diff --git a/vlib/builtin/js/array.js.v b/vlib/builtin/js/array.js.v
index d20f4084ed..cf006bdb22 100644
--- a/vlib/builtin/js/array.js.v
+++ b/vlib/builtin/js/array.js.v
@@ -147,7 +147,7 @@ pub fn (a array) repeat(count int) array {
 	}
 }
 
-#function makeEmptyArray() { return new array(new array_buffer({})); }
+#function makeEmptyArray() { return new array(new array_buffer({ arr: [], len: new int(0), index_start: new int(0), cap: new int(0) })); }
 #function makeEmtpyJSArray() { return new Array(); }
 
 fn JS.makeEmptyArray() array
diff --git a/vlib/builtin/js/builtin.js.v b/vlib/builtin/js/builtin.js.v
index 1d6fdd254d..b5092ba8aa 100644
--- a/vlib/builtin/js/builtin.js.v
+++ b/vlib/builtin/js/builtin.js.v
@@ -104,66 +104,3 @@ pub fn (f float_literal) str() string {
 
 	return res
 }
-
-pub fn tof64(n JS.Number) f64 {
-	res := f64(0.0)
-	#res.val = n;
-
-	return res
-}
-
-pub fn tof32(n JS.Number) f32 {
-	res := f32(0.0)
-	#res.val = n;
-
-	return res
-}
-
-pub fn toi(n JS.Number) int {
-	res := int(0)
-	#res.val = Math.floor(n);
-
-	return res
-}
-
-pub fn f64tonum(n f64) JS.Number {
-	mut res := JS.Number{}
-	#res = n.val;
-
-	return res
-}
-
-pub fn itonum(n int) JS.Number {
-	mut res := JS.Number{}
-	#res = n.val;
-
-	return res
-}
-
-pub fn i64tobigint(n i64) JS.BigInt {
-	mut res := JS.BigInt{}
-	#res = n.val;
-
-	return res
-}
-
-pub fn u64tobigint(n u64) JS.BigInt {
-	mut res := JS.BigInt{}
-	#res = n.val;
-
-	return res
-}
-
-pub fn tobool(b JS.Boolean) bool {
-	res := false
-	#res.val = b;
-
-	return res
-}
-
-pub fn booltojs(b bool) JS.Boolean {
-	mut res := JS.Boolean{}
-	#res = b.val;
-
-	return res
-}
diff --git a/vlib/builtin/js/jsfns.js.v b/vlib/builtin/js/jsfns.js.v
index 8bb8eba296..bc1e0ece5e 100644
--- a/vlib/builtin/js/jsfns.js.v
+++ b/vlib/builtin/js/jsfns.js.v
@@ -7,21 +7,62 @@
 
 module builtin
 
-pub struct JS.BigInt {}
+pub interface JS.Object {}
 
-pub struct JS.Number {}
+pub interface JS.BigInt {
+	JS.Any
+}
 
-pub struct JS.String {
+pub interface JS.Number {
+	JS.Any
+}
+
+pub interface JS.String {
+	JS.Any
+	length JS.Number
+	charAt(index JS.Number) JS.String
+	charCodeAt(index JS.Number) JS.Number
+	toUpperCase() JS.String
+	toLowerCase() JS.String
+	concat(a JS.String) JS.String
+	includes(substr JS.String) JS.Boolean
+	endsWith(substr JS.String) JS.Boolean
+	startsWith(substr JS.String) JS.Boolean
+	slice(a JS.Number, b JS.Number) JS.String
+	split(dot JS.String) JS.Array
+	indexOf(needle JS.String) JS.Number
+	lastIndexOf(needle JS.String) JS.Number
+}
+
+pub interface JS.Boolean {
+	JS.Any
+}
+
+pub interface JS.Map {
+	JS.Any
+	size JS.Number
+	clear()
+	delete(key JS.Any) JS.Boolean
+	get(key JS.Any) JS.Any
+	has(key JS.Any) JS.Any
+	set(key JS.Any, val JS.Any)
+}
+
+#function Any(val) { return val; }
+
+pub interface JS.Any {}
+
+pub interface JS.Array {
+	JS.Any // map(fn (JS.Any) JS.Any) JS.Array
+	map(JS.Any) JS.Array
+	push(JS.Any) JS.Any
+	pop() JS.Any
+	at(JS.Number) JS.Any
+mut:
 	length JS.Number
 }
 
