diff --git a/CHANGELOG.md b/CHANGELOG.md
index 88fee573..232beff7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -86,6 +86,7 @@ _unreleased_
   - Add `input(prompt) string` function _[JS]_
   - Add `exists_dir(path)` that checks if path exists and is a directory _[JS]_
   - Fix js error with `mkdir()` and `mkdir_all()` if path already exists
+- strings: JS implementation of the string builder
 
 ### General Fixes
 - `parser`: Fix precedence of PrefixExpr
@@ -223,7 +224,7 @@ _16 July 2023_
   - Add `user_os()` function
   - Fix `cp()` for directories
 - builtin: New `toI32()` string method
-- Add `strings` package containing a simple string builder _(C backend only)_
+- _[C]_ Add `strings` package containing a simple string builder
 
 ### C Backend (experimental)
 - Properly implement backend selection
diff --git a/lib/bait/gen/js/fun.bt b/lib/bait/gen/js/fun.bt
index db4a183d..00759822 100644
--- a/lib/bait/gen/js/fun.bt
+++ b/lib/bait/gen/js/fun.bt
@@ -85,8 +85,9 @@ fun (mut g Gen) call_expr(node ast.CallExpr) {
 		}
 
 		sym := g.table.get_sym(node.left_type)
-		if sym.kind == .array and ['push', 'push_many', 'push_many_with_len'].contains(node.name) {
-			g.gen_array_method(node.name, node, sym)
+		final_sym := g.table.get_final_sym(node.left_type)
+		if final_sym.kind == .array and ['push', 'push_many', 'push_many_with_len'].contains(node.name) {
+			g.gen_array_method(node.name, node, final_sym)
 			return
 		}
 
diff --git a/lib/builtin/string.js.bt b/lib/builtin/string.js.bt
index 8e2e0c9c..f3a83e45 100644
--- a/lib/builtin/string.js.bt
+++ b/lib/builtin/string.js.bt
@@ -3,7 +3,7 @@
 package builtin
 
 struct string {
-pub:
+pub mut:
 	str #JS.String
 	length i32
 }
diff --git a/lib/strings/builder.js.bt b/lib/strings/builder.js.bt
new file mode 100644
index 00000000..e4acb6ad
--- /dev/null
+++ b/lib/strings/builder.js.bt
@@ -0,0 +1,31 @@
+// SPDX-FileCopyrightText: 2023-present Lukas Neubert <lukas.neubert@proton.me>
+// SPDX-License-Identifier: MPL-2.0
+package strings
+
+type Builder := []u8
+
+pub fun new_builder(initial_size i32) Builder {
+	return []u8 as Builder
+}
+
+pub fun (mut b Builder) clear() {
+	#JS.'b.data = []
+	b.length = 0'
+}
+
+pub fun (mut b Builder) write(data []u8) {
+	b.push_many(data)
+}
+
+pub fun (mut b Builder) write_str(s string) {
+	for c in s {
+		b.push(c)
+	}
+}
+
+pub fun (b Builder) str() string {
+	return string{
+		str = #JS.'String.fromCharCode(...b.data)' as #JS.String
+		length = b.length
+	}
+}
diff --git a/lib/strings/builder_test.bt b/lib/strings/builder_test.bt
new file mode 100644
index 00000000..da376890
--- /dev/null
+++ b/lib/strings/builder_test.bt
@@ -0,0 +1,19 @@
+// SPDX-FileCopyrightText: 2023-present Lukas Neubert <lukas.neubert@proton.me>
+// SPDX-License-Identifier: MPL-2.0
+import strings
+
+fun test_builder() {
+	mut b := strings.new_builder(50)
+
+	b.write([104 as u8, 101 as u8]) // FIXME second u8 cast should not be needed
+	assert b.str() == 'he'
+
+	b.write_str('llo')
+	assert b.str() == 'hello'
+
+	b.clear()
+	assert b.str().length == 0
+
+	b.write_str('foo')
+	assert b.str() == 'foo'
+}