diff --git a/vlib/builtin/js/builtin.js.v b/vlib/builtin/js/builtin.js.v
index b97445ec99..99f451814c 100644
--- a/vlib/builtin/js/builtin.js.v
+++ b/vlib/builtin/js/builtin.js.v
@@ -78,3 +78,21 @@ fn js_stacktrace() string {
 
 	return stacktrace
 }
+
+// Check for nil value
+pub fn isnil(val voidptr) bool {
+	res := false
+	// This one is kinda weird. In C and native backend we can cast booleans and integers to pointers
+	// so we just check *for* all possible NULL-like values here.
+	#val = val.valueOf()
+	#res.val = val === null || val === undefined || val === false || val === 0 || val === BigInt(0)
+
+	return res
+}
+
+pub fn (f float_literal) str() string {
+	res := ''
+	#res.str += f.valueOf()
+
+	return res
+}
diff --git a/vlib/builtin/js/byte.js.v b/vlib/builtin/js/byte.js.v
index 78b3f6fb2e..c2bee28d0f 100644
--- a/vlib/builtin/js/byte.js.v
+++ b/vlib/builtin/js/byte.js.v
@@ -7,14 +7,6 @@ pub fn (b byte) is_space() bool {
 	return result
 }
 
-pub fn (c byte) is_letter() bool {
-	result := false
-
-	#result.val = (c.val >= `a`.charCodeAt() && c.val <= `z`.charCodeAt()) || (c.val >= `A`.charCodeAt() && c.val <= `Z`.charCodeAt())
-
-	return result
-}
-
 pub fn (c byte) str() string {
 	res := ''
 	#res.str = c.val.toString()
@@ -37,3 +29,42 @@ pub fn (c byte) repeat(count int) string {
 
 	return res
 }
+
+pub fn (c byte) is_digit() bool {
+	return c >= `0` && c <= `9`
+}
+
+// is_hex_digit returns `true` if the byte is either in range 0-9, a-f or A-F and `false` otherwise.
+// Example: assert byte(`F`) == true
+[inline]
+pub fn (c byte) is_hex_digit() bool {
+	return c.is_digit() || (c >= `a` && c <= `f`) || (c >= `A` && c <= `F`)
+}
+
+// is_oct_digit returns `true` if the byte is in range 0-7 and `false` otherwise.
+// Example: assert byte(`7`) == true
+[inline]
+pub fn (c byte) is_oct_digit() bool {
+	return c >= `0` && c <= `7`
+}
+
+// is_bin_digit returns `true` if the byte is a binary digit (0 or 1) and `false` otherwise.
+// Example: assert byte(`0`) == true
+[inline]
+pub fn (c byte) is_bin_digit() bool {
+	return c == `0` || c == `1`
+}
+
+// is_letter returns `true` if the byte is in range a-z or A-Z and `false` otherwise.
+// Example: assert byte(`V`) == true
+[inline]
+pub fn (c byte) is_letter() bool {
+	return (c >= `a` && c <= `z`) || (c >= `A` && c <= `Z`)
+}
+
+// is_alnum returns `true` if the byte is in range a-z, A-Z, 0-9 and `false` otherwise.
+// Example: assert byte(`V`) == true
+[inline]
+pub fn (c byte) is_alnum() bool {
+	return c.is_letter() || c.is_digit()
+}
diff --git a/vlib/builtin/js/int.js.v b/vlib/builtin/js/int.js.v
index dd67017078..379fc95645 100644
--- a/vlib/builtin/js/int.js.v
+++ b/vlib/builtin/js/int.js.v
@@ -23,35 +23,35 @@ pub fn (i u16) str() string {
 
 pub fn (i int) str() string {
 	mut res := ''
-	#res = new string( i )
+	#res = new string( i+'' )
 
 	return res
 }
 
 pub fn (i i64) str() string {
 	mut res := ''
-	#res = new string( i )
+	#res = new string( i + '')
 
 	return res
 }
 
 pub fn (i u32) str() string {
 	mut res := ''
-	#res = new string( i )
+	#res = new string( i + '')
 
 	return res
 }
 
 pub fn (i u64) str() string {
 	mut res := ''
-	#res = new string( i )
+	#res = new string( i + '')
 
 	return res
 }
 
 pub fn (i bool) str() string {
 	mut res := ''
-	#res = new string( i )
+	#res = new string( i + '')
 
 	return res
 }
diff --git a/vlib/builtin/js/string.js.v b/vlib/builtin/js/string.js.v
index 43a35a96e5..16e290db3e 100644
--- a/vlib/builtin/js/string.js.v
+++ b/vlib/builtin/js/string.js.v
@@ -232,6 +232,13 @@ pub fn (s string) u64() u64 {
 	return u64(JS.parseInt(s))
 }
 