-pub struct JS.Boolean {}
-
-pub struct JS.Array {
-	length JS.Number
-}
-
-pub struct JS.Map {}
+pub fn JS.Array.prototype.constructor(...any) JS.Array
 
 // browser: https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/Global_Objects/Error
 // node: https://nodejs.org/api/errors.html#errors_class_error
@@ -110,18 +151,3 @@ fn JS.Math.tan(f64) f64
 // JSON
 fn JS.JSON.stringify(any) string
 fn JS.JSON.parse(string) any
-
-// String
-fn (v JS.String) slice(a int, b int) JS.String
-fn (v JS.String) split(dot JS.String) []JS.String
-fn (s JS.String) indexOf(needle JS.String) int
-fn (s JS.String) lastIndexOf(needle JS.String) int
-
-fn (s JS.String) charAt(i int) JS.String
-fn (s JS.String) charCodeAt(i int) byte
-fn (s JS.String) toUpperCase() JS.String
-fn (s JS.String) toLowerCase() JS.String
-fn (s JS.String) concat(a JS.String) JS.String
-fn (s JS.String) includes(substr JS.String) bool
-fn (s JS.String) endsWith(substr JS.String) bool
-fn (s JS.String) startsWith(substr JS.String) bool
diff --git a/vlib/builtin/js/string.js.v b/vlib/builtin/js/string.js.v
index 7fecc19ef9..65ac080267 100644
--- a/vlib/builtin/js/string.js.v
+++ b/vlib/builtin/js/string.js.v
@@ -25,7 +25,7 @@ pub fn (s string) substr(start int, end int) string {
 }
 
 pub fn (s string) after(dot string) string {
-	return string(s.str.slice(s.str.lastIndexOf(dot.str) + 1, int(s.str.length)))
+	return string(s.str.slice(JS.Number(int(s.str.lastIndexOf(dot.str)) + 1), s.str.length))
 }
 
 pub fn (s string) after_char(dot byte) string {
@@ -34,7 +34,7 @@ pub fn (s string) after_char(dot byte) string {
 }
 
 pub fn (s string) all_after(dot string) string {
-	pos := if dot.len == 0 { -1 } else { s.str.indexOf(dot.str) }
+	pos := if dot.len == 0 { -1 } else { int(s.str.indexOf(dot.str)) }
 	if pos == -1 {
 		return s.clone()
 	}
@@ -43,7 +43,7 @@ pub fn (s string) all_after(dot string) string {
 
 // why does this exist?
 pub fn (s string) all_after_last(dot string) string {
-	pos := if dot.len == 0 { -1 } else { s.str.lastIndexOf(dot.str) }
+	pos := if dot.len == 0 { -1 } else { int(s.str.lastIndexOf(dot.str)) }
 	if pos == -1 {
 		return s.clone()
 	}
@@ -51,7 +51,7 @@ pub fn (s string) all_after_last(dot string) string {
 }
 
 pub fn (s string) all_before(dot string) string {
-	pos := if dot.len == 0 { -1 } else { s.str.indexOf(dot.str) }
+	pos := if dot.len == 0 { -1 } else { int(s.str.indexOf(dot.str)) }
 	if pos == -1 {
 		return s.clone()
 	}
@@ -60,7 +60,7 @@ pub fn (s string) all_before(dot string) string {
 }
 
 pub fn (s string) all_before_last(dot string) string {
-	pos := if dot.len == 0 { -1 } else { s.str.lastIndexOf(dot.str) }
+	pos := if dot.len == 0 { -1 } else { int(s.str.lastIndexOf(dot.str)) }
 	if pos == -1 {
 		return s.clone()
 	}
@@ -72,16 +72,27 @@ pub fn (s string) bool() bool {
 }
 
 pub fn (s string) split(dot string) []string {
-	mut arr := s.str.split(dot.str).map(string(it))
-	#arr = new array(new array_buffer({arr,index_start: new int(0),len: new int(arr.length)}))
+	tmparr := s.str.split(dot.str).map(fn (it JS.Any) JS.Any {
+		res := ''
+		#res.str = it
+
+		return res
+	})
+	_ := tmparr
+	mut arr := []string{}
+	#arr = new array(new array_buffer({arr: tmparr,index_start: new int(0),len: new int(tmparr.length)}))
 
