diff --git a/src/codegen/js_backend.rs b/src/codegen/js_backend.rs
index db5f6f7..a495ad0 100644
--- a/src/codegen/js_backend.rs
+++ b/src/codegen/js_backend.rs
@@ -31,6 +31,7 @@
 //! ```
 
 use crate::ast::*;
+use crate::modules::Module;
 use std::collections::{HashMap, HashSet};
 
 /// JavaScript code generation errors
@@ -75,6 +76,10 @@ pub struct JsBackend {
     extern_fns: HashMap<String, String>,
     /// Extern let names mapped to their JS names
     extern_lets: HashMap<String, String>,
+    /// Module functions: (alias, fn_name) → mangled JS name
+    module_functions: HashMap<(String, String), String>,
+    /// Known module aliases
+    imported_modules: HashSet<String>,
 }
 
 impl JsBackend {
@@ -99,13 +104,18 @@ impl JsBackend {
             used_effects: HashSet::new(),
             extern_fns: HashMap::new(),
             extern_lets: HashMap::new(),
+            module_functions: HashMap::new(),
+            imported_modules: HashSet::new(),
         }
     }
 
     /// Generate JavaScript code from a Lux program
-    pub fn generate(&mut self, program: &Program) -> Result<String, JsGenError> {
+    pub fn generate(&mut self, program: &Program, modules: &HashMap<String, Module>) -> Result<String, JsGenError> {
         self.output.clear();
 
+        // Process imported modules before the main program
+        self.process_imported_modules(program, modules)?;
+
         // First pass: collect all function names, types, and effects
         for decl in &program.declarations {
             match decl {
@@ -143,6 +153,9 @@ impl JsBackend {
         // Emit runtime helpers (tree-shaken based on used effects)
         self.emit_runtime();
 
+        // Emit imported module code (type constructors and functions)
+        self.emit_module_code(modules, program)?;
+
         // Emit type constructors
         for decl in &program.declarations {
             if let Declaration::Type(t) = decl {
@@ -323,6 +336,216 @@ impl JsBackend {
         }
     }
 
