diff --git a/vlib/builtin/js/builtin.js.v b/vlib/builtin/js/builtin.js.v
index 4290f4b989..c150f3c40c 100644
--- a/vlib/builtin/js/builtin.js.v
+++ b/vlib/builtin/js/builtin.js.v
@@ -36,3 +36,11 @@ pub fn eprint(s any) {
 		panic('Cannot `eprint` in a browser, use `println` instead')
 	}
 }
+
+// Exits the process in node, and halts execution in the browser
+// because `process.exit` is undefined. Workaround for not having
+// a 'real' way to exit in the browser.
+pub fn exit(c int) {
+	JS.process.exit(c)
+	js_throw('exit($c)')
+}
diff --git a/vlib/builtin/js/builtin.v b/vlib/builtin/js/builtin.v
index 5f97a29226..d1829def62 100644
--- a/vlib/builtin/js/builtin.v
+++ b/vlib/builtin/js/builtin.v
@@ -6,14 +6,6 @@ module builtin
 
 fn (a any) toString()
 
-// Exits the process in node, and halts execution in the browser
-// because `process.exit` is undefined. Workaround for not having
-// a 'real' way to exit in the browser.
-pub fn exit(c int) {
-	JS.process.exit(c)
-	js_throw('exit($c)')
-}
-
 pub fn unwrap(opt any) any {
 	o := &Option(opt)
 	if o.state != 0 {
diff --git a/vlib/builtin/js/string.js.v b/vlib/builtin/js/string.js.v
index ec95bd6bb6..8b2933de7c 100644
--- a/vlib/builtin/js/string.js.v
+++ b/vlib/builtin/js/string.js.v
@@ -567,16 +567,19 @@ pub fn (s string) replace_each(vals []string) string {
 
 	mut idxs := []RepIndex{}
 	mut idx := 0
+	mut new_len := s.len
 	s_ := s.clone()
 	#function setCharAt(str,index,chr) {
 	#if(index > str.length-1) return str;
 	#return str.substring(0,index) + chr + str.substring(index+1);
 	#}
 
-	for rep_i := 0; rep_i < vals.len; rep_i += 2 {
+	for rep_i := 0; rep_i < vals.len; rep_i = rep_i + 2 {
 		rep := vals[rep_i]
+
 		mut with_ := vals[rep_i + 1]
 		with_ = with_
+
 		for {
 			idx = s_.index_after(rep, idx)
 			if idx == -1 {
@@ -589,11 +592,16 @@ pub fn (s string) replace_each(vals []string) string {
 				#s_.str = setCharAt(s_.str,idx + i, String.fromCharCode(127))
 			}
 
-			idxs << RepIndex{
-				idx: idx
-				val_idx: rep_i
+			rep_idx := RepIndex{
+				idx: 0
+				val_idx: 0
 			}
+			// todo: primitives should always be copied
+			#rep_idx.idx = idx.val
+			#rep_idx.val_idx = new int(rep_i.val)
+			idxs << rep_idx
 			idx += rep.len
+			new_len += with_.len - rep.len
 		}
 	}
 
@@ -604,6 +612,9 @@ pub fn (s string) replace_each(vals []string) string {
 	idxs.sort(a.idx < b.idx)
 
 	mut b := ''
+	#for (let i = 0; i < new_len.val;i++) b.str += String.fromCharCode(127)
+
+	new_len = new_len
 	mut idx_pos := 0
 	mut cur_idx := idxs[idx_pos]
 	mut b_i := 0
@@ -613,6 +624,7 @@ pub fn (s string) replace_each(vals []string) string {
 			with_ := vals[cur_idx.val_idx + 1]
 			for j in 0 .. with_.len {
 				mut j_ := j
+
 				j_ = j_
 				#b.str = setCharAt(b.str,b_i, with_.str[j])
 				//#b.str[b_i] = with_.str[j]
diff --git a/vlib/builtin/string.v b/vlib/builtin/string.v
index 7caf9e5063..1a4815dc2b 100644
--- a/vlib/builtin/string.v
+++ b/vlib/builtin/string.v
@@ -357,9 +357,9 @@ pub fn (s string) replace_each(vals []string) string {
 		// vals: ['rep1, 'with1', 'rep2', 'with2']
 		rep := vals[rep_i]
 		with := vals[rep_i + 1]
+
 		for {
 			idx = s_.index_after(rep, idx)
-
 			if idx == -1 {
 				break
 			}
@@ -371,6 +371,7 @@ pub fn (s string) replace_each(vals []string) string {
 			}
 			// We need to remember both the position in the string,
 			// and which rep/with pair it refers to.
+
 			idxs << RepIndex{
 				idx: idx
 				val_idx: rep_i
@@ -380,6 +381,7 @@ pub fn (s string) replace_each(vals []string) string {
 			new_len += with.len - rep.len
 		}
 	}
+
 	// Dont change the string if there's nothing to replace
 	if idxs.len == 0 {
 		return s.clone()
diff --git a/vlib/os/environment.js.v b/vlib/os/environment.js.v
index 0e7961f56d..ac760a59c4 100644
--- a/vlib/os/environment.js.v
+++ b/vlib/os/environment.js.v
@@ -1,9 +1,9 @@
 module os
 