 	return arr
 }
 
 pub fn (s string) bytes() []byte {
 	sep := ''
-	mut arr := s.str.split(sep.str).map(it.charCodeAt(0))
-	#arr = new array(new array_buffer({arr,index_start: new int(0),len: new int(arr.length)}))
+	tmparr := s.str.split(sep.str).map(fn (it JS.Any) JS.Any {
+		return JS.Any(byte(JS.String(it).charCodeAt(0)))
+	})
+	_ := tmparr
+	mut arr := []byte{}
+	#arr = new array(new array_buffer({arr: tmparr,index_start: new int(0),len: new int(tmparr.length)}))
 
 	return arr
 }
@@ -96,13 +107,14 @@ pub fn (s string) clone() string {
 }
 
 pub fn (s string) contains(substr string) bool {
-	return s.str.includes(substr.str)
+	return bool(s.str.includes(substr.str))
 }
 
 pub fn (s string) contains_any(chars string) bool {
 	sep := ''
-	for x in chars.str.split(sep.str) {
-		if s.str.includes(x) {
+	res := chars.str.split(sep.str)
+	for i in 0 .. int(res.length) {
+		if bool(s.str.includes(JS.String(res.at(JS.Number(i))))) {
 			return true
 		}
 	}
@@ -114,7 +126,7 @@ pub fn (s string) contains_any_substr(chars []string) bool {
 		return true
 	}
 	for x in chars {
-		if s.str.includes(x.str) {
+		if bool(s.str.includes(x.str)) {
 			return true
 		}
 	}
@@ -124,7 +136,12 @@ pub fn (s string) contains_any_substr(chars []string) bool {
 pub fn (s string) count(substr string) int {
 	// TODO: "error: `[]JS.String` is not a struct" when returning arr.length or arr.len
 	arr := s.str.split(substr.str)
-	return native_str_arr_len(arr)
+	len := int(arr.length)
+	if len == 0 {
+		return 0
+	} else {
+		return len - 1
+	}
 }
 
 pub fn (s string) ends_with(p string) bool {
@@ -135,7 +152,7 @@ pub fn (s string) ends_with(p string) bool {
 }
 
 pub fn (s string) starts_with(p string) bool {
-	return s.str.startsWith(p.str)
+	return bool(s.str.startsWith(p.str))
 }
 
 pub fn (s string) fields() []string {
@@ -170,7 +187,7 @@ pub fn (s string) fields() []string {
 }
 
 pub fn (s string) find_between(start string, end string) string {
-	return string(s.str.slice(s.str.indexOf(start.str) + 1, s.str.indexOf(end.str)))
+	return string(s.str.slice(JS.Number(int(s.str.indexOf(start.str)) + 1), s.str.indexOf(end.str)))
 }
 
 // unnecessary in the JS backend, implemented for api parity.
diff --git a/vlib/jsdom/matrix.js.v b/vlib/jsdom/matrix.js.v
index d80f714337..6b47623d57 100644
--- a/vlib/jsdom/matrix.js.v
+++ b/vlib/jsdom/matrix.js.v
@@ -1,37 +1,36 @@
 module jsdom
 
-pub struct JS.DOMMatrix {
-pub:
-	is_2d       JS.Boolean [noinit]
-	is_identity JS.Boolean [noinit]
-pub mut:
-	m11 JS.Number [noinit]
-	m12 JS.Number [noinit]
-	m13 JS.Number [noinit]
-	m14 JS.Number [noinit]
-	m21 JS.Number [noinit]
-	m22 JS.Number [noinit]
-	m23 JS.Number [noinit]
-	m24 JS.Number [noinit]
-	m31 JS.Number [noinit]
-	m32 JS.Number [noinit]
-	m33 JS.Number [noinit]
-	m34 JS.Number [noinit]
-	m41 JS.Number [noinit]
-	m42 JS.Number [noinit]
-	m43 JS.Number [noinit]
-	m44 JS.Number [noinit]
-	a   JS.Number [noinit]
-	b   JS.Number [noinit]
-	c   JS.Number [noinit]
-	d   JS.Number [noinit]
-	e   JS.Number [noinit]
-	f   JS.Number [noinit]
+pub interface JS.DOMMatrix {
+	is_2d JS.Boolean
+	is_identity JS.Boolean
+mut:
+	m11 JS.Number
+	m12 JS.Number
+	m13 JS.Number
+	m14 JS.Number
+	m21 JS.Number
+	m22 JS.Number
+	m23 JS.Number
+	m24 JS.Number
+	m31 JS.Number
+	m32 JS.Number
+	m33 JS.Number
+	m34 JS.Number
+	m41 JS.Number
+	m42 JS.Number
+	m43 JS.Number
+	m44 JS.Number
+	a JS.Number
+	b JS.Number
+	c JS.Number
+	d JS.Number
+	e JS.Number
+	f JS.Number
 }
 
