diff --git a/pkg/_js_interop_checks/lib/src/transformations/js_util_wasm_optimizer.dart b/pkg/_js_interop_checks/lib/src/transformations/js_util_wasm_optimizer.dart
index 878fcfa28c3..1e7169f29fa 100644
--- a/pkg/_js_interop_checks/lib/src/transformations/js_util_wasm_optimizer.dart
+++ b/pkg/_js_interop_checks/lib/src/transformations/js_util_wasm_optimizer.dart
@@ -240,7 +240,8 @@ class JsUtilWasmOptimizer extends Transformer {
   /// [String] function name representing the name of the wrapping function.
   /// TODO(joshualitt): Share callback trampolines if the [FunctionType]
   /// matches.
-  String _createFunctionTrampoline(Procedure node, FunctionType function) {
+  String _createFunctionTrampoline(
+      Procedure node, FunctionType function, Expression argument) {
     int fileOffset = node.fileOffset;
 
     // Create arguments for each positional parameter in the function. These
@@ -257,11 +258,50 @@ class JsUtilWasmOptimizer extends Transformer {
     List<Expression> callbackArguments = [];
     DartType nullableObjectType =
         _objectClass.getThisType(_coreTypes, Nullability.nullable);
-    for (DartType type in function.positionalParameters) {
+    for (int i = 0; i < function.positionalParameters.length; i++) {
+      DartType type = function.positionalParameters[i];
+      Expression? defaultExpression;
+      bool hasDefault = i >= function.requiredParameterCount;
+      if (hasDefault) {
+        // We can only generate default values if we have a statically typed
+        // function argument.
+        Expression? initializer;
+        if (argument is ConstantExpression) {
+          Procedure callbackTarget = (argument.constant as TearOffConstant)
+              .targetReference
+              .asProcedure;
+          initializer =
+              callbackTarget.function.positionalParameters[i].initializer;
+        } else if (argument is FunctionExpression) {
+          initializer = argument.function.positionalParameters[i].initializer;
+        } else {
+          throw 'Cannot pass default arguments.';
+        }
+
+        // The initializer for a default argument must be a
+        // [ConstantExpression].
+        ConstantExpression init = initializer as ConstantExpression;
+        defaultExpression = ConstantExpression(init.constant, init.type);
+      }
       VariableDeclaration variable =
           VariableDeclaration('x${parameterId++}', type: nullableObjectType);
       positionalParameters.add(variable);
-      callbackArguments.add(AsExpression(VariableGet(variable), type));
+      Expression body;
+      if (hasDefault) {
+        body = ConditionalExpression(
+            StaticInvocation(
+                _coreTypes.identicalProcedure,
+                Arguments([
+                  VariableGet(variable),
+                  ConstantExpression(NullConstant())
+                ])),
+            defaultExpression ?? ConstantExpression(NullConstant()),
+            VariableGet(variable),
+            nullableObjectType);
+      } else {
+        body = VariableGet(variable);
+      }
+      callbackArguments.add(AsExpression(body, type));
     }
 
     // Create a new procedure for the callback trampoline. This procedure will
@@ -308,11 +348,17 @@ class JsUtilWasmOptimizer extends Transformer {
   /// [_createFunctionTrampoline] followed by `_wrapDartFunction`.
   StaticInvocation _allowInterop(
       Procedure node, FunctionType type, Expression argument) {
-    String functionTrampolineName = _createFunctionTrampoline(node, type);
+    String functionTrampolineName =
+        _createFunctionTrampoline(node, type, argument);
     return StaticInvocation(
         _wrapDartFunctionTarget,
-        Arguments([argument, StringLiteral(functionTrampolineName)],
-            types: [type]));
+        Arguments([
+          argument,
+          StringLiteral(functionTrampolineName),
+          ConstantExpression(IntConstant(type.positionalParameters.length))
+        ], types: [
+          type
+        ]));
   }
 
