Support ES6 Set/Map in stringifyWithFunctions

src/jsifier.mjs

@@ -33,6 +33,7 @@ import {
   compileTimeContext,
   printErr,
   readFile,
+  runInMacroContext,
   warn,
   warnOnce,
   warningOccured,
@@ -79,6 +80,19 @@ function stringifyWithFunctions(obj) {
   if (Array.isArray(obj)) {
     return '[' + obj.map(stringifyWithFunctions).join(',') + ']';
   }
+
+  // preserve the type of the object if it is one of [Map, Set, WeakMap, WeakSet].
+  const builtinContainers = runInMacroContext('[Map, Set, WeakMap, WeakSet]', {
+    filename: '<internal>',
+  });
+  for (const container of builtinContainers) {
+    if (obj instanceof container) {
+      const className = container.name;
+      assert(!obj.size, `cannot stringify ${className} with data`);
+      return `new ${className}`;
+    }
+  }
+
   var rtn = '{\n';
   for (const [key, value] of Object.entries(obj)) {
     var str = stringifyWithFunctions(value);

test/test_other.py

@@ -4974,6 +4974,34 @@ def test_jslib_has_library(self):
     ''')
     self.do_runf(test_file('hello_world.c'), emcc_args=['-L', '-lfoo.js'])
 
+  def test_jslib_new_objects_basic(self):
+    create_file('lib.js', '''
+      addToLibrary({
+        $obj: {
+          a: new Map(),
+          b: new Set(),
+          c: new WeakMap(),
+          d: new WeakSet()
+        }
+      });
+      ''')
+    self.run_process([EMCC, test_file('hello_world.c'), '--js-library=lib.js', '-sEXPORTED_FUNCTIONS=obj,_main'])
+    self.assertContained("a:new Map,", read_file('a.out.js'))
+    self.assertContained("b:new Set,", read_file('a.out.js'))
+    self.assertContained("c:new WeakMap,", read_file('a.out.js'))
+    self.assertContained("d:new WeakSet,", read_file('a.out.js'))
+
+  def test_jslib_new_objects_non_empty(self):
+    create_file('lib.js', '''
+      addToLibrary({
+        $obj: {
+          bad: new Map([[1,2],[3,4]])
+        }
+      });
+      ''')
+    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--js-library=lib.js', '-sEXPORTED_FUNCTIONS=obj,_main'])
+    self.assertContained('cannot stringify Map with data', err)
+
   def test_EMCC_BUILD_DIR(self):
     # EMCC_BUILD_DIR was necessary in the past since we used to force the cwd to be src/ for
     # technical reasons.