+pub fn (s string) byte() u64 {
+	res := byte(0)
+	#res.val = byte(JS.parseInt(s))
+
+	return res
+}
+
 // trim_right strips any of the characters given in `cutset` from the right of the string.
 // Example: assert ' Hello V d'.trim_right(' d') == ' Hello V'
 pub fn (s string) trim_right(cutset string) string {
@@ -897,3 +904,6 @@ pub fn (s []string) join(sep string) string {
 	}
 	return res
 }
+
+// There's no better way to find length of JS String in bytes.
+#Object.defineProperty(string.prototype,"len", { get: function() {return new int(new TextEncoder().encode(this.str).length);}, set: function(l) {/* ignore */ } });
diff --git a/vlib/builtin/js/string_test.js.v b/vlib/builtin/js/string_test.js.v
index 82d214b498..71c608658e 100644
--- a/vlib/builtin/js/string_test.js.v
+++ b/vlib/builtin/js/string_test.js.v
@@ -359,11 +359,10 @@ fn test_reassign() {
 	assert b == 'hi!'
 }
 
-/*
 fn test_runes() {
 	s := 'привет'
 	println(s.len)
-	//assert s.len == 12
+	// assert s.len == 12
 	s2 := 'privet'
 	assert s2.len == 6
 	u := s.runes()
@@ -386,7 +385,7 @@ fn test_runes() {
 	last := u[u.len - 1]
 	assert first.str().len == 2
 	assert last.str().len == 2
-}*/
+}
 
 fn test_contains() {
 	s := 'view.v'
@@ -420,7 +419,6 @@ fn test_arr_contains() {
 	assert ints.contains(2)
 }
 
-/*
 fn test_to_num() {
 	s := '7'
 	assert s.int() == 7
@@ -434,7 +432,7 @@ fn test_to_num() {
 	big := '93993993939322'
 	assert big.u64() == 93993993939322
 	assert big.i64() == 93993993939322
-}*/
+}
 
 /*
 fn test_inter_format_string() {
@@ -473,7 +471,6 @@ fn test_hash() {
 	assert s5.hash() % ((1 << 20) - 1) == s.hash() % ((1 << 20) - 1)
 	assert s5.hash() % ((1 << 20) - 1) == 592861
 }*/
-
 fn test_trim() {
 	assert 'banana'.trim('bna') == ''
 	assert 'abc'.trim('ac') == 'b'
diff --git a/vlib/strings/builder.js.v b/vlib/strings/builder.js.v
index 7134b2a015..a2cc91854f 100644
--- a/vlib/strings/builder.js.v
+++ b/vlib/strings/builder.js.v
@@ -97,3 +97,25 @@ pub fn (mut b Builder) write_runes(runes []rune) {
 		b << res.bytes()
 	}
 }
+
+// after(6) returns 'world'
+// buf == 'hello world'
+pub fn (mut b Builder) after(n int) string {
+	if n >= b.len {
+		return ''
+	}
+
+	x := b.slice(n, b.len)
+	return x.bytestr()
+}
+
+// last_n(5) returns 'world'
+// buf == 'hello world'
+pub fn (b &Builder) last_n(n int) string {
+	if n >= b.len {
+		return ''
+	}
+
+	x := b.slice(b.len - n, b.len)
+	return x.bytestr()
+}
diff --git a/vlib/v/gen/js/comptime.v b/vlib/v/gen/js/comptime.v
index 69d60412d4..ce4c43a9da 100644
--- a/vlib/v/gen/js/comptime.v
+++ b/vlib/v/gen/js/comptime.v
@@ -272,7 +272,7 @@ fn (mut g JsGen) comp_if_to_ifdef(name string, is_comptime_optional bool) ?strin
 			return 'false'
 		}
 		'no_bounds_checking' {
-			return 'CUSTOM_DEFINE_no_bounds_checking'
+			return 'checkDefine("CUSTOM_DEFINE_no_bounds_checking")'
 		}
 		'freestanding' {
 			return '_VFREESTANDING'
@@ -301,7 +301,7 @@ fn (mut g JsGen) comp_if_to_ifdef(name string, is_comptime_optional bool) ?strin
 		else {
 			if is_comptime_optional
 				|| (g.pref.compile_defines_all.len > 0 && name in g.pref.compile_defines_all) {
-				return 'CUSTOM_DEFINE_$name'
+				return 'checkDefine("CUSTOM_DEFINE_$name")'
 			}
 			return error('bad os ifdef name "$name"') // should never happen, caught in the checker
 		}
diff --git a/vlib/v/gen/js/fn.v b/vlib/v/gen/js/fn.v
index dd52f292eb..734042d207 100644
--- a/vlib/v/gen/js/fn.v
+++ b/vlib/v/gen/js/fn.v
@@ -221,6 +221,7 @@ fn (mut g JsGen) method_call(node ast.CallExpr) {
 	name = g.generic_fn_name(node.concrete_types, name, false)
 	g.write('${name}(')
 	g.expr(it.left)
+	g.gen_deref_ptr(it.left_type)
 	g.write(',')
 	for i, arg in it.args {
 		g.expr(arg.expr)
diff --git a/vlib/v/gen/js/js.v b/vlib/v/gen/js/js.v
index 4af8632679..d37175b3f6 100644
--- a/vlib/v/gen/js/js.v
+++ b/vlib/v/gen/js/js.v
@@ -144,7 +144,6 @@ pub fn gen(files []&ast.File, table &ast.Table, pref &pref.Preferences) string {
 		if g.file.mod.name == 'builtin' && !g.generated_builtin {
 			g.gen_builtin_type_defs()
 			g.writeln('Object.defineProperty(array.prototype,"len", { get: function() {return new int(this.arr.arr.length);}, set: function(l) { this.arr.arr.length = l.valueOf(); } }); ')
-			g.writeln('Object.defineProperty(string.prototype,"len", { get: function() {return new int(this.str.length);}, set: function(l) {/* ignore */ } }); ')
 			g.writeln('Object.defineProperty(map.prototype,"len", { get: function() {return new int(this.map.size);}, set: function(l) { this.map.size = l.valueOf(); } }); ')
 			g.writeln('Object.defineProperty(array.prototype,"length", { get: function() {return new int(this.arr.arr.length);}, set: function(l) { this.arr.arr.length = l.valueOf(); } }); ')
 			g.generated_builtin = true
@@ -410,19 +409,14 @@ pub fn (mut g JsGen) init() {
 