   StaticGet get _globalThis => StaticGet(_globalThisMember);
diff --git a/pkg/dart2wasm/bin/run_wasm.js b/pkg/dart2wasm/bin/run_wasm.js
index cc42c376af7..9edf69fd82c 100644
--- a/pkg/dart2wasm/bin/run_wasm.js
+++ b/pkg/dart2wasm/bin/run_wasm.js
@@ -150,8 +150,13 @@ var dart2wasm = {
     },
     stringFromDartString: stringFromDartString,
     stringToDartString: stringToDartString,
-    wrapDartFunction: function(dartFunction, exportFunctionName) {
+    wrapDartFunction: function(dartFunction, exportFunctionName, argCount) {
         var wrapped = function (...args) {
+            // Pad `undefined` for optional arguments that aren't passed so that
+            // the trampoline can replace these values with defaults.
+            while (args.length < argCount) {
+                args.push(undefined);
+            }
             return dartInstance.exports[`${exportFunctionName}`](
                 dartFunction, ...args.map(dartInstance.exports.$dartifyRaw));
         }
diff --git a/sdk/lib/_internal/wasm/lib/js_helper.dart b/sdk/lib/_internal/wasm/lib/js_helper.dart
index ec24b666803..8e493286682 100644
--- a/sdk/lib/_internal/wasm/lib/js_helper.dart
+++ b/sdk/lib/_internal/wasm/lib/js_helper.dart
@@ -510,15 +510,18 @@ List<Object?> toDartList(WasmExternRef? ref) => List<Object?>.generate(
     objectLength(ref).round(), (int n) => dartifyRaw(objectReadIndex(ref, n)));
 
 @pragma("wasm:import", "dart2wasm.wrapDartFunction")
-external WasmExternRef? _wrapDartFunctionRaw(
-    WasmExternRef? dartFunction, WasmExternRef? trampolineName);
+external WasmExternRef? _wrapDartFunctionRaw(WasmExternRef? dartFunction,
+    WasmExternRef? trampolineName, WasmExternRef? argCount);
 
-F _wrapDartFunction<F extends Function>(F f, String trampolineName) {
+F _wrapDartFunction<F extends Function>(
+    F f, String trampolineName, int argCount) {
   if (functionToJSWrapper.containsKey(f)) {
     return f;
   }
   JSValue wrappedFunction = JSValue(_wrapDartFunctionRaw(
-      f.toJS().toExternRef(), trampolineName.toJS().toExternRef())!);
+      f.toJS().toExternRef(),
+      trampolineName.toJS().toExternRef(),
+      argCount.toDouble().toJS().toExternRef())!);
   functionToJSWrapper[f] = wrappedFunction;
   return f;
 }
diff --git a/tests/web/wasm/callback_test.dart b/tests/web/wasm/callback_test.dart
index 2530977d3b9..24d35d371e4 100644
--- a/tests/web/wasm/callback_test.dart
+++ b/tests/web/wasm/callback_test.dart
@@ -11,51 +11,148 @@ import 'package:js/js.dart';
 @JS()
 external void eval(String code);
 
-typedef SumStringCallback = String Function(String a, String b);
+typedef SumTwoPositionalFun = String Function(String a, String b);
+typedef SumOnePositionalAndOneOptionalFun = String Function(String a,
+    [String? b]);
+typedef SumTwoOptionalFun = String Function([String? a, String? b]);
+typedef SumOnePositionalAndOneOptionalNonNullFun = String Function(String a,
+    [String b]);
+typedef SumTwoOptionalNonNullFun = String Function([String a, String b]);
 
 @JS()
 @staticInterop
 class DartFromJSCallbackHelper {
-  external factory DartFromJSCallbackHelper.factory(SumStringCallback summer);
+  external factory DartFromJSCallbackHelper.factory(
+      SumTwoPositionalFun sumTwoPositional,
+      SumOnePositionalAndOneOptionalFun sumOnePositionalOneOptional,
+      SumTwoOptionalFun sumTwoOptional,
+      SumOnePositionalAndOneOptionalNonNullFun
+          sumOnePositionalAndOneOptionalNonNull,
+      SumTwoOptionalNonNullFun sumTwoOptionalNonNull);
 }
 
 extension DartFromJSCallbackHelperMethods on DartFromJSCallbackHelper {
   external String doSum1();
   external String doSum2(String a, String b);
   external String doSum3(Object summer);
+
+  external String doSumOnePositionalAndOneOptionalA(String a);
+  external String doSumOnePositionalAndOneOptionalB(String a, String b);
+  external String doSumTwoOptionalA();
+  external String doSumTwoOptionalB(String a);
+  external String doSumTwoOptionalC(String a, String b);
+
+  external String doSumOnePositionalAndOneOptionalANonNull(String a);
+  external String doSumOnePositionalAndOneOptionalBNonNull(String a, String b);
+  external String doSumTwoOptionalANonNull();
+  external String doSumTwoOptionalBNonNull(String a);
+  external String doSumTwoOptionalCNonNull(String a, String b);
 }
 
