Add isInfinite() (#29)

readme.md

@@ -30,6 +30,7 @@ import {isDefined} from 'ts-extras';
 - [`isDefined`](source/is-defined.ts) - Check whether a value is defined (not `null` or `undefined`).
 - [`isEmpty`](source/is-empty.ts) - Check whether an array is empty.
 - [`assertError`](source/assert-error.ts) - Assert that the given value is an `Error`.
+- [`isInfinite`](source/is-infinite.ts) - Check whether a value is infinite.
 
 **Improved builtin**
 

source/index.ts

@@ -4,6 +4,7 @@ export {assertError} from './assert-error.js';
 export {isDefined} from './is-defined.js';
 export {isEmpty} from './is-empty.js';
 export {isFinite} from './is-finite.js';
+export {isInfinite} from './is-infinite.js';
 export {isInteger} from './is-integer.js';
 export {isSafeInteger} from './is-safe-integer.js';
 export {objectEntries} from './object-entries.js';

source/is-infinite.ts

@@ -0,0 +1,10 @@
+import {NegativeInfinity, PositiveInfinity} from 'type-fest';
+
+/**
+Check whether a value is infinite.
+
+@category Type guard
+*/
+export function isInfinite(value: unknown): value is NegativeInfinity | PositiveInfinity {
+	return !Number.isNaN(value) && !Number.isFinite(value);
+}

test/is-infinite.ts

@@ -0,0 +1,18 @@
+import test from 'ava';
+import {expectTypeOf} from 'expect-type';
+import {NegativeInfinity, PositiveInfinity} from 'type-fest';
+import {isInfinite} from '../source/index.js';
+
+test('isInfinite()', t => {
+	t.false(isInfinite(123));
+	t.true(isInfinite(Number.POSITIVE_INFINITY));
+	t.true(isInfinite(Number.NEGATIVE_INFINITY));
+
+	const number_ = 123 as number;
+
+	if (isInfinite(number_)) {
+		expectTypeOf<PositiveInfinity | NegativeInfinity>(number_);
+	} else {
+		expectTypeOf<number>(number_);
+	}
+});