 		g.definitions.writeln('const \$os = {')
 		g.definitions.writeln('  endianess: "LE",')
-
 		g.definitions.writeln('}')
 	} else {
 		g.definitions.writeln('const \$os = require("os");')
 		g.definitions.writeln('const \$process = process;')
 	}
-	g.definitions.writeln('function alias(value) { return value; } ')
-
-	g.definitions.writeln('function \$v_fmt(value) { let res = "";
-		if (Object.getPrototypeOf(s).hasOwnProperty("str") && typeof s.str == "function") res = s.str().str
-		else res = s.toString()
-		return res
-  } ')
+	g.definitions.writeln('function checkDefine(key) {')
+	g.definitions.writeln('\tif (globalThis.hasOwnProperty(key)) { return !!globalThis[key]; } return false;')
+	g.definitions.writeln('}')
 }
 
 pub fn (g JsGen) hashes() string {
@@ -848,7 +842,7 @@ fn (mut g JsGen) expr(node ast.Expr) {
 			// TODO
 		}
 		ast.CTempVar {
-			g.write('/* ast.CTempVar: node.name */')
+			g.write('$node.name')
 		}
 		ast.DumpExpr {
 			g.write('/* ast.DumpExpr: $node.expr */')
@@ -1008,6 +1002,60 @@ fn (mut g JsGen) expr(node ast.Expr) {
 	}
 }
 
