Add truthy type guard

This adds an explicit type guard for truthiness. Generally you could use the `Boolean` constructor, but typescript doesn't treat that as a type guard right now (see [#16655](microsoft/TypeScript#16655)).

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "fast-ts-helpers",
-	"version": "0.0.0-dev.4",
+	"version": "0.0.0-dev.5",
 	"private": true,
 	"description": "A package containing some helpful functions, constants, types, and react hooks.",
 	"license": "MIT",

src/cacheKey.ts

@@ -1,5 +1,6 @@
 import stringify from "json-stable-stringify"
 
+import { truthy } from "./truthy"
 import { uuid } from "./uuid"
 
 /**
@@ -35,7 +36,7 @@ export function cacheKey<Type>(obj: Type) {
 	}
 
 	function hasCacheKey<T>(obj: T): obj is T & { __cacheKey: string } {
-		return Boolean(obj) && `__cacheKey` in obj
+		return truthy(obj) && `__cacheKey` in obj
 	}
 
 	return stringify(obj, { replacer })

src/concatUrls.ts

@@ -4,19 +4,18 @@ import { FalsibleList } from "./FalsibleList"
 /** Concatenate the passed URL parts into one URL using `/` as the separator, with a leadin slash and without a trailing slash. */
 export function concatUrls(...urls: FalsibleList<string>[]) {
 	const filtered = flatfilter(urls)
-	let protocol = ``
 
-	const first = filtered[0]
+	if (filtered.length > 0) {
+		let protocol = ``
+		const first = filtered[0]
 
-	if (Boolean(first) && /^https?:\/\//.test(first)) {
-		protocol = /^https?:\//.exec(first)![0]
-		filtered[0] = first.replace(/^https?:\/\//, ``)
+		if (/^https?:\/\//.test(first)) {
+			protocol = /^.+?:\/(?=\/)/.exec(first)![0]
+			filtered[0] = first.replace(/^.+?:\/\//, ``)
+		}
+
+		return `${protocol}/${filtered.join(`/`).replace(/^\/+|\/+$|\/+(\/)/g, `$1`)}`
 	}
 
-	return (
-		protocol +
-		(filtered.length > 0
-			? `/${filtered.join(`/`).replace(/^\/+|\/+$|\/+(\/)/g, `$1`)}`
-			: ``)
-	)
+	return ``
 }

src/focus.ts

@@ -1,5 +1,6 @@
-import { focusableSelector } from "./focusableSelector"
 import { Falsible } from "./Falsible"
+import { focusableSelector } from "./focusableSelector"
+import { truthy } from "./truthy"
 
 /**
  * Transfer the focus to an element, optionally preventing scrolling. You can use this to focus on elements that aren't actually focusable too, for accessibility.
@@ -8,7 +9,7 @@ import { Falsible } from "./Falsible"
  * @param preventScroll Whether scrolling should be prevented. Defaults to `true`.
  */
 export function focus(element: Falsible<HTMLElement | SVGElement>, preventScroll = true) {
-	if (!Boolean(element)) {
+	if (!truthy(element)) {
 		return
 	}
 

src/onEnterKeyDown.ts

@@ -1,7 +1,7 @@
 import { Falsible } from "./Falsible"
-
-import { onNonRepeatedKeyDown } from "./onNonRepeatedKeyDown"
 import { memoize } from "./memoize"
+import { onNonRepeatedKeyDown } from "./onNonRepeatedKeyDown"
+import { truthy } from "./truthy"
 
 /**
  * Respond to the event where the user presses the enter key, responding only to the actula press, not the repeated events if the user holds it.
@@ -19,7 +19,7 @@ export const onEnterKeyDown = memoize(
 	) => {
 		return onNonRepeatedKeyDown(
 			(event) => {
-				if (event.key === `Enter` && Boolean(action)) {
+				if (event.key === `Enter` && truthy(action)) {
 					action(event)
 				}
 			},

src/onNonRepeatedKeyDown.ts

@@ -1,8 +1,8 @@
-import { Falsible } from "./Falsible"
-
-import { memoize } from "./memoize"
 import { combine } from "./combine"
 import { deleteOwnProperties } from "./deleteOwnProperties"
+import { Falsible } from "./Falsible"
+import { memoize } from "./memoize"
+import { truthy } from "./truthy"
 
 /** The keys that are currently pressed. */
 const keysDown: Record<string, boolean> = {}
@@ -23,7 +23,7 @@ export const onNonRepeatedKeyDown = memoize(
 	) => {
 		return {
 			onKeyDown: combine(
-				Boolean(action) &&
+				truthy(action) &&
 					((event: React.KeyboardEvent) => {
 						if (!keysDown[event.key]) {
 							keysDown[event.key] = true
@@ -34,7 +34,7 @@ export const onNonRepeatedKeyDown = memoize(
 				extraOnKeyDown
 			),
 			onKeyUp: combine(
-				Boolean(action) &&
+				truthy(action) &&
 					((event: React.KeyboardEvent) => {
 						if (event.key === `Meta`) {
 							deleteOwnProperties(keysDown)

src/parseDate.ts

@@ -1,3 +1,5 @@
+import { truthy } from "./truthy"
+
 /**
  * Parse a date, number, or string into a Date.
  *
@@ -41,7 +43,7 @@ export function parseDate(date: string | number | Date | undefined) {
 	}
 
 	// If the datePart is set, parse that date part
-	if (Boolean(datePart)) {
+	if (truthy(datePart)) {
 		const [rawYear, rawMonth, rawDay] = datePart.split(`-`).map(parseFloat)
 
 		year = rawYear
@@ -58,7 +60,7 @@ export function parseDate(date: string | number | Date | undefined) {
 	let zoneHalf: string | undefined
 
 	// If the time part is defined, get the hour, minute, and second values
-	if (Boolean(timePart)) {
+	if (truthy(timePart)) {
 		const parts = timePart.split(/Z|\+|(?=-)/)
 
 		const timeHalf = parts[0]!
@@ -82,12 +84,12 @@ export function parseDate(date: string | number | Date | undefined) {
 	const manualTime = new Date(year, month, day, hours, minutes, seconds).getTime()
 
 	// If the time part is defined, assume UTC. The other if statement below will offset accordingly.
-	if (Boolean(timePart)) {
+	if (truthy(timePart)) {
 		offset = now.getTimezoneOffset() * 60 * 1000
 	}
 
 	// If the zone half of the time part is defined, do that offset here
-	if (Boolean(zoneHalf)) {
+	if (truthy(zoneHalf)) {
 		const [offsetHours, offsetMinutes] = zoneHalf.split(`:`).map(parseFloat)
 
 		offset += offsetHours * 60 * 60 * 1000

src/truthy.ts

@@ -0,0 +1,5 @@
+import { Falsible } from "./Falsible"
+
+export function truthy<T>(value?: Falsible<T>): value is T {
+	return Boolean(value)
+}