+    /// Process imported modules: collect function names, types, and mappings
+    fn process_imported_modules(
+        &mut self,
+        program: &Program,
+        modules: &HashMap<String, Module>,
+    ) -> Result<(), JsGenError> {
+        let mut processed = HashSet::new();
+        for import in &program.imports {
+            let module_path = import.path.segments.iter()
+                .map(|s| s.name.as_str())
+                .collect::<Vec<_>>()
+                .join("/");
+            let alias = if let Some(ref a) = import.alias {
+                a.name.clone()
+            } else {
+                import.path.segments.last()
+                    .map(|s| s.name.clone())
+                    .unwrap_or_else(|| module_path.clone())
+            };
+            self.imported_modules.insert(alias.clone());
+            self.process_single_module(&alias, &module_path, modules, &mut processed)?;
+        }
+        Ok(())
+    }
+
+    /// Process a single module and its transitive imports
+    fn process_single_module(
+        &mut self,
+        alias: &str,
+        module_path: &str,
+        modules: &HashMap<String, Module>,
+        processed: &mut HashSet<String>,
+    ) -> Result<(), JsGenError> {
+        if processed.contains(module_path) {
+            return Ok(());
+        }
+        processed.insert(module_path.to_string());
+
+        let module = match modules.get(module_path) {
+            Some(m) => m,
+            None => return Ok(()), // Module not found — might be a built-in
+        };
+
+        // Process transitive imports first
+        for sub_import in &module.program.imports {
+            let sub_path = sub_import.path.segments.iter()
+                .map(|s| s.name.as_str())
+                .collect::<Vec<_>>()
+                .join("/");
+            let sub_alias = sub_import.path.segments.last()
+                .map(|s| s.name.clone())
+                .unwrap_or_else(|| sub_path.clone());
+            self.process_single_module(&sub_alias, &sub_path, modules, processed)?;
+        }
+
+        // Collect types from the module
+        for decl in &module.program.declarations {
+            if let Declaration::Type(t) = decl {
+                self.collect_type(t)?;
+            }
+        }
+
+        // Collect functions from the module
+        for decl in &module.program.declarations {
+            if let Declaration::Function(f) = decl {
+                if f.visibility == Visibility::Public || module.exports.contains(&f.name.name) {
+                    let mangled = format!("{}_{}_lux", alias, f.name.name);
+                    self.functions.insert(mangled.clone());
+                    self.module_functions.insert(
+                        (alias.to_string(), f.name.name.clone()),
+                        mangled,
+                    );
+
+                    if !f.effects.is_empty() {
+                        self.effectful_functions.insert(format!("{}_{}_lux", alias, f.name.name));
+                    }
+                }
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Emit code for imported modules (type constructors, functions, handlers, let bindings)
+    fn emit_module_code(
+        &mut self,
+        modules: &HashMap<String, Module>,
+        program: &Program,
+    ) -> Result<(), JsGenError> {
+        if self.module_functions.is_empty() {
+            return Ok(());
+        }
+
+        self.writeln("// === Imported Modules ===");
+        self.writeln("");
+
+        let mut processed = HashSet::new();
+        for import in &program.imports {
+            let module_path = import.path.segments.iter()
+                .map(|s| s.name.as_str())
+                .collect::<Vec<_>>()
+                .join("/");
+            let alias = if let Some(ref a) = import.alias {
+                a.name.clone()
+            } else {
+                import.path.segments.last()
+                    .map(|s| s.name.clone())
+                    .unwrap_or_else(|| module_path.clone())
+            };
+            self.emit_module_code_recursive(&alias, &module_path, modules, &mut processed)?;
+        }
+
+        Ok(())
+    }
+
+    fn emit_module_code_recursive(
+        &mut self,
+        alias: &str,
+        module_path: &str,
+        modules: &HashMap<String, Module>,
+        processed: &mut HashSet<String>,
+    ) -> Result<(), JsGenError> {
+        if processed.contains(module_path) {
+            return Ok(());
+        }
+        processed.insert(module_path.to_string());
+
+        let module = match modules.get(module_path) {
+            Some(m) => m.clone(),
+            None => return Ok(()),
+        };
+
+        // Process transitive imports first
+        for sub_import in &module.program.imports {
+            let sub_path = sub_import.path.segments.iter()
+                .map(|s| s.name.as_str())
+                .collect::<Vec<_>>()
+                .join("/");
+            let sub_alias = sub_import.path.segments.last()
+                .map(|s| s.name.clone())
+                .unwrap_or_else(|| sub_path.clone());
+            self.emit_module_code_recursive(&sub_alias, &sub_path, modules, processed)?;
+        }
+
+        // Emit type constructors
+        for decl in &module.program.declarations {
+            if let Declaration::Type(t) = decl {
+                self.emit_type_constructors(t)?;
+            }
+        }
+
+        // Emit functions with module-prefixed names
+        for decl in &module.program.declarations {
+            if let Declaration::Function(f) = decl {
+                if f.visibility == Visibility::Public || module.exports.contains(&f.name.name) {
+                    let mangled = format!("{}_{}_lux", alias, f.name.name);
+                    self.emit_function_with_name(f, &mangled)?;
+                }
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Emit a function with a custom mangled name (for module functions)
+    fn emit_function_with_name(
+        &mut self,
+        func: &FunctionDecl,
+        mangled_name: &str,
+    ) -> Result<(), JsGenError> {
+        let is_effectful = !func.effects.is_empty();
+
+        // Build parameter list
+        let mut params: Vec<String> = func.params.iter().map(|p| p.name.name.clone()).collect();
+
+        // Effectful functions get handlers as first parameter
+        if is_effectful {
+            params.insert(0, "handlers".to_string());
+        }
+
+        // Function declaration
+        self.writeln(&format!(
+            "function {}({}) {{",
+            mangled_name,
+            params.join(", ")
+        ));
+        self.indent += 1;
+
+        // Set context for effect handling
+        let prev_has_handlers = self.has_handlers;
+        self.has_handlers = is_effectful;
+
+        // Save and clear var substitutions for this function scope
+        let saved_substitutions = self.var_substitutions.clone();
+        self.var_substitutions.clear();
+
+        // Emit function body
+        let body_code = self.emit_expr(&func.body)?;
+        self.writeln(&format!("return {};", body_code));
+
+        self.has_handlers = prev_has_handlers;
+        self.var_substitutions = saved_substitutions;
+
+        self.indent -= 1;
+        self.writeln("}");
+        self.writeln("");
+
+        Ok(())
+    }
+
     /// Emit the Lux runtime, tree-shaken based on used effects
     fn emit_runtime(&mut self) {
         let uses_console = self.used_effects.contains("Console");
@@ -1315,6 +1538,34 @@ impl JsBackend {
                         if module_name.name == "Ref" {
                             return self.emit_ref_operation(&field.name, args);
                         }
+
+                        // Check for user-defined module function
+                        let key = (module_name.name.clone(), field.name.clone());
+                        if let Some(js_name) = self.module_functions.get(&key).cloned() {
+                            let arg_strs: Result<Vec<_>, _> = args.iter().map(|a| self.emit_expr(a)).collect();
+                            let args_str = arg_strs?.join(", ");
+                            let is_effectful = self.effectful_functions.contains(&js_name);
+
+                            if is_effectful && self.has_handlers {
+                                let handlers_name = self.var_substitutions
+                                    .get("handlers")
+                                    .cloned()
+                                    .unwrap_or_else(|| "handlers".to_string());
+                                return if args_str.is_empty() {
+                                    Ok(format!("{}({})", js_name, handlers_name))
+                                } else {
+                                    Ok(format!("{}({}, {})", js_name, handlers_name, args_str))
+                                };
+                            } else if is_effectful {
+                                return if args_str.is_empty() {
+                                    Ok(format!("{}(Lux.defaultHandlers)", js_name))
+                                } else {
+                                    Ok(format!("{}(Lux.defaultHandlers, {})", js_name, args_str))
+                                };
+                            } else {
+                                return Ok(format!("{}({})", js_name, args_str));
+                            }
+                        }
                     }
                 }
 
