Add whereDefined and mapCall methods to Collection class

src/util/collection/Collection.ts

@@ -14,7 +14,7 @@ type MaybeCollectionIndex = CollectionIndex | undefined
 type ComparisonFunction<T> = (item: CollectionItem<T>, otherItem: CollectionItem<T>) => number
 
 import { WhereOperator, applyWhere, whereMatch } from './where'
-import {Awaitable, Either, isLeft, right, unright} from '../support/types'
+import {Awaitable, Either, isLeft, Maybe, MethodsOf, right, unright} from '../support/types'
 
 const collect = <T>(items: CollectionItem<T>[]): Collection<T> => Collection.collect(items)
 const toString = (item: unknown): string => String(item)
@@ -293,7 +293,7 @@ class Collection<T> {
      * Stops executing if a single truth case is found.
      * @param func
      */
-    some(func: (item: T) => boolean): boolean {
+    some(func: (item: T) => Maybe<boolean>): boolean {
         return this.storedItems.some(func)
     }
 
@@ -355,6 +355,23 @@ class Collection<T> {
         return right(new Collection<TRight>(newItems))
     }
 
+    /**
+     * Map a method on the underlying type, passing it any required parameters.
+     * This is delightfully type-safe.
+     * @param method
+     * @param params
+     */
+    mapCall<T2 extends MethodsOf<T>>(method: T2, ...params: Parameters<T[T2]>): Collection<ReturnType<T[T2]>> {
+        return this.map(x => x[method](...params))
+    }
+
+    /**
+     * Map each element in the collection to a string.
+     */
+    strings(): Collection<string> {
+        return this.map<string>(x => String(x))
+    }
+
     /**
      * Create a new collection by mapping the items in this collection using the given function,
      * excluding any for which the function returns undefined.
@@ -441,8 +458,12 @@ class Collection<T> {
      * Return a new collection filtered by the given function.
      * @param func
      */
-    filter<T2>(func: KeyFunction<T, T2>): Collection<T> {
-        return new Collection(this.storedItems.filter(func))
+    filter<T2>(func?: KeyFunction<T, T2>): Collection<T> {
+        return new Collection(this.storedItems.filter(func ?? Boolean))
+    }
+
+    whereDefined(): Collection<NonNullable<T>> {
+        return this.filter() as unknown as Collection<NonNullable<T>>
     }
 
     /**
@@ -780,7 +801,7 @@ class Collection<T> {
      * Return the max value of the given key.
      * @param key
      */
-    max<T2>(key: KeyOperator<T, T2>): number {
+    max<T2 = T>(key: KeyOperator<T, T2>): number {
         const values = this.allAsNumbers(key)
         return Math.max(...values)
     }

src/util/support/types.ts

@@ -68,3 +68,7 @@ export interface TypeTag<S extends string> {
 export type PrefixTypeArray<T, TArr extends unknown[]> = [T, ...TArr]
 export type SuffixTypeArray<TArr extends unknown[], T> = [...TArr, T]
 export type TypeArraySignature<TArr extends unknown[], TReturn> = (...params: TArr) => TReturn
+
+export type MethodsOf<T, TMethod = (...args: any[]) => any> = {
+    [K in keyof T]: T[K] extends TMethod ? K : never
+}[keyof T]