fix: type ironing with string buffer

packages/hash-node/src/index.ts

@@ -1,6 +1,10 @@
 import { Hash as IHash, SourceData } from "@aws-sdk/types";
 import { Buffer } from "buffer";
-import { fromArrayBuffer, fromString } from "@aws-sdk/util-buffer-from";
+import {
+  fromArrayBuffer,
+  fromString,
+  StringEncoding
+} from "@aws-sdk/util-buffer-from";
 import { createHash, createHmac, Hash as NodeHash, Hmac } from "crypto";
 
 export class Hash implements IHash {
@@ -21,7 +25,7 @@ export class Hash implements IHash {
   }
 }
 
-function castSourceData(toCast: SourceData, encoding?: string): Buffer {
+function castSourceData(toCast: SourceData, encoding?: StringEncoding): Buffer {
   if (Buffer.isBuffer(toCast)) {
     return toCast;
   }

packages/util-buffer-from/src/index.ts

@@ -27,27 +27,23 @@ export function fromArrayBuffer(
   }
   return new Buffer(input);
 }
-
-export function fromString(
-  input: string,
-  encoding?:
-    | "ascii"
-    | "utf8"
-    | "utf16le"
-    | "ucs2"
-    | "base64"
-    | "latin1"
-    | "binary"
-    | "hex"
-    | string
-): Buffer {
+export type StringEncoding =
+  | "ascii"
+  | "utf8"
+  | "utf16le"
+  | "ucs2"
+  | "base64"
+  | "latin1"
+  | "binary"
+  | "hex";
+export function fromString(input: string, encoding?: StringEncoding): Buffer {
   if (typeof input !== "string") {
     throw new Error("argument passed to fromString was not a string");
   }
 
   if (typeof Buffer.from === "function" && Buffer.from !== Uint8Array.from) {
-    return Buffer.from(input, encoding);
+    return Buffer.from(input, encoding as any);
   }
 
-  return new Buffer(input, encoding);
+  return new Buffer(input, encoding as any);
 }