Add the ability to provide custom messages

readme.md

@@ -236,6 +236,42 @@ ow(1, 'input', ow.number.validate(value => ({
 //=> ArgumentError: Expected number `input` to be greater than 10, got 1
 ```
 
+#### message(string | fn)
+
+Provide a custom message:
+
+```ts
+ow('🌈', 'unicorn', ow.string.equals('🦄').message('Expected unicorn, got rainbow'));
+//=> ArgumentError: Expected unicorn, got rainbow
+```
+
+You can also pass in a function which receives the value as the first parameter and the label as the second parameter and is expected to return the message.
+
+```ts
+ow('🌈', ow.string.minLength(5).message((value, label) => `Expected ${label}, to have a minimum length of 5, got \`${value}\``));
+//=> ArgumentError: Expected string, to be have a minimum length of 5, got `🌈`
+```
+
+It's also possible to add a separate message per validation:
+
+```ts
+ow(
+	'1234',
+	ow.string
+		.minLength(5).message((value, label) => `Expected ${label}, to be have a minimum length of 5, got \`${value}\``)
+		.url.message('This is no url')
+);
+//=> ArgumentError: Expected string, to be have a minimum length of 5, got `1234`
+
+ow(
+	'12345',
+	ow.string
+		.minLength(5).message((value, label) => `Expected ${label}, to be have a minimum length of 5, got \`${value}\``)
+		.url.message('This is no url')
+);
+//=> ArgumentError: This is no url
+```
+
 This can be useful for creating your own reusable validators which can be extracted to a separate npm package.
 
 ## Maintainers
@@ -247,4 +283,3 @@ This can be useful for creating your own reusable validators which can be extrac
 
 - [@sindresorhus/is](https://github.com/sindresorhus/is) - Type check values
 - [ngx-ow](https://github.com/SamVerschueren/ngx-ow) - Angular form validation on steroids
-

source/predicates/predicate.ts

@@ -4,6 +4,11 @@ import {not} from '../operators/not';
 import {BasePredicate, testSymbol} from './base-predicate';
 import {Main} from '..';
 
+/**
+@hidden
+ */
+export type ValidatorMessage<T> = (value: T, label?: string) => string;
+
 /**
 @hidden
 */
@@ -164,6 +169,25 @@ export class Predicate<T = unknown> implements BasePredicate<T> {
 		});
 	}
 
+	/**
+	Provide an new error message to be thrown when the validation fails.
+
+	@param newMessage - Either a string containing the new message or a function returning the new message
+	*/
+	message(newMessage: string | ValidatorMessage<T>) {
+		const {validators} = this.context;
+
+		validators[validators.length - 1].message = (value, label) => {
+			if (typeof newMessage === 'function') {
+				return newMessage(value, label);
+			}
+
+			return newMessage;
+		};
+
+		return this;
+	}
+
 	/**
 	Register a new validator.
 

test/custom-message.ts

@@ -0,0 +1,37 @@
+import test from 'ava';
+import {default as ow, Predicate} from '../source';
+
+class CustomPredicate extends Predicate<any> {
+	constructor() {
+		super('string');
+	}
+
+	get unicorn() {
+		return this.addValidator({
+			message: (value, label) => `Expected ${label} to be \`🦄\`, got \`${value}\``,
+			validator: value => value === '🦄'
+		});
+	}
+}
+
+test('custom validate message', t => {
+	t.throws(() => {
+		ow('🌈', 'unicorn', new CustomPredicate().unicorn.message('Expect unicorn, got rainbow'));
+	}, 'Expect unicorn, got rainbow');
+
+	t.throws(() => {
+		ow('🌈', 'unicorn', new CustomPredicate().unicorn.message((value, label) => `Expected ${label}, to be \`🦄\` got \`${value}\``));
+	}, 'Expected string `unicorn`, to be `🦄` got `🌈`');
+
+	t.throws(() => {
+		ow('🌈', ow.string.minLength(5).message((value, label) => `Expected ${label}, to be have a minimum length of 5, got \`${value}\``));
+	}, 'Expected string, to be have a minimum length of 5, got `🌈`');
+
+	t.throws(() => {
+		ow('1234', ow.string.minLength(5).message((value, label) => `Expected ${label}, to be have a minimum length of 5, got \`${value}\``).url.message('This is no url'));
+	}, 'Expected string, to be have a minimum length of 5, got `1234`');
+
+	t.throws(() => {
+		ow('12345', ow.string.minLength(5).message((value, label) => `Expected ${label}, to be have a minimum length of 5, got \`${value}\``).url.message('This is no url'));
+	}, 'This is no url');
+});