-String sumString(String a, String b) {
+String sumTwoPositional(String a, String b) {
+  return a + b;
+}
+
+String sumOnePositionalAndOneOptional(String a, [String? b]) {
+  return a + (b ?? 'bar');
+}
+
+String sumTwoOptional([String? a, String? b]) {
+  return (a ?? 'foo') + (b ?? 'bar');
+}
+
+String sumOnePositionalAndOneOptionalNonNull(String a, [String b = 'bar']) {
+  return a + b;
+}
+
+String sumTwoOptionalNonNull([String a = 'foo', String b = 'bar']) {
   return a + b;
 }
 
 void staticInteropCallbackTest() {
   eval(r'''
-    globalThis.DartFromJSCallbackHelper = function(summer) {
+    globalThis.DartFromJSCallbackHelper = function(
+        sumTwoPositional, sumOnePositionalOneOptional, sumTwoOptional,
+        sumOnePositionalAndOneOptionalNonNull, sumTwoOptionalNonNull) {
       this.a = 'hello ';
       this.b = 'world!';
       this.sum = null;
-      this.summer = summer;
+      this.sumTwoPositional = sumTwoPositional;
+      this.sumOnePositionalOneOptional = sumOnePositionalOneOptional;
+      this.sumTwoOptional = sumTwoOptional;
+      this.sumOnePositionalAndOneOptionalNonNull = sumOnePositionalAndOneOptionalNonNull;
+      this.sumTwoOptionalNonNull = sumTwoOptionalNonNull;
       this.doSum1 = () => {
-        return this.summer(this.a, this.b);
+        return this.sumTwoPositional(this.a, this.b);
       }
       this.doSum2 = (a, b) => {
-        return this.summer(a, b);
+        return this.sumTwoPositional(a, b);
       }
       this.doSum3 = (summer) => {
         return summer(this.a, this.b);
       }
+      this.doSumOnePositionalAndOneOptionalA = (a) => {
+        return sumOnePositionalOneOptional(a);
+      }
+      this.doSumOnePositionalAndOneOptionalB = (a, b) => {
+        return sumOnePositionalOneOptional(a, b);
+      }
+      this.doSumTwoOptionalA = () => {
+        return sumTwoOptional();
+      }
+      this.doSumTwoOptionalB = (a) => {
+        return sumTwoOptional(a);
+      }
+      this.doSumTwoOptionalC = (a, b) => {
+        return sumTwoOptional(a, b);
+      }
+      this.doSumOnePositionalAndOneOptionalANonNull = (a) => {
+        return sumOnePositionalAndOneOptionalNonNull(a);
+      }
+      this.doSumOnePositionalAndOneOptionalBNonNull = (a, b) => {
+        return sumOnePositionalAndOneOptionalNonNull(a, b);
+      }
+      this.doSumTwoOptionalANonNull = () => {
+        return sumTwoOptionalNonNull();
+      }
+      this.doSumTwoOptionalBNonNull = (a) => {
+        return sumTwoOptionalNonNull(a);
+      }
+      this.doSumTwoOptionalCNonNull = (a, b) => {
+        return sumTwoOptionalNonNull(a, b);
+      }
+
     }
   ''');
 