 pub struct DOMMatrix {
 mut:
-	matrix JS.DOMMatrix [noinit]
+	matrix JS.DOMMatrix = JS.DOMMatrix(voidptr(0))
 }
 
 pub fn (matrix DOMMatrix) str() string {
@@ -48,7 +47,7 @@ pub fn new_matrix(init []f64) DOMMatrix {
 		_ := val
 		#tmp.push(val);
 	}
-	mut m := JS.DOMMatrix{}
+	mut m := JS.DOMMatrix(voidptr(0))
 	#m = new DOMMatrix(tmp);
 
 	return DOMMatrix{m}
@@ -190,177 +189,177 @@ pub fn (m DOMMatrix) is_2d() bool {
 }
 
 pub fn (m DOMMatrix) a() f64 {
-	return tof64(m.matrix.a)
+	return f64(m.matrix.a)
 }
 
 pub fn (m DOMMatrix) b() f64 {
-	return tof64(m.matrix.b)
+	return f64(m.matrix.b)
 }
 
 pub fn (m DOMMatrix) c() f64 {
-	return tof64(m.matrix.c)
+	return f64(m.matrix.c)
 }
 
 pub fn (m DOMMatrix) d() f64 {
-	return tof64(m.matrix.d)
+	return f64(m.matrix.d)
 }
 
 pub fn (m DOMMatrix) e() f64 {
-	return tof64(m.matrix.e)
+	return f64(m.matrix.e)
 }
 
 pub fn (m DOMMatrix) f() f64 {
-	return tof64(m.matrix.f)
+	return f64(m.matrix.f)
 }
 
 pub fn (mut m DOMMatrix) set_a(a f64) {
-	m.matrix.a = f64tonum(a)
+	m.matrix.a = JS.Number(a)
 }
 
 pub fn (mut m DOMMatrix) set_b(b f64) {
-	m.matrix.b = f64tonum(b)
+	m.matrix.b = JS.Number(b)
 }
 
 pub fn (mut m DOMMatrix) set_c(c f64) {
-	m.matrix.c = f64tonum(c)
+	m.matrix.c = JS.Number(c)
 }
 
 pub fn (mut m DOMMatrix) set_d(d f64) {
-	m.matrix.d = f64tonum(d)
+	m.matrix.d = JS.Number(d)
 }
 
 pub fn (mut m DOMMatrix) set_e(e f64) {
-	m.matrix.e = f64tonum(e)
+	m.matrix.e = JS.Number(e)
 }
 
 pub fn (mut m DOMMatrix) set_f(f f64) {
-	m.matrix.f = f64tonum(f)
+	m.matrix.f = JS.Number(f)
 }
 
 pub fn (m DOMMatrix) m11() f64 {
-	return tof64(m.matrix.m11)
+	return f64(m.matrix.m11)
 }
 
 pub fn (m DOMMatrix) m12() f64 {
-	return tof64(m.matrix.m12)
+	return f64(m.matrix.m12)
 }
 
 pub fn (m DOMMatrix) m13() f64 {
-	return tof64(m.matrix.m13)
+	return f64(m.matrix.m13)
 }
 
 pub fn (m DOMMatrix) m14() f64 {
-	return tof64(m.matrix.m14)
+	return f64(m.matrix.m14)
 }
 
 pub fn (m DOMMatrix) m21() f64 {
-	return tof64(m.matrix.m21)
+	return f64(m.matrix.m21)
 }
 
 pub fn (m DOMMatrix) m22() f64 {
-	return tof64(m.matrix.m22)
+	return f64(m.matrix.m22)
 }
 
 pub fn (m DOMMatrix) m23() f64 {
-	return tof64(m.matrix.m23)
+	return f64(m.matrix.m23)
 }
 
 pub fn (m DOMMatrix) m24() f64 {
-	return tof64(m.matrix.m24)
+	return f64(m.matrix.m24)
 }
 