@@ -1471,6 +1722,34 @@ impl JsBackend {
                     return self.emit_html_operation(&operation.name, args);
                 }
 
+                // Check for user-defined module function
+                let key = (effect.name.clone(), operation.name.clone());
+                if let Some(js_name) = self.module_functions.get(&key).cloned() {
+                    let arg_strs: Result<Vec<_>, _> = args.iter().map(|a| self.emit_expr(a)).collect();
+                    let args_str = arg_strs?.join(", ");
+                    let is_effectful = self.effectful_functions.contains(&js_name);
+
+                    if is_effectful && self.has_handlers {
+                        let handlers_name = self.var_substitutions
+                            .get("handlers")
+                            .cloned()
+                            .unwrap_or_else(|| "handlers".to_string());
+                        return if args_str.is_empty() {
+                            Ok(format!("{}({})", js_name, handlers_name))
+                        } else {
+                            Ok(format!("{}({}, {})", js_name, handlers_name, args_str))
+                        };
+                    } else if is_effectful {
+                        return if args_str.is_empty() {
+                            Ok(format!("{}(Lux.defaultHandlers)", js_name))
+                        } else {
+                            Ok(format!("{}(Lux.defaultHandlers, {})", js_name, args_str))
+                        };
+                    } else {
+                        return Ok(format!("{}({})", js_name, args_str));
+                    }
+                }
+
                 let arg_strs: Result<Vec<_>, _> = args.iter().map(|a| self.emit_expr(a)).collect();
                 let args_str = arg_strs?.join(", ");
 
