Node FS: Guard on require() existing before requiring. See #17851

Author: kripken

src/node_shell_read.js

@@ -4,9 +4,20 @@
  * SPDX-License-Identifier: MIT
  */
 
-// These modules will usually be used on Node.js.
-var fs = require('fs');
-var nodePath = require('path');
+// These modules will usually be used on Node.js. Load them eagerly to avoid
+// the complexity of lazy-loading. However, for now we must guard on require()
+// actually existing: if the JS is put in a .mjs file (ES6 module) and run on
+// node, then we'll detect node as the environment and get here, but require()
+// does not exist (since ES6 modules should use |import|). If the code actually
+// uses the node filesystem then it will crash, of course, but in the case of
+// code that never uses it we don't want to crash here, so the guarding if lets
+// such code work properly. See discussion in
+// https://github.com/emscripten-core/emscripten/pull/17851
+var fs, nodePath;
+if (typeof require === 'function') {
+  fs = require('fs');
+  nodePath = require('path');
+}
 
 read_ = (filename, binary) => {
 #if SUPPORT_BASE64_EMBEDDING