 pub fn (m DOMMatrix) m31() f64 {
-	return tof64(m.matrix.m31)
+	return f64(m.matrix.m31)
 }
 
 pub fn (m DOMMatrix) m32() f64 {
-	return tof64(m.matrix.m32)
+	return f64(m.matrix.m32)
 }
 
 pub fn (m DOMMatrix) m33() f64 {
-	return tof64(m.matrix.m33)
+	return f64(m.matrix.m33)
 }
 
 pub fn (m DOMMatrix) m34() f64 {
-	return tof64(m.matrix.m34)
+	return f64(m.matrix.m34)
 }
 
 pub fn (m DOMMatrix) m41() f64 {
-	return tof64(m.matrix.m41)
+	return f64(m.matrix.m41)
 }
 
 pub fn (m DOMMatrix) m42() f64 {
-	return tof64(m.matrix.m42)
+	return f64(m.matrix.m42)
 }
 
 pub fn (m DOMMatrix) m43() f64 {
-	return tof64(m.matrix.m43)
+	return f64(m.matrix.m43)
 }
 
 pub fn (m DOMMatrix) m44() f64 {
-	return tof64(m.matrix.m44)
+	return f64(m.matrix.m44)
 }
 
 pub fn (mut m DOMMatrix) set_m11(x f64) {
-	m.matrix.m11 = f64tonum(x)
+	m.matrix.m11 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m12(x f64) {
-	m.matrix.m12 = f64tonum(x)
+	m.matrix.m12 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m13(x f64) {
-	m.matrix.m13 = f64tonum(x)
+	m.matrix.m13 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m14(x f64) {
-	m.matrix.m14 = f64tonum(x)
+	m.matrix.m14 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m21(x f64) {
-	m.matrix.m21 = f64tonum(x)
+	m.matrix.m21 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m22(x f64) {
-	m.matrix.m22 = f64tonum(x)
+	m.matrix.m22 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m23(x f64) {
-	m.matrix.m23 = f64tonum(x)
+	m.matrix.m23 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m24(x f64) {
-	m.matrix.m24 = f64tonum(x)
+	m.matrix.m24 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m31(x f64) {
-	m.matrix.m31 = f64tonum(x)
+	m.matrix.m31 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m32(x f64) {
-	m.matrix.m32 = f64tonum(x)
+	m.matrix.m32 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m33(x f64) {
-	m.matrix.m33 = f64tonum(x)
+	m.matrix.m33 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m34(x f64) {
-	m.matrix.m34 = f64tonum(x)
+	m.matrix.m34 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m41(x f64) {
-	m.matrix.m41 = f64tonum(x)
+	m.matrix.m41 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m42(x f64) {
-	m.matrix.m42 = f64tonum(x)
+	m.matrix.m42 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m43(x f64) {
-	m.matrix.m43 = f64tonum(x)
+	m.matrix.m43 = JS.Number(x)
 }
 
 pub fn (mut m DOMMatrix) set_m44(x f64) {
-	m.matrix.m44 = f64tonum(x)
+	m.matrix.m44 = JS.Number(x)
 }
diff --git a/vlib/jsdom/path2d.js.v b/vlib/jsdom/path2d.js.v
index 1fd64f6fd0..4883d63322 100644
--- a/vlib/jsdom/path2d.js.v
+++ b/vlib/jsdom/path2d.js.v
@@ -51,11 +51,11 @@ pub fn (path Path2D) rect(x f64, y f64, width f64, height f64) {
 }
 
 pub fn (path Path2D) line_to(x f64, y f64) {
-	path.path.lineTo(f64tonum(x), f64tonum(y))
+	path.path.lineTo(JS.Number(x), JS.Number(y))
 }
 
 pub fn (path Path2D) move_to(x f64, y f64) {
-	path.path.lineTo(f64tonum(x), f64tonum(y))
+	path.path.lineTo(JS.Number(x), JS.Number(y))
 }
 
 pub fn (path Path2D) close_path() {
diff --git a/vlib/jsdom/point.js.v b/vlib/jsdom/point.js.v
index eb7704c323..21997b52c6 100644
--- a/vlib/jsdom/point.js.v
+++ b/vlib/jsdom/point.js.v
@@ -16,44 +16,44 @@ mut:
 pub fn new_dompoint(x f64, y f64, z f64, w f64) DOMPoint {
 	mut point := DOMPoint{}
 