 $if js_node {
-	#const $ENV = $process.env
+	#global.$ENV = $process.env
 } $else {
-	#const $ENV = {}
+	#global.$ENV = {}
 }
 
 // setenv sets the value of an environment variable with `name` to `value`.
diff --git a/vlib/v/gen/js/js.v b/vlib/v/gen/js/js.v
index 3aeb9fe099..d86064659d 100644
--- a/vlib/v/gen/js/js.v
+++ b/vlib/v/gen/js/js.v
@@ -81,6 +81,11 @@ mut:
 	defer_ifdef           string
 }
 
+fn (mut g JsGen) write_tests_definitions() {
+	g.definitions.writeln('globalThis.g_test_oks = 0;')
+	g.definitions.writeln('globalThis.g_test_fails = 0;')
+}
+
 pub fn gen(files []&ast.File, table &ast.Table, pref &pref.Preferences) string {
 	mut g := &JsGen{
 		definitions: strings.new_builder(100)
@@ -105,6 +110,8 @@ pub fn gen(files []&ast.File, table &ast.Table, pref &pref.Preferences) string {
 		mut sg := sourcemap.generate_empty_map()
 		g.sourcemap = sg.add_map('', '', g.pref.sourcemap_src_included, 0, 0)
 	}
+	mut tests_inited := false
+
 	// Get class methods
 	for file in files {
 		g.file = file
@@ -133,15 +140,22 @@ pub fn gen(files []&ast.File, table &ast.Table, pref &pref.Preferences) string {
 			g.writeln('Object.defineProperty(array.prototype,"length", { get: function() {return new builtin.int(this.arr.length);}, set: function(l) { this.arr.length = l.valueOf(); } }); ')
 			g.generated_builtin = true
 		}
-
+		if g.is_test && !tests_inited {
+			g.write_tests_definitions()
+			tests_inited = true
+		}
 		g.stmts(file.stmts)
 		g.writeln('try { init() } catch (_) {}')
 		// store the current namespace
 		g.escape_namespace()
 	}
+	if g.pref.is_test {
+		g.gen_js_main_for_tests()
+	}
 	// resolve imports
 	deps_resolved := graph.resolve()
 	nodes := deps_resolved.nodes
+
 	mut out := g.hashes() + g.definitions.str()
 	// equality check for js objects
 	// TODO: Fix msvc bug that's preventing $embed_file('fast_deep_equal.js')
@@ -262,6 +276,67 @@ fn (g JsGen) create_sourcemap() string {
 	return out
 }
 
