diff --git a/valuescript_common/src/instruction_byte.rs b/valuescript_common/src/instruction_byte.rs
index 7fad347..6b5c447 100644
--- a/valuescript_common/src/instruction_byte.rs
+++ b/valuescript_common/src/instruction_byte.rs
@@ -41,22 +41,23 @@ pub enum InstructionByte {
   SubCall = 0x26,
   Jmp = 0x27,
   JmpIf = 0x28,
-  UnaryPlus = 0x29,
-  UnaryMinus = 0x2a,
-  New = 0x2b,
-  Throw = 0x2c,
-  Import = 0x2d,
-  ImportStar = 0x2e,
-  SetCatch = 0x2f,
-  UnsetCatch = 0x30,
-  ConstSubCall = 0x31,
-  RequireMutableThis = 0x32,
-  ThisSubCall = 0x33,
-  Next = 0x34,
-  UnpackIterRes = 0x35,
-  Cat = 0x36,
-  Yield = 0x37,
-  YieldStar = 0x38,
+  JmpIfNot = 0x29,
+  UnaryPlus = 0x2a,
+  UnaryMinus = 0x2b,
+  New = 0x2c,
+  Throw = 0x2d,
+  Import = 0x2e,
+  ImportStar = 0x2f,
+  SetCatch = 0x30,
+  UnsetCatch = 0x31,
+  ConstSubCall = 0x32,
+  RequireMutableThis = 0x33,
+  ThisSubCall = 0x34,
+  Next = 0x35,
+  UnpackIterRes = 0x36,
+  Cat = 0x37,
+  Yield = 0x38,
+  YieldStar = 0x39,
 }
 
 impl InstructionByte {
@@ -105,22 +106,23 @@ impl InstructionByte {
       0x26 => SubCall,
       0x27 => Jmp,
       0x28 => JmpIf,
-      0x29 => UnaryPlus,
-      0x2a => UnaryMinus,
-      0x2b => New,
-      0x2c => Throw,
-      0x2d => Import,
-      0x2e => ImportStar,
-      0x2f => SetCatch,
-      0x30 => UnsetCatch,
-      0x31 => ConstSubCall,
-      0x32 => RequireMutableThis,
-      0x33 => ThisSubCall,
-      0x34 => Next,
-      0x35 => UnpackIterRes,
-      0x36 => Cat,
-      0x37 => Yield,
-      0x38 => YieldStar,
+      0x29 => JmpIfNot,
+      0x2a => UnaryPlus,
+      0x2b => UnaryMinus,
+      0x2c => New,
+      0x2d => Throw,
+      0x2e => Import,
+      0x2f => ImportStar,
+      0x30 => SetCatch,
+      0x31 => UnsetCatch,
+      0x32 => ConstSubCall,
+      0x33 => RequireMutableThis,
+      0x34 => ThisSubCall,
+      0x35 => Next,
+      0x36 => UnpackIterRes,
+      0x37 => Cat,
+      0x38 => Yield,
+      0x39 => YieldStar,
 
       _ => panic!("Unrecognized instruction: {}", byte),
     };
diff --git a/valuescript_compiler/src/assembler.rs b/valuescript_compiler/src/assembler.rs
index ffeb71b..8be5631 100644
--- a/valuescript_compiler/src/assembler.rs
+++ b/valuescript_compiler/src/assembler.rs
@@ -235,7 +235,7 @@ impl Assembler {
       Jmp(label_ref) => {
         self.label_ref(label_ref);
       }
-      JmpIf(value, label_ref) => {
+      JmpIf(value, label_ref) | JmpIfNot(value, label_ref) => {
         self.value(value);
         self.label_ref(label_ref);
       }
diff --git a/valuescript_compiler/src/assembly_parser.rs b/valuescript_compiler/src/assembly_parser.rs
index 543ea31..f89127f 100644
--- a/valuescript_compiler/src/assembly_parser.rs
+++ b/valuescript_compiler/src/assembly_parser.rs
@@ -238,6 +238,7 @@ impl<'a> AssemblyParser<'a> {
       ("subcall", InstructionByte::SubCall),
       ("jmp", InstructionByte::Jmp),
       ("jmpif", InstructionByte::JmpIf),
+      ("jmpif_not", InstructionByte::JmpIfNot),
       ("unary+", InstructionByte::UnaryPlus),
       ("unary-", InstructionByte::UnaryMinus),
       ("new", InstructionByte::New),
@@ -731,6 +732,7 @@ impl<'a> AssemblyParser<'a> {
       ),
       Jmp => Instruction::Jmp(self.assemble_label_read()),
       JmpIf => Instruction::JmpIf(self.assemble_value(), self.assemble_label_read()),
+      JmpIfNot => Instruction::JmpIfNot(self.assemble_value(), self.assemble_label_read()),
       UnaryPlus => Instruction::UnaryPlus(self.assemble_value(), self.assemble_register()),
       UnaryMinus => Instruction::UnaryMinus(self.assemble_value(), self.assemble_register()),
       New => Instruction::New(
diff --git a/valuescript_compiler/src/instruction.rs b/valuescript_compiler/src/instruction.rs
index e716a94..30234e3 100644
--- a/valuescript_compiler/src/instruction.rs
+++ b/valuescript_compiler/src/instruction.rs
@@ -45,6 +45,7 @@ pub enum Instruction {
   SubCall(Register, Value, Value, Register),
   Jmp(LabelRef),
   JmpIf(Value, LabelRef),
+  JmpIfNot(Value, LabelRef),
   UnaryPlus(Value, Register),
   UnaryMinus(Value, Register),
   New(Value, Value, Register),
@@ -190,7 +191,7 @@ impl Instruction {
         visit(InstructionFieldMut::LabelRef(label_ref));
       }
 