-	point.point.x = f64tonum(x)
-	point.point.y = f64tonum(y)
-	point.point.z = f64tonum(z)
-	point.point.w = f64tonum(w)
+	point.point.x = JS.Number(x)
+	point.point.y = JS.Number(y)
+	point.point.z = JS.Number(z)
+	point.point.w = JS.Number(w)
 
 	return point
 }
 
 pub fn (p DOMPoint) x() f64 {
-	return tof64(p.point.x)
+	return f64(p.point.x)
 }
 
 pub fn (p DOMPoint) y() f64 {
-	return tof64(p.point.y)
+	return f64(p.point.y)
 }
 
 pub fn (p DOMPoint) z() f64 {
-	return tof64(p.point.z)
+	return f64(p.point.z)
 }
 
 pub fn (p DOMPoint) w() f64 {
-	return tof64(p.point.w)
+	return f64(p.point.w)
 }
 
 pub fn (mut p DOMPoint) set_x(x f64) {
-	p.point.x = f64tonum(x)
+	p.point.x = JS.Number(x)
 }
 
 pub fn (mut p DOMPoint) set_y(y f64) {
-	p.point.y = f64tonum(y)
+	p.point.y = JS.Number(y)
 }
 
 pub fn (mut p DOMPoint) set_z(z f64) {
-	p.point.z = f64tonum(z)
+	p.point.z = JS.Number(z)
 }
 
 pub fn (mut p DOMPoint) set_w(w f64) {
-	p.point.w = f64tonum(w)
+	p.point.w = JS.Number(w)
 }
 
 pub fn (p DOMPoint) matrix_transform(matrix DOMMatrix) DOMPoint {
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 2d641ca248..7cd6b6680c 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -458,6 +458,7 @@ pub fn (mut c Checker) expand_iface_embeds(idecl &ast.InterfaceDecl, level int,
 pub fn (mut c Checker) interface_decl(mut node ast.InterfaceDecl) {
 	c.check_valid_pascal_case(node.name, 'interface name', node.pos)
 	mut decl_sym := c.table.get_type_symbol(node.typ)
+	is_js := node.language == .js
 	if mut decl_sym.info is ast.Interface {
 		if node.ifaces.len > 0 {
 			all_ifaces := c.expand_iface_embeds(node, 0, node.ifaces)
@@ -552,8 +553,26 @@ pub fn (mut c Checker) interface_decl(mut node ast.InterfaceDecl) {
 				c.check_valid_snake_case(method.name, 'method name', method.pos)
 			}
 			c.ensure_type_exists(method.return_type, method.return_type_pos) or { return }
-			for param in method.params {
+			if is_js {
+				mtyp := c.table.get_type_symbol(method.return_type)
+				if (mtyp.language != .js && !method.return_type.is_void())
+					&& !mtyp.name.starts_with('JS.') {
+					c.error('method $method.name returns non JS type', method.pos)
+				}
+			}
+			for j, param in method.params {
+				if j == 0 && is_js {
+					continue // no need to check first param
+				}
 				c.ensure_type_exists(param.typ, param.pos) or { return }
+				if is_js {
+					ptyp := c.table.get_type_symbol(param.typ)
+					if ptyp.kind != .function && ptyp.language != .js
+						&& !ptyp.name.starts_with('JS.') {
+						c.error('method `$method.name` accepts non JS type as parameter',
+							method.pos)
+					}
+				}
 			}
 			for field in node.fields {
 				field_sym := c.table.get_type_symbol(field.typ)
@@ -573,6 +592,12 @@ pub fn (mut c Checker) interface_decl(mut node ast.InterfaceDecl) {
 				c.check_valid_snake_case(field.name, 'field name', field.pos)
 			}
 			c.ensure_type_exists(field.typ, field.pos) or { return }
+			if is_js {
+				tsym := c.table.get_type_symbol(field.typ)
+				if tsym.language != .js && !tsym.name.starts_with('JS.') {
+					c.error('field `$field.name` uses non JS type', field.pos)
+				}
+			}
 			if field.typ == node.typ {
 				c.error('recursive interface fields are not allowed because they cannot be initialised',
 					field.type_pos)
@@ -1916,7 +1941,7 @@ pub fn (mut c Checker) method_call(mut node ast.CallExpr) ast.Type {
 	}
 	node.left_type = left_type
 	// Set default values for .return_type & .receiver_type too,
-	// or there will be hard to diagnose 0 type panics in cgen.
+	// or there will be hard tRo diagnose 0 type panics in cgen.
 	node.return_type = left_type
 	node.receiver_type = left_type
 
