TypeScriptToLua · Perryvw · Oct 5, 2019 · Oct 3, 2019 · Oct 3, 2019 · Oct 5, 2019
diff --git a/src/lualib/ArrayReduce.ts b/src/lualib/ArrayReduce.ts
@@ -3,25 +3,26 @@ function __TS__ArrayReduce<T>(
    this: void,
    arr: T[],
    callbackFn: (accumulator: T, currentValue: T, index: number, array: T[]) => T,
-    initial?: T
+    ...initial: Vararg<T>
 ): T {
    const len = arr.length;

-    if (len === 0 && initial === undefined) {
-        // tslint:disable-next-line: no-string-throw
-        throw "Reduce of empty array with no initial value";
-    }
-
    let k = 0;
-    let accumulator = initial;
-    if (initial === undefined) {
+    let accumulator = undefined;
+
+    // Check if initial value is present in function call
+    if (select("#", ...initial) !== 0) {
+        accumulator = select(1, ...initial);
+    } else if (len > 0) {
        accumulator = arr[0];
-        k++;
+        k = 1;
+    } else {
+        // tslint:disable-next-line: no-string-throw
+        throw "Reduce of empty array with no initial value";
    }

-    while (k < len) {
-        accumulator = callbackFn(accumulator, arr[k], k, arr);
-        k = k + 1;
+    for (const i of forRange(k, len - 1)) {
+        accumulator = callbackFn(accumulator, arr[i], i, arr);
    }

    return accumulator;

diff --git a/src/lualib/declarations/global.d.ts b/src/lualib/declarations/global.d.ts
@@ -16,3 +16,6 @@ declare function rawset<T, K extends keyof T>(table: T, key: K, val: T[K]): void
 declare function next<K, V>(table: Record<any, V>, index?: K): [K, V];
 declare function pcall(func: () => any): any;
 declare function unpack<T>(list: T[], i?: number, j?: number): T[];
+
+declare function select<T>(index: number, ...args: T[]): T;
+declare function select<T>(index: "#", ...args: T[]): number;
diff --git a/src/lualib/declarations/tstl.d.ts b/src/lualib/declarations/tstl.d.ts
@@ -1,5 +1,11 @@
 /** @noSelfInFile */

+/** @vararg */
+interface Vararg<T> extends Array<T> {}
+
+/** @forRange */
+declare function forRange(start: number, limit: number, step?: number): number[];
+
 interface LuaClass {
    prototype: LuaObject;
    ____super?: LuaClass;

diff --git a/test/unit/builtins/array.spec.ts b/test/unit/builtins/array.spec.ts
@@ -486,6 +486,22 @@ test.each<[[(total: number, currentItem: number, index: number, array: number[])
    util.testExpression`[1, 3, 5, 7].reduce(${util.valuesToString(args)})`.expectToMatchJsResult();
 });

+test("array.reduce empty undefined initial", () => {
+    util.testExpression`[].reduce(() => {}, undefined)`.expectToMatchJsResult();
+});
+
+test("array.reduce empty no initial", () => {
+    util.testExpression`[].reduce(() => {})`.expectToMatchJsResult(true);
+});
+
+test("array.reduce undefined returning callback", () => {
+    util.testFunction`
+        const calls: Array<{ a: void, b: string }> = [];
+        ["a", "b"].reduce<void>((a, b) => { calls.push({ a, b }) }, undefined);
+        return calls;
+    `.expectToMatchJsResult();
+});
+
 const genericChecks = [
    "function generic<T extends number[]>(array: T)",
    "function generic<T extends [...number[]]>(array: T)",