Add symbol, undefined and null predicates (#28)

source/index.ts

@@ -20,6 +20,18 @@ export interface Ow {
 	 * Test the value to be a boolean.
 	 */
 	boolean: BooleanPredicate;
+	/**
+	 * Test the value to be undefined.
+	 */
+	undefined: Predicate<undefined>;
+	/**
+	 * Test the value to be null.
+	 */
+	null: Predicate<null>;
+	/**
+	 * Test the value to be a Symbol.
+	 */
+	symbol: Predicate<Symbol>;
 	/**
 	 * Test the value to be an array.
 	 */
@@ -49,6 +61,15 @@ Object.defineProperties(main, {
 	boolean: {
 		get: () => new BooleanPredicate()
 	},
+	undefined: {
+		get: () => new Predicate('undefined')
+	},
+	null: {
+		get: () => new Predicate('null')
+	},
+	symbol: {
+		get: () => new Predicate('symbol')
+	},
 	array: {
 		get: () => new ArrayPredicate()
 	},

source/lib/predicates/predicate.ts

@@ -12,7 +12,7 @@ export interface Context {
 
 export const validatorSymbol = Symbol('validators');
 
-export abstract class Predicate<T = any> {
+export class Predicate<T = any> {
 	constructor(
 		type: string,
 		private context: Context = {

source/test/null.ts

@@ -0,0 +1,11 @@
+import test from 'ava';
+import m from '..';
+
+test('undefined', t => {
+	const x = null;
+
+	t.notThrows(() => m(null, m.null));
+	t.notThrows(() => m(x, m.null));
+	t.throws(() => m(undefined, m.null), 'Expected argument to be of type `null` but received type `undefined`');
+	t.throws(() => m('foo', m.null), 'Expected argument to be of type `null` but received type `string`');
+});

source/test/symbol.ts

@@ -0,0 +1,8 @@
+import test from 'ava';
+import m from '..';
+
+test('symbol', t => {
+	t.notThrows(() => m(Symbol.iterator, m.symbol));
+	t.notThrows(() => m(Symbol('foo'), m.symbol));
+	t.throws(() => m(12, m.symbol), 'Expected argument to be of type `symbol` but received type `number`');
+});

source/test/undefined.ts

@@ -0,0 +1,13 @@
+import test from 'ava';
+import m from '..';
+
+test('undefined', t => {
+	let x;						// tslint:disable-line:prefer-const
+	const y = 12;
+
+	t.notThrows(() => m(undefined, m.undefined));
+	t.notThrows(() => m(x, m.undefined));
+	t.throws(() => m(y, m.undefined), 'Expected argument to be of type `undefined` but received type `number`');
+	t.throws(() => m(null, m.undefined), 'Expected argument to be of type `undefined` but received type `null`');
+	t.throws(() => m('foo', m.undefined), 'Expected argument to be of type `undefined` but received type `string`');
+});