diff --git a/src/org/mozilla/javascript/IRFactory.java b/src/org/mozilla/javascript/IRFactory.java
index de26e57a19..6c9d0594c0 100644
--- a/src/org/mozilla/javascript/IRFactory.java
+++ b/src/org/mozilla/javascript/IRFactory.java
@@ -641,6 +641,32 @@ private Node transformFunction(FunctionNode fn) {
                 decompiler.addToken(Token.EOL);
             }
 
+            /* Process default parameters */
+            if (fn.getDefaultParams() != null) {
+                Object[] defaultParams = fn.getDefaultParams();
+                for (int i = defaultParams.length - 1; i > 0; ) {
+                    AstNode rhs = (AstNode) defaultParams[i];
+                    String name = (String) defaultParams[i - 1];
+                    /* TODO: default params - error handling */
+                    body.addChildToFront(
+                            createIf(
+                                    createBinary(
+                                            Token.SHEQ,
+                                            new Name(fn.getPosition(), name),
+                                            new Name(fn.getPosition(), "undefined")),
+                                    new Node(
+                                            Token.EXPR_VOID,
+                                            createAssignment(
+                                                    Token.ASSIGN,
+                                                    new Name(fn.getPosition(), name),
+                                                    transform(rhs)),
+                                            body.getLineno()),
+                                    null,
+                                    body.getLineno()));
+                    i -= 2;
+                }
+            }
+
             if (destructuring != null) {
                 body.addChildToFront(new Node(Token.EXPR_VOID, destructuring, lineno));
             }
diff --git a/src/org/mozilla/javascript/Parser.java b/src/org/mozilla/javascript/Parser.java
index 8a31a212a4..5ea39165fc 100644
--- a/src/org/mozilla/javascript/Parser.java
+++ b/src/org/mozilla/javascript/Parser.java
@@ -848,6 +848,29 @@ private void parseFunctionParams(FunctionNode fnNode) throws IOException {
                             addError("msg.dup.param.strict", paramName);
                         paramNames.add(paramName);
                     }
+                    if (matchToken(Token.ASSIGN, true)) {
+                        /* TODO: Same for arrowFunctionParams below. Refactor into a helper. */
+                        AstNode rhs = assignExpr();
+                        Object[] existing = fnNode.getDefaultParams();
+                        Object[] current;
+                        int current_size = 0;
+                        if (existing == null) {
+                            existing = new Object[2];
+                            fnNode.setDefaultParams(existing);
+                            current = existing;
+                        } else {
+                            current =
+                                    new Object
+                                            [existing.length
+                                                    + 2]; /* TODO: A more flexible structure? */
+                            System.arraycopy(existing, 0, current, 0, existing.length);
+                            current_size = existing.length;
+                            existing = null;
+                        }
+                        current[current_size] = paramName;
+                        current[current.length - 1] = rhs;
+                        fnNode.setDefaultParams(current);
+                    }
                 } else {
                     fnNode.addParam(makeErrorNode());
                 }
@@ -1046,7 +1069,8 @@ private void arrowFunctionParams(
             FunctionNode fnNode,
             AstNode params,
             Map<String, Node> destructuring,
-            Set<String> paramNames) {
+            Set<String> paramNames)
+            throws IOException {
         if (params instanceof ArrayLiteral || params instanceof ObjectLiteral) {
             markDestructuring(params);
             fnNode.addParam(params);
diff --git a/src/org/mozilla/javascript/ast/FunctionNode.java b/src/org/mozilla/javascript/ast/FunctionNode.java
index 7c5d07d6e8..c08aab2e2f 100644
--- a/src/org/mozilla/javascript/ast/FunctionNode.java
+++ b/src/org/mozilla/javascript/ast/FunctionNode.java
@@ -82,6 +82,16 @@ public static enum Form {
     private int rp = -1;
     private boolean hasRestParameter;
 
+    public Object[] getDefaultParams() {
+        return defaultParams;
+    }
+
+    public void setDefaultParams(Object[] defaultParams) {
+        this.defaultParams = defaultParams;
+    }
+
+    Object[] defaultParams;
+
     // codegen variables
     private int functionType;
     private boolean needsActivation;
diff --git a/testsrc/org/mozilla/javascript/tests/FunctionTest.java b/testsrc/org/mozilla/javascript/tests/FunctionTest.java
index 6edf137ec7..c1b0e44a35 100644
--- a/testsrc/org/mozilla/javascript/tests/FunctionTest.java
+++ b/testsrc/org/mozilla/javascript/tests/FunctionTest.java
@@ -95,6 +95,31 @@ public void secondFunctionWithSameNameStrict() throws Exception {
         assertEvaluates("functionfunc(){result+=norm(func);}outer", script);
     }
 
+    @Test
+    public void functionDefaultArgsBasic() throws Exception {
+        final String script = "function foo(a = 2) {" + "   return a;" + "}";
+        assertIntEvaluates(32, script + "\nfoo(32)");
+        assertIntEvaluates(2, script + "\nfoo()");
+        assertIntEvaluates(2, script + "\nfoo(undefined)");
+    }
+
+    @Test
+    public void functionDefaultArgsMulti() throws Exception {
+        final String script = "function foo(a = 2, b = 23) {" + "   return a + b;" + "}";
+        assertIntEvaluates(55, script + "\nfoo(32)");
+        assertIntEvaluates(25, script + "\nfoo()");
+        assertIntEvaluates(34, script + "\nfoo(32, 2)");
+        assertIntEvaluates(34, script + "\nfoo(undefined, undefined)");
+    }
+
+    @Test
+    public void functionDefaultArgsUsage() throws Exception {
+        final String script = "function foo(a = 2, b = a * 2) {" + "   return a + b;" + "}";
+        assertIntEvaluates(96, script + "\nfoo(32)");
+        assertIntEvaluates(6, script + "\nfoo()");
+        assertIntEvaluates(34, script + "\nfoo(32, 2)");
+    }
+
     @Test
     public void functioNamesExceptionsStrict() throws Exception {
         final String script =
@@ -149,4 +174,16 @@ private static void assertEvaluates(final Object expected, final String source)
                     return null;
                 });
     }
+
+    private static void assertIntEvaluates(final Object expected, final String source) {
+        Utils.runWithAllOptimizationLevels(
+                cx -> {
+                    final Scriptable scope = cx.initStandardObjects();
+                    final Object rep = cx.evaluateString(scope, source, "test.js", 0, null);
+                    if (rep instanceof Double)
+                        assertEquals((int) expected, ((Double) rep).intValue());
+                    else assertEquals(expected, rep);
+                    return null;
+                });
+    }
 }