+struct UnsupportedAssertCtempTransform {
+	msg  string
+	code int
+}
+
+const unsupported_ctemp_assert_transform = IError(UnsupportedAssertCtempTransform{})
+
+fn (mut g JsGen) assert_subexpression_to_ctemp(expr ast.Expr, expr_type ast.Type) ?ast.Expr {
+	match expr {
+		ast.CallExpr {
+			return g.new_ctemp_var_then_gen(expr, expr_type)
+		}
+		ast.ParExpr {
+			if expr.expr is ast.CallExpr {
+				return g.new_ctemp_var_then_gen(expr.expr, expr_type)
+			}
+		}
+		ast.SelectorExpr {
+			if expr.expr is ast.CallExpr {
+				sym := g.table.get_final_type_symbol(g.unwrap_generic(expr.expr.return_type))
+				if sym.kind == .struct_ {
+					if (sym.info as ast.Struct).is_union {
+						return js.unsupported_ctemp_assert_transform
+					}
+				}
+				return g.new_ctemp_var_then_gen(expr, expr_type)
+			}
+		}
+		else {}
+	}
+	return js.unsupported_ctemp_assert_transform
+}
+
+fn (mut g JsGen) new_ctemp_var(expr ast.Expr, expr_type ast.Type) ast.CTempVar {
+	return ast.CTempVar{
+		name: g.new_tmp_var()
+		typ: expr_type
+		is_ptr: expr_type.is_ptr()
+		orig: expr
+	}
+}
+
+fn (mut g JsGen) new_ctemp_var_then_gen(expr ast.Expr, expr_type ast.Type) ast.CTempVar {
+	x := g.new_ctemp_var(expr, expr_type)
+	g.gen_ctemp_var(x)
+	return x
+}
+
+fn (mut g JsGen) gen_ctemp_var(tvar ast.CTempVar) {
+	g.write('let $tvar.name = ')
+	g.expr(tvar.orig)
+	g.writeln(';')
+}
+
 fn (mut g JsGen) gen_assert_metainfo(node ast.AssertStmt) string {
 	mod_path := g.file.path
 	fn_name := g.fn_decl.name
@@ -1029,12 +1077,12 @@ fn (mut g JsGen) gen_assert_metainfo(node ast.AssertStmt) string {
 			g.writeln('\t${metaname}.op = new string("$expr_op_str");')
 			g.writeln('\t${metaname}.llabel = new string("$expr_left_str");')
 			g.writeln('\t${metaname}.rlabel = new string("$expr_right_str");')
-			g.write('\t${metaname}.lvalue = new string("')
+			g.write('\t${metaname}.lvalue = ')
 			g.gen_assert_single_expr(node.expr.left, node.expr.left_type)
-			g.writeln('");')
-			g.write('\t${metaname}.rvalue = new string("')
+			g.writeln(';')
+			g.write('\t${metaname}.rvalue = ')
 			g.gen_assert_single_expr(node.expr.right, node.expr.right_type)
-			g.writeln('");')
+			g.writeln(';')
 		}
 		ast.CallExpr {
 			g.writeln('\t${metaname}.op = new string("call");')
@@ -1049,28 +1097,69 @@ fn (mut g JsGen) gen_assert_single_expr(expr ast.Expr, typ ast.Type) {
 	unknown_value := '*unknown value*'
 	match expr {
 		ast.CastExpr, ast.IfExpr, ast.IndexExpr, ast.MatchExpr {
-			g.write(unknown_value)
+			g.write('new string("$unknown_value")')
 		}
 		ast.PrefixExpr {
-			g.write(unknown_value)
+			if expr.right is ast.CastExpr {
+				// TODO: remove this check;
+				// vlib/builtin/map_test.v (a map of &int, set to &int(0)) fails
+				// without special casing ast.CastExpr here
+				g.write('new string("$unknown_value")')
+			} else {
+				g.gen_expr_to_string(expr, typ)
+			}
 		}
 		ast.TypeNode {
 			sym := g.table.get_type_symbol(g.unwrap_generic(typ))
-			g.write('$sym.name')
+			g.write('new string("$sym.name"')
 		}
 		else {
-			g.write(unknown_value)
+			mut should_clone := true
+			if typ == ast.string_type && expr is ast.StringLiteral {
+				should_clone = false
+			}
+			if expr is ast.CTempVar {
+				if expr.orig is ast.CallExpr {
+					should_clone = false
+					if expr.orig.or_block.kind == .propagate {
+						should_clone = true
+					}
+					if expr.orig.is_method && expr.orig.args.len == 0
+						&& expr.orig.name == 'type_name' {
+						should_clone = true
+					}
+				}
+			}
+			if should_clone {
+				g.write('string_clone(')
+			}
+			g.gen_expr_to_string(expr, typ)
+			if should_clone {
+				g.write(')')
+			}
 		}
 	}
 	// g.writeln(' /* typeof: ' + expr.type_name() + ' type: ' + typ.str() + ' */ ')
 }
 
 // TODO
-fn (mut g JsGen) gen_assert_stmt(a ast.AssertStmt) {
-	if !a.is_used {
+fn (mut g JsGen) gen_assert_stmt(orig_node ast.AssertStmt) {
+	mut node := orig_node
+	if !node.is_used {
 		return
 	}
+
 	g.writeln('// assert')
+
+	if mut node.expr is ast.InfixExpr {
+		if subst_expr := g.assert_subexpression_to_ctemp(node.expr.left, node.expr.left_type) {
+			node.expr.left = subst_expr
+		}
+		if subst_expr := g.assert_subexpression_to_ctemp(node.expr.right, node.expr.right_type) {
+			node.expr.right = subst_expr
+		}
+	}
+	mut a := node
 	g.write('if( ')
 	g.expr(a.expr)
 	g.write('.valueOf() ) {')