♻️ Update

Author: UltiRequiem

src/language/average.ts

@@ -0,0 +1,3 @@
+export function average(array: number[]) {
+  return array.reduce((a, b) => a + b) / array.length;
+}

src/language/reduce.ts

@@ -1,18 +1,20 @@
-export type ReduceCallback<T> = (
-  accumulator: T,
+// I don't like TypeScript Anymore, my mood: 😔 -> 😩 -> 😭
+
+export type ReduceCallback<T, U> = (
+  accumulator: U,
   value: T,
   index: number,
   array: readonly T[],
 ) => unknown;
 
-export function reduce<T, F extends ReduceCallback<T>>(
-  array: T[],
-  callback: F,
-  initialValue?: unknown,
-) {
+export function reduce<
+  T extends unknown[],
+  F extends ReduceCallback<T, U>,
+  U = unknown,
+>(array: readonly T[], callback: F, initialValue?: U | T) {
   const copy = [...array];
 
-  if (array.length >= 1) {
+  if (array.length >= 1 && initialValue === undefined) {
     initialValue = copy.shift();
   }
 
@@ -21,7 +23,7 @@ export function reduce<T, F extends ReduceCallback<T>>(
   const entries = copy.entries();
 
   for (const [index, element] of entries) {
-    previous = callback(previous as T, element, index, array);
+    previous = callback(previous as T & U, element, index, array) as U;
   }
 
   return previous as ReturnType<F>;