@@ -3026,7 +3305,7 @@ mod tests {
 
         let mut backend = JsBackend::new();
         let js_code = backend
-            .generate(&program)
+            .generate(&program, &std::collections::HashMap::new())
             .map_err(|e| format!("Codegen error: {}", e))?;
 
         let output = Command::new("node")
@@ -4286,7 +4565,7 @@ line3"
 
         let program = Parser::parse_source(source).expect("Should parse");
         let mut backend = JsBackend::new();
-        let js_code = backend.generate(&program).expect("Should generate");
+        let js_code = backend.generate(&program, &std::collections::HashMap::new()).expect("Should generate");
 
         // Core runtime is always present
         assert!(js_code.contains("const Lux = {"), "Lux object should be defined");
@@ -4318,7 +4597,7 @@ line3"
 
         let program = Parser::parse_source(source).expect("Should parse");
         let mut backend = JsBackend::new();
-        let js_code = backend.generate(&program).expect("Should generate");
+        let js_code = backend.generate(&program, &std::collections::HashMap::new()).expect("Should generate");
 
         assert!(js_code.contains("Console:"), "Console handler should exist");
         assert!(js_code.contains("Dom:"), "Dom handler should exist");
@@ -4341,7 +4620,7 @@ line3"
 
         let program = Parser::parse_source(source).expect("Should parse");
         let mut backend = JsBackend::new();
-        let js_code = backend.generate(&program).expect("Should generate");
+        let js_code = backend.generate(&program, &std::collections::HashMap::new()).expect("Should generate");
 
         // Only Console should be present
         assert!(js_code.contains("Console:"), "Console handler should exist");
diff --git a/src/main.rs b/src/main.rs
index eca6308..1833b8e 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -997,7 +997,7 @@ fn compile_to_js(path: &str, output_path: Option<&str>, run_after: bool) {
 
     // Generate JavaScript code
     let mut backend = JsBackend::new();
-    let js_code = match backend.generate(&program) {
+    let js_code = match backend.generate(&program, loader.module_cache()) {
         Ok(code) => code,
         Err(e) => {
             eprintln!("{} JS codegen: {}", c(colors::RED, "error:"), e);
@@ -4253,7 +4253,7 @@ c")"#;
         let tokens = Lexer::new(source).tokenize().unwrap();
         let program = Parser::new(tokens).parse_program().unwrap();
         let mut backend = JsBackend::new();
-        let js = backend.generate(&program).unwrap();
+        let js = backend.generate(&program, &std::collections::HashMap::new()).unwrap();
 
         // getElementById should appear as-is (no _lux suffix)
         assert!(js.contains("getElementById("), "JS should call getElementById directly: {}", js);
@@ -4280,7 +4280,7 @@ c")"#;
         let tokens = Lexer::new(source).tokenize().unwrap();
         let program = Parser::new(tokens).parse_program().unwrap();
         let mut backend = JsBackend::new();
-        let js = backend.generate(&program).unwrap();
+        let js = backend.generate(&program, &std::collections::HashMap::new()).unwrap();
 
         assert!(js.contains("Lux.Some"), "JS should contain Lux.Some for List.get: {}", js);
         assert!(js.contains("Lux.None"), "JS should contain Lux.None for List.get: {}", js);
@@ -4301,7 +4301,7 @@ c")"#;
         let tokens = Lexer::new(source).tokenize().unwrap();
         let program = Parser::new(tokens).parse_program().unwrap();
         let mut backend = JsBackend::new();
-        let js = backend.generate(&program).unwrap();
+        let js = backend.generate(&program, &std::collections::HashMap::new()).unwrap();
 
         // Should contain the let binding
         assert!(js.contains("const main"), "JS should contain 'const main': {}", js);
@@ -4338,7 +4338,7 @@ c")"#;
         let tokens = Lexer::new(source).tokenize().unwrap();
         let program = Parser::new(tokens).parse_program().unwrap();
         let mut backend = JsBackend::new();
-        let js = backend.generate(&program).unwrap();
+        let js = backend.generate(&program, &std::collections::HashMap::new()).unwrap();
 
         // Handler should be emitted as a const object
         assert!(js.contains("const consoleLogger_lux"), "JS should contain handler const: {}", js);