@@ -3067,6 +3092,11 @@ fn (mut c Checker) type_implements(typ ast.Type, interface_type ast.Type, pos to
 	utyp := c.unwrap_generic(typ)
 	typ_sym := c.table.get_type_symbol(utyp)
 	mut inter_sym := c.table.get_type_symbol(interface_type)
+
+	// small hack for JS.Any type. Since `any` in regular V is getting deprecated we have our own JS.Any type for JS backend.
+	if typ_sym.name == 'JS.Any' {
+		return true
+	}
 	if mut inter_sym.info is ast.Interface {
 		mut generic_type := interface_type
 		mut generic_info := inter_sym.info
@@ -3105,7 +3135,8 @@ fn (mut c Checker) type_implements(typ ast.Type, interface_type ast.Type, pos to
 		// `none` "implements" the Error interface
 		return true
 	}
-	if typ_sym.kind == .interface_ && inter_sym.kind == .interface_ {
+	if typ_sym.kind == .interface_ && inter_sym.kind == .interface_ && styp != 'JS.Any'
+		&& inter_sym.name != 'JS.Any' {
 		c.error('cannot implement interface `$inter_sym.name` with a different interface `$styp`',
 			pos)
 	}
diff --git a/vlib/v/gen/js/builtin_types.v b/vlib/v/gen/js/builtin_types.v
index 46e04d2e80..e9f4ee2354 100644
--- a/vlib/v/gen/js/builtin_types.v
+++ b/vlib/v/gen/js/builtin_types.v
@@ -40,7 +40,7 @@ fn (mut g JsGen) to_js_typ_val(t ast.Type) string {
 			styp = 'new map(new Map())'
 		}
 		.array {
-			styp = '$prefix${g.sym_to_js_typ(sym)}()'
+			styp = 'empty_array()'
 		}
 		.struct_ {
 			styp = 'new ${g.js_name(sym.name)}(${g.to_js_typ_def_val(sym.name)})'
diff --git a/vlib/v/gen/js/fn.v b/vlib/v/gen/js/fn.v
index cd5bf1a5f6..2019624743 100644
--- a/vlib/v/gen/js/fn.v
+++ b/vlib/v/gen/js/fn.v
@@ -459,6 +459,7 @@ fn (mut g JsGen) generic_fn_name(types []ast.Type, before string, is_decl bool)
 }
 
 fn (mut g JsGen) gen_method_decl(it ast.FnDecl, typ FnGenType) {
+	cur_fn_decl := g.fn_decl
 	unsafe {
 		g.fn_decl = &it
 	}
@@ -528,7 +529,7 @@ fn (mut g JsGen) gen_method_decl(it ast.FnDecl, typ FnGenType) {
 	mut args := it.params
 
 	g.fn_args(args, it.is_variadic)
-	g.write(') {')
+	g.writeln(') {')
 	for i, arg in args {
 		is_varg := i == args.len - 1 && it.is_variadic
 		arg_name := g.js_name(arg.name)
@@ -584,7 +585,7 @@ fn (mut g JsGen) gen_method_decl(it ast.FnDecl, typ FnGenType) {
 		}
 	}
 
-	g.fn_decl = voidptr(0)
+	g.fn_decl = cur_fn_decl
 }
 
 fn (mut g JsGen) fn_args(args []ast.Param, is_variadic bool) {
@@ -609,6 +610,7 @@ fn (mut g JsGen) gen_anon_fn(mut fun ast.AnonFn) {
 	}
 	fun.has_gen = true
 	it := fun.decl
+	cur_fn_decl := g.fn_decl
 	unsafe {
 		g.fn_decl = &it
 	}
@@ -672,5 +674,5 @@ fn (mut g JsGen) gen_anon_fn(mut fun ast.AnonFn) {
 	g.dec_indent()
 	g.writeln('}})()')
 
