Add the ability to provide custom messages

readme.md

@@ -236,6 +236,31 @@ ow(1, 'input', ow.number.validate(value => ({
 //=> ArgumentError: Expected number `input` to be greater than 10, got 1
 ```
 
+#### message(fn | string)
+
+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 as `message` value which accepts the label as argument.
+
+```ts
+ow('🌈', ow.string.minLength(5).message((value, label) => `Expected ${label}, to be have a minimum length of 5, got \`${value}\``));
+//=> ArgumentError: Expected string, to be have a minimum length of 5, got `🌈`
+```
+
+If you want to add a different message per validation it is possible
+
+```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

source/predicates/predicate.ts

@@ -4,6 +4,8 @@ import {not} from '../operators/not';
 import {BasePredicate, testSymbol} from './base-predicate';
 import {Main} from '..';
 
+export type ValidatorMessage<T> = (value: T, label?: string) => string;
+
 /**
 @hidden
 */
@@ -164,6 +166,25 @@ export class Predicate<T = unknown> implements BasePredicate<T> {
 		});
 	}
 
+	/**
+	Override the message thrown
+
+	@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');
+});