Make assert, truthy and falsy typeguards

test-types/import-in-cts/assertions-as-type-guards.cts

@@ -4,6 +4,15 @@ import {expectType} from 'tsd';
 type Expected = {foo: 'bar'};
 const expected: Expected = {foo: 'bar'};
 
+test('assert', t => {
+	const actual = expected as Expected | undefined;
+	if (t.truthy(actual)) {
+		expectType<Expected>(actual);
+	} else {
+		expectType<undefined>(actual);
+	}
+});
+
 test('deepEqual', t => {
 	const actual: unknown = {};
 	if (t.deepEqual(actual, expected)) {
@@ -32,9 +41,28 @@ test('false', t => {
 	}
 });
 
+test('falsy', t => {
+	type Actual = Expected | undefined | null | false | 0 | '' | 0n;
+	const actual = undefined as Actual;
+	if (t.falsy(actual)) {
+		expectType<Exclude<Actual, Expected>>(actual);
+	} else {
+		expectType<Expected>(actual);
+	}
+});
+
 test('true', t => {
 	const actual: unknown = false;
 	if (t.true(actual)) {
 		expectType<true>(actual);
 	}
 });
+
+test('truthy', t => {
+	const actual = expected as Expected | undefined;
+	if (t.truthy(actual)) {
+		expectType<Expected>(actual);
+	} else {
+		expectType<undefined>(actual);
+	}
+});

test-types/module/assertions-as-type-guards.ts

@@ -5,6 +5,15 @@ import test from '../../entrypoints/main.mjs';
 type Expected = {foo: 'bar'};
 const expected: Expected = {foo: 'bar'};
 
+test('assert', t => {
+	const actual = expected as Expected | undefined;
+	if (t.truthy(actual)) {
+		expectType<Expected>(actual);
+	} else {
+		expectType<undefined>(actual);
+	}
+});
+
 test('deepEqual', t => {
 	const actual: unknown = {};
 	if (t.deepEqual(actual, expected)) {
@@ -33,9 +42,28 @@ test('false', t => {
 	}
 });
 
+test('falsy', t => {
+	type Actual = Expected | undefined | null | false | 0 | '' | 0n;
+	const actual = undefined as Actual;
+	if (t.falsy(actual)) {
+		expectType<Exclude<Actual, Expected>>(actual);
+	} else {
+		expectType<Expected>(actual);
+	}
+});
+
 test('true', t => {
 	const actual: unknown = false;
 	if (t.true(actual)) {
 		expectType<true>(actual);
 	}
 });
+
+test('truthy', t => {
+	const actual = expected as Expected | undefined;
+	if (t.truthy(actual)) {
+		expectType<Expected>(actual);
+	} else {
+		expectType<undefined>(actual);
+	}
+});

types/assertions.d.cts

@@ -125,12 +125,14 @@ export type Assertions = {
 	truthy: TruthyAssertion;
 };
 
+type Falsy = false | 0 | -0 | 0n | '' | null | undefined;
+
 export type AssertAssertion = {
 	/**
 	 * Assert that `actual` is [truthy](https://developer.mozilla.org/en-US/docs/Glossary/Truthy), returning a boolean
 	 * indicating whether the assertion passed.
 	 */
-	(actual: any, message?: string): boolean;
+	<T>(actual: T, message?: string): actual is Exclude<T, Falsy>;
 
 	/** Skip this assertion. */
 	skip(actual: any, message?: string): void;
@@ -192,7 +194,7 @@ export type FalsyAssertion = {
 	 * Assert that `actual` is [falsy](https://developer.mozilla.org/en-US/docs/Glossary/Falsy), returning a boolean
 	 * indicating whether the assertion passed.
 	 */
-	(actual: any, message?: string): boolean;
+	<T>(actual: T, message?: string): actual is T extends Falsy ? T :never;
 
 	/** Skip this assertion. */
 	skip(actual: any, message?: string): void;
@@ -337,7 +339,7 @@ export type TruthyAssertion = {
 	 * Assert that `actual` is [truthy](https://developer.mozilla.org/en-US/docs/Glossary/Truthy), returning a boolean
 	 * indicating whether the assertion passed.
 	 */
-	(actual: any, message?: string): boolean;
+	<T>(actual: T, message?: string): actual is Exclude<T, Falsy>;
 
 	/** Skip this assertion. */
 	skip(actual: any, message?: string): void;