-	g.fn_decl = voidptr(0)
+	g.fn_decl = cur_fn_decl
 }
diff --git a/vlib/v/gen/js/js.v b/vlib/v/gen/js/js.v
index 7fe6037fe1..962739d153 100644
--- a/vlib/v/gen/js/js.v
+++ b/vlib/v/gen/js/js.v
@@ -173,11 +173,13 @@ pub fn gen(files []&ast.File, table &ast.Table, pref &pref.Preferences) string {
 	}
 	g.enter_namespace('main')
 	// generate JS methods for interface methods
-	for _, iface_types in g.table.iface_types {
+	for iface_name, iface_types in g.table.iface_types {
+		iface := g.table.find_type(iface_name) or { panic('unreachable: interface must exist') }
 		for ty in iface_types {
 			sym := g.table.get_type_symbol(ty)
-			for method in sym.methods {
-				p_sym := g.table.get_type_symbol(method.params[0].typ)
+
+			for method in iface.methods {
+				p_sym := g.table.get_type_symbol(ty)
 				mname := g.js_name(p_sym.name) + '_' + method.name
 				g.write('${g.js_name(sym.name)}.prototype.$method.name = function(')
 				for i, param in method.params {
@@ -1702,6 +1704,10 @@ fn (mut g JsGen) gen_import_stmt(it ast.Import) {
 }
 
 fn (mut g JsGen) gen_interface_decl(it ast.InterfaceDecl) {
+	if it.language != .v {
+		// JS interfaces do not need codegen
+		return
+	}
 	// JS is dynamically typed, so we don't need any codegen at all
 	// We just need the JSDoc so TypeScript type checking works
 	g.doc.gen_interface(it)
@@ -1829,6 +1835,17 @@ fn (mut g JsGen) gen_struct_decl(node ast.StructDecl) {
 		etyp := g.typ(embed.typ)
 		g.writeln('...${g.js_name(etyp)}.prototype,')
 	}
+	for iface, iface_types in g.table.iface_types {
+		if iface.starts_with('JS.') {
+			for ty in iface_types {
+				sym := g.table.get_type_symbol(ty)
+
+				if sym.name == node.name {
+					g.writeln('...${g.js_name(iface)}.prototype,')
+				}
+			}
+		}
+	}
 	fns := g.method_fn_decls[name]
 	// gen toString method
 	fn_names := fns.map(it.name)
@@ -2335,7 +2352,9 @@ fn (mut g JsGen) match_expr_sumtype(node ast.MatchExpr, is_expr bool, cond_var M
 					g.write(' instanceof ')
 					g.expr(branch.exprs[sumtype_index])
 				} else if sym.kind == .interface_ {
-					g.write('.val')
+					if !sym.name.starts_with('JS.') {
+						g.write('.val')
+					}
 					if branch.exprs[sumtype_index] is ast.TypeNode {
 						g.write(' instanceof ')
 						g.expr(branch.exprs[sumtype_index])
diff --git a/vlib/v/parser/struct.v b/vlib/v/parser/struct.v
index d4b7105f4f..7453e6e61c 100644
--- a/vlib/v/parser/struct.v
+++ b/vlib/v/parser/struct.v
@@ -450,7 +450,12 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 	name_pos := p.tok.position()
 	p.check_for_impure_v(language, name_pos)
 	modless_name := p.check_name()
-	interface_name := p.prepend_mod(modless_name).clone()
+	mut interface_name := ''
+	if language == .js {
+		interface_name = 'JS.' + modless_name
+	} else {
+		interface_name = p.prepend_mod(modless_name)
+	}
 	generic_types, _ := p.parse_generic_types()
 	// println('interface decl $interface_name')
 	p.check(.lcbr)
@@ -472,6 +477,7 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 			is_generic: generic_types.len > 0
 			generic_types: generic_types
 		}
+		language: language
 	)
 	if reg_idx == -1 {
 		p.error_with_pos('cannot register interface `$interface_name`, another type with this name exists',
@@ -492,8 +498,11 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 	for p.tok.kind != .rcbr && p.tok.kind != .eof {
 		if p.tok.kind == .name && p.tok.lit.len > 0 && p.tok.lit[0].is_capital() {
 			iface_pos := p.tok.position()
-			iface_name := p.tok.lit
+			mut iface_name := p.tok.lit
 			iface_type := p.parse_type()
+			if iface_name == 'JS' {
+				iface_name = p.table.get_type_symbol(iface_type).name
+			}
 			comments := p.eat_comments()
 			ifaces << ast.InterfaceEmbedding{
 				name: iface_name