1 files changed, 120 insertions, 4 deletions
diff --git a/std/node/_fs/_fs_common.ts b/std/node/_fs/_fs_common.ts
index ebdd28d64..1f00bc481 100644
--- a/std/node/_fs/_fs_common.ts
+++ b/std/node/_fs/_fs_common.ts
@@ -1,21 +1,137 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 
-export type CallbackWithError = (err?: Error) => void;
+import { notImplemented } from "../_utils.ts";
+
+export type CallbackWithError = (err?: Error | null) => void;
 
 export interface FileOptions {
   encoding?: string;
-  mode?: number;
   flag?: string;
 }
 
+export interface WriteFileOptions extends FileOptions {
+  mode?: number;
+}
+
 export function isFileOptions(
-  fileOptions: string | FileOptions | undefined
+  fileOptions: string | WriteFileOptions | undefined
 ): fileOptions is FileOptions {
   if (!fileOptions) return false;
 
   return (
     (fileOptions as FileOptions).encoding != undefined ||
     (fileOptions as FileOptions).flag != undefined ||
-    (fileOptions as FileOptions).mode != undefined
+    (fileOptions as WriteFileOptions).mode != undefined
   );
 }
+
+export function getEncoding(
+  optOrCallback?: FileOptions | WriteFileOptions | Function | string
+): string | null {
+  if (!optOrCallback || typeof optOrCallback === "function") {
+    return null;
+  }
+
+  const encoding =
+    typeof optOrCallback === "string" ? optOrCallback : optOrCallback.encoding;
+  if (!encoding) return null;
+  if (encoding === "utf8" || encoding === "utf-8") {
+    return "utf8";
+  }
+  if (encoding === "buffer") {
+    return "buffer";
+  }
+
+  const notImplementedEncodings = [
+    "utf16le",
+    "latin1",
+    "base64",
+    "hex",
+    "ascii",
+    "binary",
+    "ucs2",
+  ];
+
+  if (notImplementedEncodings.includes(encoding)) {
+    notImplemented(`"${encoding}" encoding`);
+  }
+
+  throw new Error(`The value "${encoding}" is invalid for option "encoding"`);
+}
+
+export function getOpenOptions(flag: string | undefined): Deno.OpenOptions {
+  if (!flag) {
+    return { create: true, append: true };
+  }
+
+  let openOptions: Deno.OpenOptions;
+  switch (flag) {
+    case "a": {
+      // 'a': Open file for appending. The file is created if it does not exist.
+      openOptions = { create: true, append: true };
+      break;
+    }
+    case "ax": {
+      // 'ax': Like 'a' but fails if the path exists.
+      openOptions = { createNew: true, write: true, append: true };
+      break;
+    }
+    case "a+": {
+      // 'a+': Open file for reading and appending. The file is created if it does not exist.
+      openOptions = { read: true, create: true, append: true };
+      break;
+    }
+    case "ax+": {
+      // 'ax+': Like 'a+' but fails if the path exists.
+      openOptions = { read: true, createNew: true, append: true };
+      break;
+    }
+    case "r": {
+      // 'r': Open file for reading. An exception occurs if the file does not exist.
+      openOptions = { read: true };
+      break;
+    }
+    case "r+": {
+      // 'r+': Open file for reading and writing. An exception occurs if the file does not exist.
+      openOptions = { read: true, write: true };
+      break;
+    }
+    case "w": {
+      // 'w': Open file for writing. The file is created (if it does not exist) or truncated (if it exists).
+      openOptions = { create: true, write: true, truncate: true };
+      break;
+    }
+    case "wx": {
+      // 'wx': Like 'w' but fails if the path exists.
+      openOptions = { createNew: true, write: true };
+      break;
+    }
+    case "w+": {
+      // 'w+': Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists).
+      openOptions = { create: true, write: true, truncate: true, read: true };
+      break;
+    }
+    case "wx+": {
+      // 'wx+': Like 'w+' but fails if the path exists.
+      openOptions = { createNew: true, write: true, read: true };
+      break;
+    }
+    case "as": {
+      // 'as': Open file for appending in synchronous mode. The file is created if it does not exist.
+      openOptions = { create: true, append: true };
+    }
+    case "as+": {
+      // 'as+': Open file for reading and appending in synchronous mode. The file is created if it does not exist.
+      openOptions = { create: true, read: true, append: true };
+    }
+    case "rs+": {
+      // 'rs+': Open file for reading and writing in synchronous mode. Instructs the operating system to bypass the local file system cache.
+      openOptions = { create: true, read: true, write: true };
+    }
+    default: {
+      throw new Error(`Unrecognized file system flag: ${flag}`);
+    }
+  }
+
+  return openOptions;
+}