diff --git a/src/main.rs b/src/main.rs
index 533745b..59260eb 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -98,7 +98,7 @@ fn main() -> Result<(), NiceError> {
             log::info!("Done ir2ast @ {}", Time(start.elapsed()));
 
             let ast = if opt_ast {
-                let ast = opt_ast::run(g, ast);
+                let ast = opt_ast::run(g, ast, opt_ast::Opt { minify });
                 log::info!("Done ast optimization @ {}", Time(start.elapsed()));
                 ast
             } else {
diff --git a/src/opt_ast.rs b/src/opt_ast.rs
index d0c586d..7e7bb49 100644
--- a/src/opt_ast.rs
+++ b/src/opt_ast.rs
@@ -4,16 +4,27 @@ use swc_ecma_visit::FoldWith;
 use crate::swc_globals;
 
 mod if2cond;
+mod merge_vars;
 mod swc;
 
 #[cfg(test)]
 mod tests;
 
+pub struct Opt {
+    pub minify: bool,
+}
+
 #[inline(never)] // for better profiling
-pub fn run(g: &swc_globals::Initialized, ast: ast::Program) -> ast::Program {
+pub fn run(g: &swc_globals::Initialized, ast: ast::Program, options: Opt) -> ast::Program {
     let ast = swc::run_passes(g, ast);
 
     let ast = ast.fold_with(&mut if2cond::If2Cond);
 
+    let ast = if options.minify {
+        ast.fold_with(&mut merge_vars::MergeVars)
+    } else {
+        ast
+    };
+
     swc::run_passes(g, ast)
 }
diff --git a/src/opt_ast/merge_vars.rs b/src/opt_ast/merge_vars.rs
new file mode 100644
index 0000000..31c563a
--- /dev/null
+++ b/src/opt_ast/merge_vars.rs
@@ -0,0 +1,48 @@
+use std::mem;
+
+use swc_ecma_ast as ast;
+use swc_ecma_visit::{Fold, FoldWith};
+
+/// Merges adjacent variable declarations.
+pub struct MergeVars;
+
+impl Fold for MergeVars {
+    fn fold_stmts(&mut self, stmts: Vec<ast::Stmt>) -> Vec<ast::Stmt> {
+        let stmts = stmts.fold_children_with(self);
+
+        let var_to_stmt = |var| ast::Stmt::Decl(ast::Decl::Var(var));
+
+        let mut out = Vec::with_capacity(stmts.len());
+        let mut buffered_var = None;
+        for stmt in stmts {
+            match (stmt, &mut buffered_var) {
+                (ast::Stmt::Decl(ast::Decl::Var(cur)), buf @ None) => {
+                    // no buffer yet, buffer this decl
+                    *buf = Some(cur);
+                }
+                (ast::Stmt::Decl(ast::Decl::Var(cur)), Some(buf)) if cur.kind == buf.kind => {
+                    // same kind, add to buffer
+                    buf.decls.extend(cur.decls);
+                }
+                (ast::Stmt::Decl(ast::Decl::Var(cur)), Some(_)) => {
+                    // different kinds, swap into buffer
+                    let buffered_var = mem::replace(&mut buffered_var, Some(cur));
+                    if let Some(buf) = buffered_var {
+                        out.push(var_to_stmt(buf));
+                    }
+                }
+                (stmt, _) => {
+                    // not a var decl, flush buffer
+                    if let Some(buf) = buffered_var.take() {
+                        out.push(var_to_stmt(buf));
+                    }
+                    out.push(stmt);
+                }
+            }
+        }
+        if let Some(buf) = buffered_var {
+            out.push(var_to_stmt(buf));
+        }
+        out
+    }
+}
diff --git a/src/opt_ast/tests/merge_vars.rs b/src/opt_ast/tests/merge_vars.rs
new file mode 100644
index 0000000..76f575d
--- /dev/null
+++ b/src/opt_ast/tests/merge_vars.rs
@@ -0,0 +1,77 @@
+use crate::opt_ast::merge_vars;
+
+case!(basic, || merge_vars::MergeVars, r#"
+    var x;
+    var y;
+    var z;
+    let a;
+    let b;
+    let c;
+    const d;
+    const e;
+    const f;
+"#, @r###"
+var x, y, z;
+let a, b, c;
+const d, e, f;
+"###);
+
+case!(basic_values, || merge_vars::MergeVars, r#"
+    var x;
+    var y = 1;
+    var z;
+    let a;
+    let b = 2;
+    let c;
+    const d;
+    const e = 3;
+    const f;
+"#, @r###"
+var x, y = 1, z;
+let a, b = 2, c;
+const d, e = 3, f;
+"###);
+
+case!(inner_scopes, || merge_vars::MergeVars, r#"
+    if (foo) {
+        var a;
+        var b = 1;
+    }
+"#, @r###"
+if (foo) {
+    var a, b = 1;
+}
+"###);
+
+case!(inner_fn_scopes, || merge_vars::MergeVars, r#"
+    function foo() {
+        var a;
+        var b = 1;
+    }
+"#, @r###"
+function foo() {
+    var a, b = 1;
+}
+"###);
+
+case!(bail_nondecl, || merge_vars::MergeVars, r#"
+    var a;
+    foo();
+    var b = 1;
+"#, @r###"
+var a;
+foo();
+var b = 1;
+"###);
+
+case!(bail_different_types, || merge_vars::MergeVars, r#"
+    var x;
+    let y;
+    const z;
+    var a;
+"#, @r###"
+var x;
+let y;
+const z;
+var a;
+"###);
diff --git a/src/opt_ast/tests/mod.rs b/src/opt_ast/tests/mod.rs
index 17b64db..7f06c85 100644
--- a/src/opt_ast/tests/mod.rs
+++ b/src/opt_ast/tests/mod.rs
@@ -19,7 +19,7 @@ macro_rules! case {
             use crate::{emit, opt_ast, parse, swc_globals};
             swc_globals::with(|g| {
                 let (ast, files) = parse::parse(g, $string)?;
-                let ast = opt_ast::run(g, ast);
+                let ast = opt_ast::run(g, ast, opt_ast::Opt { minify: false });
                 let js = emit::emit(g, ast, files, emit::Opt { minify: false })?;
                 insta::assert_snapshot!(js, @ $expected);
                 Ok(())
@@ -29,4 +29,5 @@ macro_rules! case {
 }
 
 mod if2cond;
+mod merge_vars;
 mod swc;
diff --git a/src/tests/all_phases.rs b/src/tests/all_phases.rs
index f098bd7..a459984 100644
--- a/src/tests/all_phases.rs
+++ b/src/tests/all_phases.rs
@@ -23,7 +23,7 @@ macro_rules! case {
                         minify: false,
                     },
                 );
-                let ast = opt_ast::run(g, ast);
+                let ast = opt_ast::run(g, ast, opt_ast::Opt { minify: false });
                 let js = emit::emit(g, ast, files, emit::Opt { minify: false })?;
                 insta::assert_snapshot!(js, @ $expected);
                 Ok(())
@@ -48,7 +48,7 @@ macro_rules! extern_case {
                         minify: false,
                     },
                 );
-                let ast = opt_ast::run(g, ast);
+                let ast = opt_ast::run(g, ast, opt_ast::Opt { minify: false });
                 let js = emit::emit(g, ast, files, emit::Opt { minify: false })?;
                 insta::assert_snapshot!(stringify!($name), js);
                 Ok(())