Use an object to register a validator - fixes #2 (#6)

source/lib/predicates/predicate.ts

@@ -1,20 +1,32 @@
 import * as is from '@sindresorhus/is';
 
-export type Validator = (value: any) => string | undefined;
+export interface Validator<T> {
+	message: (value: T) => string;
+	validator: (value: T) => boolean;
+}
 
 export interface Context {
-	validators: Validator[];
+	validators: Validator<any>[];
 }
 
-export class Predicate {
+export class Predicate<T = any> {
 	constructor(
 		type: string,
-		public context: Context = { validators: [] }
+		private context: Context = { validators: [] }
 	) {
-		this.context.validators.push(value => {
-			if (!is[type](value)) {
-				return `Expected argument to be of type \`${type}\` but received type \`${is(value)}\``;
-			}
+		this.addValidator({
+			message: value => `Expected argument to be of type \`${type}\` but received type \`${is(value)}\``,
+			validator: value => is[type](value)
 		});
 	}
+
+	get validators() {
+		return this.context.validators;
+	}
+
+	protected addValidator(validator: Validator<T>) {
+		this.context.validators.push(validator);
+
+		return this;
+	}
 }

source/lib/predicates/string.ts

@@ -1,6 +1,6 @@
 import { Predicate, Context } from './predicate';
 
-export class StringPredicate extends Predicate {
+export class StringPredicate extends Predicate<string> {
 
 	constructor(context?: Context) {
 		super('string', context);
@@ -12,25 +12,19 @@ export class StringPredicate extends Predicate {
 	 * @param number The minimum length of the string.
 	 */
 	minLength(number: number) {
-		this.context.validators.push(value => {
-			if (value.length < number) {
-				return `Expected string length to be minimum ${number}`;
-			}
+		return this.addValidator({
+			message: () => `Expected string length to be minimum ${number}`,
+			validator: value => value.length >= number
 		});
-
-		return this;
 	}
 
 	/**
 	 * Test a string to be alphanumeric.
 	 */
 	get alphanumeric() {
-		this.context.validators.push(value => {
-			if (!/^[a-z\d]+$/i.test(value)) {
-				return `Expected string to contain only alphanumeric characters but received \`${value}\``;
-			}
+		return this.addValidator({
+			message: value => `Expected string to contain only alphanumeric characters but received \`${value}\``,
+			validator: value => /^[a-z\d]+$/i.test(value)
 		});
-
-		return this;
 	}
 }

source/ow.ts

@@ -12,13 +12,14 @@ export interface Ow {
 }
 
 export const ow: Ow = (value: any, predicate: Predicate) => {
-	for (const validator of predicate.context.validators) {
-		const result = validator(value);
-		if (result) {
+	for (const { validator, message } of predicate.validators) {
+		if (!validator(value)) {
 			// TODO: Modify the stack output to show the original `ow()` call instead of this `throw` statement
-			throw new ArgumentError(result, ow);
+			throw new ArgumentError(message(value), ow);
 		}
 	}
 };
 
-ow.string = new StringPredicate();
+Object.defineProperty(ow, 'string', {
+	get: () => new StringPredicate()
+});