Add string.nonBlank (#227)

source/predicates/string.ts

@@ -127,6 +127,27 @@ export class StringPredicate extends Predicate<string> {
 		});
 	}
 
+	/**
+	Test a string to contain at least 1 non-whitespace character.
+	*/
+	get nonBlank(): this {
+		return this.addValidator({
+			message: (value, label) => {
+				// Unicode's formal substitute characters can be barely legible and may not be easily recognized.
+				// Hence this alternative substitution scheme.
+				const madeVisible = value
+					.replace(/ /g, '·')
+					.replace(/\f/g, '\\f')
+					.replace(/\n/g, '\\n')
+					.replace(/\r/g, '\\r')
+					.replace(/\t/g, '\\t')
+					.replace(/\v/g, '\\v');
+				return `Expected ${label} to not be only whitespace, got \`${madeVisible}\``;
+			},
+			validator: value => value.trim() !== '',
+		});
+	}
+
 	/**
 	Test a string to be not empty.
 	*/

test/string.ts

@@ -163,6 +163,16 @@ test('string.empty', t => {
 	}, {message: 'Expected string to be empty, got `foo`'});
 });
 
+test('string.nonBlank', t => {
+	t.notThrows(() => {
+		ow('foo', ow.string.nonBlank);
+	});
+
+	t.throws(() => {
+		ow(' \n\t\f\v', ow.string.nonBlank);
+	}, {message: 'Expected string to not be only whitespace, got `·\\n\\t\\f\\v`'});
+});
+
 test('string.nonEmpty', t => {
 	t.notThrows(() => {
 		ow('foo', ow.string.nonEmpty);