-  final dartFromJSCallbackHelper = DartFromJSCallbackHelper.factory(
-      allowInterop<SumStringCallback>(sumString));
-  Expect.equals('hello world!', dartFromJSCallbackHelper.doSum1());
-  Expect.equals('foobar', dartFromJSCallbackHelper.doSum2('foo', 'bar'));
+  final helper = DartFromJSCallbackHelper.factory(
+      allowInterop<SumTwoPositionalFun>(sumTwoPositional),
+      allowInterop<SumOnePositionalAndOneOptionalFun>(
+          sumOnePositionalAndOneOptional),
+      allowInterop<SumTwoOptionalFun>(sumTwoOptional),
+      allowInterop<SumOnePositionalAndOneOptionalNonNullFun>(
+          sumOnePositionalAndOneOptionalNonNull),
+      allowInterop<SumTwoOptionalNonNullFun>(sumTwoOptionalNonNull));
+
+  Expect.equals('hello world!', helper.doSum1());
+  Expect.equals('foobar', helper.doSum2('foo', 'bar'));
+  Expect.equals('hello world!',
+      helper.doSum3(allowInterop<SumTwoPositionalFun>((a, b) => a + b)));
+
+  Expect.equals('foobar', helper.doSumOnePositionalAndOneOptionalA('foo'));
   Expect.equals(
-      'hello world!',
-      dartFromJSCallbackHelper
-          .doSum3(allowInterop<SumStringCallback>((a, b) => a + b)));
+      'foobar', helper.doSumOnePositionalAndOneOptionalB('foo', 'bar'));
+  Expect.equals('foobar', helper.doSumTwoOptionalA());
+  Expect.equals('foobar', helper.doSumTwoOptionalB('foo'));
+  Expect.equals('foobar', helper.doSumTwoOptionalC('foo', 'bar'));
+
+  Expect.equals(
+      'foobar', helper.doSumOnePositionalAndOneOptionalANonNull('foo'));
+  Expect.equals(
+      'foobar', helper.doSumOnePositionalAndOneOptionalBNonNull('foo', 'bar'));
+  Expect.equals('foobar', helper.doSumTwoOptionalANonNull());
+  Expect.equals('foobar', helper.doSumTwoOptionalBNonNull('foo'));
+  Expect.equals('foobar', helper.doSumTwoOptionalCNonNull('foo', 'bar'));
 }
 
 void allowInteropCallbackTest() {
@@ -66,17 +163,109 @@ void allowInteropCallbackTest() {
     globalThis.doSum2 = function(a, b) {
       return globalThis.summer(a, b);
     }
+    globalThis.doSumOnePositionalAndOneOptionalA = function(a) {
+      return summer(a);
+    }
+    globalThis.doSumOnePositionalAndOneOptionalB = function(a, b) {
+      return summer(a, b);
+    }
+    globalThis.doSumTwoOptionalA = function() {
+      return summer();
+    }
+    globalThis.doSumTwoOptionalB = function(a) {
+      return summer(a);
+    }
+    globalThis.doSumTwoOptionalC = function(a, b) {
+      return summer(a, b);
+    }
+    globalThis.doSumOnePositionalAndOneOptionalANonNull = function(a) {
+      return summer(a);
+    }
+    globalThis.doSumOnePositionalAndOneOptionalBNonNull = function(a, b) {
+      return summer(a, b);
+    }
+    globalThis.doSumTwoOptionalANonNull = function() {
+      return summer();
+    }
+    globalThis.doSumTwoOptionalBNonNull = function(a) {
+      return summer(a);
+    }
+    globalThis.doSumTwoOptionalCNonNull = function(a, b) {
+      return summer(a, b);
+    }
   ''');
 
-  final interopCallback = allowInterop<SumStringCallback>((a, b) => a + b);
-  Expect.equals(
-      'foobar', callMethod(globalThis, 'doSum1', [interopCallback]).toString());
-  setProperty(globalThis, 'summer', interopCallback);
-  Expect.equals(
-      'foobar', callMethod(globalThis, 'doSum2', ['foo', 'bar']).toString());
-  final roundTripCallback = getProperty(globalThis, 'summer');
-  Expect.equals('foobar',
-      (dartify(roundTripCallback) as SumStringCallback)('foo', 'bar'));
+  // General
+  {
+    final interopCallback = allowInterop<SumTwoPositionalFun>((a, b) => a + b);
+    Expect.equals('foobar',
+        callMethod(globalThis, 'doSum1', [interopCallback]).toString());
+    setProperty(globalThis, 'summer', interopCallback);
+    Expect.equals(
+        'foobar', callMethod(globalThis, 'doSum2', ['foo', 'bar']).toString());
+    final roundTripCallback = getProperty(globalThis, 'summer');
+    Expect.equals('foobar',
+        (dartify(roundTripCallback) as SumTwoPositionalFun)('foo', 'bar'));
+  }
+
+  // 1 nullable optional argument
+  {
+    final interopCallback = allowInterop<SumOnePositionalAndOneOptionalFun>(
+        (a, [b]) => a + (b ?? 'bar'));
+    setProperty(globalThis, 'summer', interopCallback);
+    Expect.equals(
+        'foobar',
+        callMethod(globalThis, 'doSumOnePositionalAndOneOptionalA', ['foo'])
+            .toString());
+    Expect.equals(
+        'foobar',
+        callMethod(
+                globalThis, 'doSumOnePositionalAndOneOptionalB', ['foo', 'bar'])
+            .toString());
+  }
+
+  // All nullable optional arguments
+  {
+    final interopCallback = allowInterop<SumTwoOptionalFun>(
+        ([a, b]) => (a ?? 'foo') + (b ?? 'bar'));
+    setProperty(globalThis, 'summer', interopCallback);
+    Expect.equals(
+        'foobar', callMethod(globalThis, 'doSumTwoOptionalA', []).toString());
+    Expect.equals('foobar',
+        callMethod(globalThis, 'doSumTwoOptionalB', ['foo']).toString());
+    Expect.equals('foobar',
+        callMethod(globalThis, 'doSumTwoOptionalC', ['foo', 'bar']).toString());
+  }
+
+  // 1 non-nullable optional argument
+  {
+    final interopCallback =
+        allowInterop<SumOnePositionalAndOneOptionalNonNullFun>(
+            (a, [b = 'bar']) => a + b);
+    setProperty(globalThis, 'summer', interopCallback);
+    Expect.equals(
+        'foobar',
+        callMethod(globalThis, 'doSumOnePositionalAndOneOptionalA', ['foo'])
+            .toString());
+    Expect.equals(
+        'foobar',
+        callMethod(
+                globalThis, 'doSumOnePositionalAndOneOptionalB', ['foo', 'bar'])
+            .toString());
+  }
+
+  // All non-nullable optional arguments
+  {
+    final interopCallback = allowInterop<SumTwoOptionalNonNullFun>(
+        ([a = 'foo', b = 'bar']) => a + b);
+    setProperty(globalThis, 'summer', interopCallback);
+    Expect.equals(
+        'foobar', callMethod(globalThis, 'doSumTwoOptionalA', []).toString());
+    Expect.equals('foobar',
+        callMethod(globalThis, 'doSumTwoOptionalB', ['foo']).toString());
+    Expect.equals('foobar',
+        callMethod(globalThis, 'doSumTwoOptionalC', ['foo', 'bar']).toString());
+  }
 }
 
