every(element => value.includes(element))
@@ -92,7 +92,7 @@ export class ArrayPredicate<T = unknown> extends Predicate<T[]> {
 
 	@param searchElements - The values that should be included in the array.
 	*/
-	includesAny(...searchElements: readonly T[]) {
+	includesAny(...searchElements: readonly T[]): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to include any element of \`${JSON.stringify(searchElements)}\`, got \`${JSON.stringify(value)}\``,
 			validator: value => searchElements.some(element => value.includes(element))
@@ -102,7 +102,7 @@ export class ArrayPredicate<T = unknown> extends Predicate<T[]> {
 	/**
 	Test an array to be empty.
 	*/
-	get empty() {
+	get empty(): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to be empty, got \`${JSON.stringify(value)}\``,
 			validator: value => value.length === 0
@@ -112,7 +112,7 @@ export class ArrayPredicate<T = unknown> extends Predicate<T[]> {
 	/**
 	Test an array to be not empty.
 	*/
-	get nonEmpty() {
+	get nonEmpty(): this {
 		return this.addValidator({
 			message: (_, label) => `Expected ${label} to not be empty`,
 			validator: value => value.length > 0
@@ -124,7 +124,7 @@ export class ArrayPredicate<T = unknown> extends Predicate<T[]> {
 
 	@param expected - Expected value to match.
 	*/
-	deepEqual(expected: readonly T[]) {
+	deepEqual(expected: readonly T[]): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to be deeply equal to \`${JSON.stringify(expected)}\`, got \`${JSON.stringify(value)}\``,
 			validator: value => isEqual(value, expected)
@@ -172,7 +172,7 @@ export class ArrayPredicate<T = unknown> extends Predicate<T[]> {
 	ow(['1', 2], ow.array.exactShape([ow.string, ow.number]));
 	```
 	*/
-	exactShape(predicates: Predicate[]) {
+	exactShape(predicates: Predicate[]): this {
 		const shape = predicates as unknown as Shape;
 
 		return this.addValidator({

source/predicates/boolean.ts

@@ -11,7 +11,7 @@ export class BooleanPredicate extends Predicate<boolean> {
 	/**
 	Test a boolean to be true.
 	*/
-	get true() {
+	get true(): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to be true, got ${value}`,
 			validator: value => value
@@ -21,7 +21,7 @@ export class BooleanPredicate extends Predicate<boolean> {
 	/**
 	Test a boolean to be false.
 	*/
-	get false() {
+	get false(): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to be false, got ${value}`,
 			validator: value => !value

source/predicates/data-view.ts

@@ -13,7 +13,7 @@ export class DataViewPredicate extends Predicate<DataView> {
 
 	@param byteLength - The byte length of the DataView.
 	*/
-	byteLength(byteLength: number) {
+	byteLength(byteLength: number): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to have byte length of \`${byteLength}\`, got \`${value.byteLength}\``,
 			validator: value => value.byteLength === byteLength
@@ -25,7 +25,7 @@ export class DataViewPredicate extends Predicate<DataView> {
 
 	@param byteLength - The minimum byte length of the DataView.
 	*/
-	minByteLength(byteLength: number) {
+	minByteLength(byteLength: number): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to have a minimum byte length of \`${byteLength}\`, got \`${value.byteLength}\``,
 			validator: value => value.byteLength >= byteLength,
@@ -38,7 +38,7 @@ export class DataViewPredicate extends Predicate<DataView> {
 
 	@param length - The minimum byte length of the DataView.
 	*/
-	maxByteLength(byteLength: number) {
+	maxByteLength(byteLength: number): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} to have a maximum byte length of \`${byteLength}\`, got \`${value.byteLength}\``,
 			validator: value => value.byteLength <= byteLength,

source/predicates/date.ts

@@ -13,7 +13,7 @@ export class DatePredicate extends Predicate<Date> {
 
 	@param date - Maximum value.
 	*/
-	before(date: Date) {
+	before(date: Date): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} ${value.toISOString()} to be before ${date.toISOString()}`,
 			validator: value => value.getTime() < date.getTime()
@@ -25,7 +25,7 @@ export class DatePredicate extends Predicate<Date> {
 
 	@param date - Minimum value.
 	*/
-	after(date: Date) {
+	after(date: Date): this {
 		return this.addValidator({
 			message: (value, label) => `Expected ${label} ${value.toISOString()} to be after ${date.toISOString()}`,
 			validator: value => value.getTime() > date.getTime()