+pub fn (mut g JsGen) gen_js_main_for_tests() {
+	g.enter_namespace('main')
+	g.writeln('(function() {  ')
+	g.inc_indent()
+	all_tfuncs := g.get_all_test_function_names()
+
+	g.writeln('')
+	g.writeln('globalThis.VTEST=1')
+	// g.writeln('let bt = start_testing($all_tfuncs.len, "$g.pref.path")')
+
+	for tname in all_tfuncs {
+		tcname := g.js_name(tname)
+
+		if g.pref.is_stats {
+			// g.writeln('bt.testing_step_start("$tcname")')
+		}
+
+		g.writeln('try { ${tcname}(); } catch (_e) {} ')
+		if g.pref.is_stats {
+			//	g.writeln('bt.testing_step_end();')
+		}
+	}
+
+	g.writeln('')
+	if g.pref.is_stats {
+		// g.writeln('bt.end_testing();')
+	}
+	g.dec_indent()
+	g.writeln('})();')
+	g.escape_namespace()
+}
+
+fn (g &JsGen) get_all_test_function_names() []string {
+	mut tfuncs := []string{}
+	mut tsuite_begin := ''
+	mut tsuite_end := ''
+	for _, f in g.table.fns {
+		if f.name.ends_with('.testsuite_begin') {
+			tsuite_begin = f.name
+			continue
+		}
+		if f.name.contains('.test_') {
+			tfuncs << f.name
+			continue
+		}
+		if f.name.ends_with('.testsuite_end') {
+			tsuite_end = f.name
+			continue
+		}
+	}
+	mut all_tfuncs := []string{}
+	if tsuite_begin.len > 0 {
+		all_tfuncs << tsuite_begin
+	}
+	all_tfuncs << tfuncs
+	if tsuite_end.len > 0 {
+		all_tfuncs << tsuite_end
+	}
+	return all_tfuncs
+}
+
 pub fn (mut g JsGen) enter_namespace(name string) {
 	if g.namespaces[name] == 0 {
 		// create a new namespace
@@ -936,6 +1011,63 @@ fn (mut g JsGen) expr(node ast.Expr) {
 	}
 }
 
+fn (mut g JsGen) gen_assert_metainfo(node ast.AssertStmt) string {
+	mod_path := g.file.path
+	fn_name := g.fn_decl.name
+	line_nr := node.pos.line_nr
+	src := node.expr.str()
+	metaname := 'v_assert_meta_info_$g.new_tmp_var()'
+	g.writeln('let $metaname = {}')
+	g.writeln('${metaname}.fpath = new builtin.string("$mod_path");')
+	g.writeln('${metaname}.line_nr = new builtin.int("$line_nr")')
+	g.writeln('${metaname}.fn_name = new builtin.string("$fn_name")')
+	metasrc := src
+	g.writeln('${metaname}.src = "$metasrc"')
+
+	match mut node.expr {
+		ast.InfixExpr {
+			expr_op_str := node.expr.op.str()
+			expr_left_str := node.expr.left.str()
+			expr_right_str := node.expr.right.str()
+			g.writeln('\t${metaname}.op = new builtin.string("$expr_op_str");')
+			g.writeln('\t${metaname}.llabel = new builtin.string("$expr_left_str");')
+			g.writeln('\t${metaname}.rlabel = new builtin.string("$expr_right_str");')
+			g.write('\t${metaname}.lvalue = new builtin.string("')
+			g.gen_assert_single_expr(node.expr.left, node.expr.left_type)
+			g.writeln('");')
+			g.write('\t${metaname}.rvalue = new builtin.string("')
+			g.gen_assert_single_expr(node.expr.right, node.expr.right_type)
+			g.writeln('");')
+		}
+		ast.CallExpr {
+			g.writeln('\t${metaname}.op = new builtin.string("call");')
+		}
+		else {}
+	}
+	return metaname
+}
+
+fn (mut g JsGen) gen_assert_single_expr(expr ast.Expr, typ ast.Type) {
+	// eprintln('> gen_assert_single_expr typ: $typ | expr: $expr | typeof(expr): ${typeof(expr)}')
+	unknown_value := '*unknown value*'
+	match expr {
+		ast.CastExpr, ast.IfExpr, ast.IndexExpr, ast.MatchExpr {
+			g.write(unknown_value)
+		}
+		ast.PrefixExpr {
+			g.write(unknown_value)
+		}
+		ast.TypeNode {
+			sym := g.table.get_type_symbol(g.unwrap_generic(typ))
+			g.write('$sym.name')
+		}
+		else {
+			g.writeln(unknown_value)
+		}
+	}
+	g.write(' /* typeof: ' + expr.type_name() + ' type: ' + typ.str() + ' */ ')
+}
+
 // TODO
 fn (mut g JsGen) gen_assert_stmt(a ast.AssertStmt) {
 	if !a.is_used {
@@ -948,12 +1080,14 @@ fn (mut g JsGen) gen_assert_stmt(a ast.AssertStmt) {
 	s_assertion := a.expr.str().replace('"', "'")
 	mut mod_path := g.file.path.replace('\\', '\\\\')
 	if g.is_test {
+		metaname_ok := g.gen_assert_metainfo(a)
 		g.writeln('	g_test_oks++;')
-		g.writeln('	cb_assertion_ok("$mod_path", ${a.pos.line_nr + 1}, "assert $s_assertion", "${g.fn_decl.name}()" );')
+		g.writeln('	cb_assertion_ok($metaname_ok);')
 		g.writeln('} else {')
+		metaname_fail := g.gen_assert_metainfo(a)
 		g.writeln('	g_test_fails++;')
-		g.writeln('	cb_assertion_failed("$mod_path", ${a.pos.line_nr + 1}, "assert $s_assertion", "${g.fn_decl.name}()" );')
-		g.writeln('	exit(1);')
+		g.writeln('	cb_assertion_failed($metaname_fail);')
+		g.writeln('	builtin.exit(1);')
 		g.writeln('}')
 		return
 	}
diff --git a/vlib/v/preludes_js/tests_assertions.v b/vlib/v/preludes_js/tests_assertions.v
index 4226ea2f51..0c96f95fb6 100644
--- a/vlib/v/preludes_js/tests_assertions.v
+++ b/vlib/v/preludes_js/tests_assertions.v
@@ -1,6 +1,6 @@
 module main
 