-      JmpIf(cond, label_ref) => {
+      JmpIf(cond, label_ref) | JmpIfNot(cond, label_ref) => {
         visit(InstructionFieldMut::Value(cond));
         visit(InstructionFieldMut::LabelRef(label_ref));
       }
@@ -313,7 +314,7 @@ impl Instruction {
 
       Jmp(_label_ref) => {}
 
-      JmpIf(cond, _label_ref) => {
+      JmpIf(cond, _label_ref) | JmpIfNot(cond, _label_ref) => {
         cond.visit_registers_mut_rev(visit);
       }
 
@@ -393,6 +394,7 @@ impl Instruction {
       SubCall(..) => InstructionByte::SubCall,
       Jmp(..) => InstructionByte::Jmp,
       JmpIf(..) => InstructionByte::JmpIf,
+      JmpIfNot(..) => InstructionByte::JmpIfNot,
       UnaryPlus(..) => InstructionByte::UnaryPlus,
       UnaryMinus(..) => InstructionByte::UnaryMinus,
       New(..) => InstructionByte::New,
@@ -531,6 +533,9 @@ impl std::fmt::Display for Instruction {
       Instruction::JmpIf(value, label_ref) => {
         write!(f, "jmpif {} {}", value, label_ref)
       }
+      Instruction::JmpIfNot(value, label_ref) => {
+        write!(f, "jmpif_not {} {}", value, label_ref)
+      }
       Instruction::UnaryPlus(value, register) => {
         write!(f, "unary+ {} {}", value, register)
       }
diff --git a/valuescript_compiler/src/optimization/kal.rs b/valuescript_compiler/src/optimization/kal.rs
index c867e7b..48d3742 100644
--- a/valuescript_compiler/src/optimization/kal.rs
+++ b/valuescript_compiler/src/optimization/kal.rs
@@ -442,7 +442,7 @@ impl FnState {
         self.set(dst.name.clone(), Kal::Unknown);
       }
 
-      JmpIf(a1, _) => {
+      JmpIf(a1, _) | JmpIfNot(a1, _) => {
         self.eval_arg(a1);
       }
 
@@ -511,6 +511,7 @@ impl FnState {
       | SubCall(_, _, _, _)
       | Jmp(_)
       | JmpIf(_, _)
+      | JmpIfNot(_, _)
       | New(_, _, _)
       | Throw(_)
       | Import(_, _)
diff --git a/valuescript_compiler/src/optimization/reduce_instructions.rs b/valuescript_compiler/src/optimization/reduce_instructions.rs
index 9860b5e..d6a5f07 100644
--- a/valuescript_compiler/src/optimization/reduce_instructions.rs
+++ b/valuescript_compiler/src/optimization/reduce_instructions.rs
@@ -80,6 +80,12 @@ fn reduce(instr: Instruction) -> Option<Instruction> {
       Value::Bool(true) => Some(Instruction::Jmp(label_ref.clone())),
       _ => Some(instr),
     },
+    JmpIfNot(cond, label_ref) => match cond {
+      // TODO: Kal::from_value(cond).is_truthy()
+      Value::Bool(false) => Some(Instruction::Jmp(label_ref.clone())),
+      Value::Bool(true) => None,
+      _ => Some(instr),
+    },
     UnpackIterRes(_, value_dst, done_dst) => {
       if value_dst.is_ignore() && done_dst.is_ignore() {
         None
diff --git a/valuescript_compiler/src/optimization/simplify.rs b/valuescript_compiler/src/optimization/simplify.rs
index 285843f..307634d 100644
--- a/valuescript_compiler/src/optimization/simplify.rs
+++ b/valuescript_compiler/src/optimization/simplify.rs
@@ -150,7 +150,10 @@ fn is_jmp_or_label(line: &FnLine) -> bool {
 
 fn is_jmp_instr(instr: &Instruction) -> bool {
   match instr {
-    Instruction::End | Instruction::Jmp(..) | Instruction::JmpIf(..) => true,
+    Instruction::End
+    | Instruction::Jmp(..)
+    | Instruction::JmpIf(..)
+    | Instruction::JmpIfNot(..) => true,
     Instruction::Mov(..)
     | Instruction::OpInc(..)
     | Instruction::OpDec(..)
diff --git a/valuescript_compiler/src/visit_pointers.rs b/valuescript_compiler/src/visit_pointers.rs
index b3f8e33..0efe9c9 100644
--- a/valuescript_compiler/src/visit_pointers.rs
+++ b/valuescript_compiler/src/visit_pointers.rs
@@ -163,7 +163,7 @@ where
         self.value(Some(owner), key);
         self.value(Some(owner), args);
       }
-      JmpIf(arg, _) => {
+      JmpIf(arg, _) | JmpIfNot(arg, _) => {
         self.value(Some(owner), arg);
       }
     };
diff --git a/valuescript_vm/src/bytecode_stack_frame.rs b/valuescript_vm/src/bytecode_stack_frame.rs
index c555444..4c7777d 100644
--- a/valuescript_vm/src/bytecode_stack_frame.rs
+++ b/valuescript_vm/src/bytecode_stack_frame.rs
@@ -412,6 +412,15 @@ impl StackFrameTrait for BytecodeStackFrame {
         }
       }
 
+      JmpIfNot => {
+        let cond = self.decoder.decode_val(&mut self.registers);
+        let dst = self.decoder.decode_pos();
+
+        if !cond.is_truthy() {
+          self.decoder.pos = dst;
+        }
+      }
+
       UnaryPlus => self.apply_unary_op(operations::op_unary_plus),
       UnaryMinus => self.apply_unary_op(operations::op_unary_minus),