src: prefer param function check over args length

PR-URL: #23835
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: John-David Dalton <john.david.dalton@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>

lib/fs.js

@@ -418,7 +418,7 @@ function open(path, flags, mode, callback) {
     callback = flags;
     flags = 'r';
     mode = 0o666;
-  } else if (arguments.length === 3) {
+  } else if (typeof mode === 'function') {
     callback = mode;
     mode = 0o666;
   }
@@ -811,7 +811,7 @@ function readdirSync(path, options) {
 }
 
 function fstat(fd, options, callback) {
-  if (arguments.length < 3) {
+  if (typeof options === 'function') {
     callback = options;
     options = {};
   }
@@ -822,7 +822,7 @@ function fstat(fd, options, callback) {
 }
 
 function lstat(path, options, callback) {
-  if (arguments.length < 3) {
+  if (typeof options === 'function') {
     callback = options;
     options = {};
   }
@@ -835,7 +835,7 @@ function lstat(path, options, callback) {
 }
 
 function stat(path, options, callback) {
-  if (arguments.length < 3) {
+  if (typeof options === 'function') {
     callback = options;
     options = {};
   }