-import os_js
+import os
 
 // VAssertMetaInfo is used during assertions. An instance of it
 // is filled in by compile time generated code, when an assertion fails.
@@ -20,7 +20,7 @@ pub:
 const use_relative_paths = can_use_relative_paths()
 
 fn can_use_relative_paths() bool {
-	return match os_js.getenv('VERROR_PATHS') {
+	return match os.getenv('VERROR_PATHS') {
 		'absolute' { false }
 		else { true }
 	}
@@ -40,11 +40,13 @@ fn myeprintln(s string) {
 // //////////////////////////////////////////////////////////////////
 // TODO copy pasta builtin.v fn ___print_assert_failure
 fn cb_assertion_failed(i VAssertMetaInfo) {
-	filepath := if use_relative_paths { i.fpath } else { os_js.real_path(i.fpath) }
+	filepath := if use_relative_paths { i.fpath } else { os.real_path(i.fpath) }
 	mut final_filepath := filepath + ':${i.line_nr + 1}:'
-	mut final_funcname := 'fn ' + i.fn_name.replace('main.', '').replace('__', '.')
+	mut final_funcname := 'fn ' + i.fn_name
 	final_src := 'assert ' + i.src
+
 	myeprintln('$final_filepath $final_funcname')
+
 	if i.op.len > 0 && i.op != 'call' {
 		mut lvtitle := '    Left value:'
 		mut rvtitle := '    Right value:'
@@ -72,15 +74,15 @@ fn cb_assertion_ok(i &VAssertMetaInfo) {
 }
 
 fn cb_propagate_test_error(line_nr int, file string, mod string, fn_name string, errmsg string) {
-	filepath := if use_relative_paths { file } else { os_js.real_path(file) }
+	filepath := if use_relative_paths { file } else { os.real_path(file) }
 	mut final_filepath := filepath + ':$line_nr:'
 	mut final_funcname := 'fn ' + fn_name.replace('main.', '').replace('__', '.')
 	final_msg := errmsg
 	myeprintln('$final_filepath $final_funcname failed propagation with error: $final_msg')
-	// TODO: implement os_js.is_file and os_js.read_lines:
+	// TODO: implement os.is_file and os.read_lines:
 	/*
-	if os_js.is_file(file) {
-		source_lines := os_js.read_lines(file) or { []string{len: line_nr + 1} }
+	if os.is_file(file) {
+		source_lines := os.read_lines(file) or { []string{len: line_nr + 1} }
 		myeprintln('${line_nr:5} | ${source_lines[line_nr - 1]}')
 	}
 	*/
diff --git a/vlib/v/preludes_js/tests_with_stats.v b/vlib/v/preludes_js/tests_with_stats.v
index 2ea2d568ba..cc6992ec01 100644
--- a/vlib/v/preludes_js/tests_with_stats.v
+++ b/vlib/v/preludes_js/tests_with_stats.v
@@ -20,7 +20,7 @@ mut:
 
 // ///////////////////////////////////////////////////////////////////
 // Called at the start of the test program produced by `v -stats file_test.v`
-fn start_testing(total_number_of_tests int, vfilename string) BenchedTests {
+pub fn start_testing(total_number_of_tests int, vfilename string) BenchedTests {
 	mut benched_tests_res := BenchedTests{}
 	benched_tests_res.total_number_of_tests = total_number_of_tests
 	benched_tests_res.test_suit_file = vfilename