child_process: revise argument processing

execFile() and fork() have complicated argument processing. Clarify code
and avoid using `arguments`.

Author: Trott

lib/child_process.js

@@ -111,28 +111,27 @@ const MAX_BUFFER = 1024 * 1024;
  *   }} [options]
  * @returns {ChildProcess}
  */
-function fork(modulePath /* , args, options */) {
+function fork(modulePath, args = [], options = {}) {
   modulePath = getValidatedPath(modulePath, 'modulePath');
 
   // Get options and args arguments.
   let execArgv;
-  let options = {};
-  let args = [];
-  let pos = 1;
-  if (pos < arguments.length && ArrayIsArray(arguments[pos])) {
-    args = arguments[pos++];
-  }
 
-  if (pos < arguments.length && arguments[pos] == null) {
-    pos++;
+  if (args == null) {
+    args = [];
+  } else if (typeof args !== 'object') {
+    throw new ERR_INVALID_ARG_VALUE('args', args);
+  } else if (!ArrayIsArray(args)) {
+    options = args;
+    args = [];
   }
 
-  if (pos < arguments.length && arguments[pos] != null) {
-    if (typeof arguments[pos] !== 'object') {
-      throw new ERR_INVALID_ARG_VALUE(`arguments[${pos}]`, arguments[pos]);
-    }
-
-    options = { ...arguments[pos++] };
+  if (options == null) {
+    options = {};
+  } else if (typeof options !== 'object') {
+    throw new ERR_INVALID_ARG_VALUE('options', options);
+  } else {
+    options = { ...options };
   }
 
   // Prepare arguments for fork:
@@ -276,31 +275,34 @@ ObjectDefineProperty(exec, promisify.custom, {
  *   ) => any} [callback]
  * @returns {ChildProcess}
  */
-function execFile(file /* , args, options, callback */) {
-  let args = [];
-  let callback;
-  let options;
-
-  // Parse the optional positional parameters.
-  let pos = 1;
-  if (pos < arguments.length && ArrayIsArray(arguments[pos])) {
-    args = arguments[pos++];
-  } else if (pos < arguments.length && arguments[pos] == null) {
-    pos++;
-  }
-
-  if (pos < arguments.length && typeof arguments[pos] === 'object') {
-    options = arguments[pos++];
-  } else if (pos < arguments.length && arguments[pos] == null) {
-    pos++;
+function execFile(file, args = [], options, callback) {
+  if (args == null) {
+    args = [];
+  } else if (typeof args === 'object') {
+    if (!ArrayIsArray(args)) {
+      callback = options;
+      options = args;
+      args = [];
+    }
+  } else if (typeof args === 'function') {
+    callback = args;
+    options = {};
+    args = [];
+  } else {
+    throw new ERR_INVALID_ARG_VALUE('args', args);
   }
 
-  if (pos < arguments.length && typeof arguments[pos] === 'function') {
-    callback = arguments[pos++];
+  if (options == null) {
+    options = {};
+  } else if (typeof options === 'function') {
+    callback = options;
+    options = {};
+  } else if (typeof options !== 'object') {
+    throw new ERR_INVALID_ARG_VALUE('options', options);
   }
 
-  if (!callback && pos < arguments.length && arguments[pos] != null) {
-    throw new ERR_INVALID_ARG_VALUE('args', arguments[pos]);
+  if (callback && typeof callback !== 'function') {
+    throw new ERR_INVALID_ARG_VALUE('callback', callback);
   }
 
   options = {