fix(util-base64): allow arrays to stand in for Uint8Array (runtime only)

.changeset/old-kangaroos-rhyme.md

@@ -0,0 +1,5 @@
+---
+"@smithy/util-base64": patch
+---
+
+allow arrays to stand in for Uint8Array in toBase64

packages/util-base64/src/toBase64.browser.spec.ts

@@ -4,6 +4,7 @@
 import type { Encoder } from "@smithy/types";
 
 import testCases from "./__mocks__/testCases.json";
+import { fromBase64 } from "./fromBase64.browser";
 import { toBase64 } from "./toBase64.browser";
 
 describe(toBase64.name, () => {
@@ -19,4 +20,11 @@ describe(toBase64.name, () => {
     expect(() => (toBase64 as Encoder)(new Date())).toThrow();
     expect(() => (toBase64 as Encoder)({})).toThrow();
   });
+
+  it("allows array to stand in for Uint8Array", () => {
+    expect(() => (toBase64 as Encoder)([])).not.toThrow();
+
+    const helloUtf8Array = fromBase64("aGVsbG8=");
+    expect(toBase64(([...helloUtf8Array] as unknown) as Uint8Array)).toEqual("aGVsbG8=");
+  });
 });

packages/util-base64/src/toBase64.browser.ts

@@ -17,9 +17,17 @@ export function toBase64(_input: Uint8Array | string): string {
   } else {
     input = _input as Uint8Array;
   }
-  if (typeof input !== "object" || typeof input.byteOffset !== "number" || typeof input.byteLength !== "number") {
+
+  const isArrayLike = typeof input === "object" && typeof input.length === "number";
+  const isUint8Array =
+    typeof input === "object" ||
+    typeof (input as Uint8Array).byteOffset === "number" ||
+    typeof (input as Uint8Array).byteLength === "number";
+
+  if (!isArrayLike && !isUint8Array) {
     throw new Error("@smithy/util-base64: toBase64 encoder function only accepts string | Uint8Array.");
   }
+
   let str = "";
   for (let i = 0; i < input.length; i += 3) {
     let bits = 0;