Add `string.url` predicate (#140)

source/predicates/string.ts

@@ -1,3 +1,4 @@
+import is from '@sindresorhus/is';
 import valiDate from 'vali-date';
 import {Predicate, PredicateOptions} from './predicate';
 
@@ -205,4 +206,14 @@ export class StringPredicate extends Predicate<string> {
 			validator: value => value.trim() !== '' && value === value.toUpperCase()
 		});
 	}
+
+	/**
+	 * Test a string to be a valid URL.
+	 */
+	get url() {
+		return this.addValidator({
+			message: (value, label) => `Expected ${label} to be a URL, got \`${value}\``,
+			validator: is.urlString
+		});
+	}
 }

test/string.ts

@@ -310,3 +310,21 @@ test('string.uppercase', t => {
 		ow('', ow.string.uppercase);
 	}, 'Expected string to be uppercase, got ``');
 });
+
+test('string.url', t => {
+	t.notThrows(() => {
+		ow('https://sindresorhus.com', ow.string.url);
+	});
+
+	t.notThrows(() => {
+		ow('file:///path/to/an/awesome/file', ow.string.url);
+	});
+
+	t.throws(() => {
+		ow('foo' as any, ow.string.url);
+	}, 'Expected string to be a URL, got `foo`');
+
+	t.throws(() => {
+		ow('foo' as any, 'bar', ow.string.url);
+	}, 'Expected string `bar` to be a URL, got `foo`');
+});