 void main() {
diff --git a/tests/web/wasm/static_interop_test.dart b/tests/web/wasm/static_interop_test.dart
index 90656c2ed77..c6423347a84 100644
--- a/tests/web/wasm/static_interop_test.dart
+++ b/tests/web/wasm/static_interop_test.dart
@@ -24,6 +24,10 @@ extension StaticJSClassMethods on StaticJSClass {
   external int? get nullableInt;
   external int nonNullableIntReturnMethod();
   external int? nullableIntReturnMethod(bool returnNull);
+  external String doSum1Or2(String a, [String? b]);
+  external String doSumUpTo2([String? a, String? b]);
+  external String doSum1Or2NonNull(String a, [String b = 'bar']);
+  external String doSumUpTo2NonNull([String a = 'foo', String b = 'bar']);
 }
 
 void createClassTest() {
@@ -49,6 +53,18 @@ void createClassTest() {
         this.nonNullableInt = 60.5;
         this.nullableInt = 100.5;
       }
+      this.doSum1Or2 = function(a, b) {
+        return a + (b ?? 'bar');
+      }
+      this.doSumUpTo2 = function(a, b) {
+        return (a ?? 'foo') + (b ?? 'bar');
+      }
+      this.doSum1Or2NonNull = function(a, b) {
+        return a + b;
+      }
+      this.doSumUpTo2NonNull = function(a, b) {
+        return a + b;
+      }
     }
   ''');
   final foo = StaticJSClass.factory('foo');
@@ -68,6 +84,18 @@ void createClassTest() {
   foo.doublifyNumbers();
   Expect.equals(100, foo.nullableInt);
   Expect.equals(60, foo.nonNullableInt);
+
+  Expect.equals('foobar', foo.doSum1Or2('foo'));
+  Expect.equals('foobar', foo.doSum1Or2('foo', 'bar'));
+  Expect.equals('foobar', foo.doSumUpTo2());
+  Expect.equals('foobar', foo.doSumUpTo2('foo'));
+  Expect.equals('foobar', foo.doSumUpTo2('foo', 'bar'));
+
+  Expect.equals('foobar', foo.doSum1Or2NonNull('foo'));
+  Expect.equals('foobar', foo.doSum1Or2NonNull('foo', 'bar'));
+  Expect.equals('foobar', foo.doSumUpTo2NonNull());
+  Expect.equals('foobar', foo.doSumUpTo2NonNull('foo'));
+  Expect.equals('foobar', foo.doSumUpTo2NonNull('foo', 'bar'));
 }
 
 @JS('JSClass.NestedJSClass')