diff --git a/vlib/v/builder/compile.v b/vlib/v/builder/compile.v
index cda440efb9..29250aec71 100644
--- a/vlib/v/builder/compile.v
+++ b/vlib/v/builder/compile.v
@@ -40,7 +40,7 @@ pub fn compile(command string, pref &pref.Preferences) {
 	mut sw := time.new_stopwatch({})
 	match pref.backend {
 		.c { b.compile_c() }
-		.js { b.compile_js() }
+		.js_node, .js_freestanding, .js_browser { b.compile_js() }
 		.native { b.compile_native() }
 	}
 	mut timers := util.get_timers()
@@ -118,7 +118,7 @@ fn (mut b Builder) run_compiled_executable_and_exit() {
 	}
 	mut exefile := os.real_path(b.pref.out_name)
 	mut cmd := '"$exefile"'
-	if b.pref.backend == .js {
+	if b.pref.backend.is_js() {
 		exefile = os.real_path('${b.pref.out_name}.js')
 		cmd = 'node "$exefile"'
 	}
@@ -200,7 +200,7 @@ pub fn (v Builder) get_builtin_files() []string {
 	for location in v.pref.lookup_path {
 		if os.exists(os.join_path(location, 'builtin')) {
 			mut builtin_files := []string{}
-			if v.pref.backend == .js {
+			if v.pref.backend.is_js() {
 				builtin_files << v.v_files_from_dir(os.join_path(location, 'builtin',
 					'js'))
 			} else {
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 13495494a4..afb5942236 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -26,7 +26,7 @@ const (
 	valid_comp_if_platforms     = ['amd64', 'i386', 'aarch64', 'arm64', 'arm32', 'rv64', 'rv32']
 	valid_comp_if_cpu_features  = ['x64', 'x32', 'little_endian', 'big_endian']
 	valid_comp_if_other         = ['js', 'debug', 'prod', 'test', 'glibc', 'prealloc',
-		'no_bounds_checking', 'freestanding', 'threads']
+		'no_bounds_checking', 'freestanding', 'threads', 'js_browser', 'js_freestanding']
 	valid_comp_not_user_defined = all_valid_comptime_idents()
 	array_builtin_methods       = ['filter', 'clone', 'repeat', 'reverse', 'map', 'slice', 'sort',
 		'contains', 'index', 'wait', 'any', 'all', 'first', 'last', 'pop']
@@ -4802,7 +4802,7 @@ fn (mut c Checker) asm_stmt(mut stmt ast.AsmStmt) {
 		c.warn('inline assembly goto is not supported, it will most likely not work',
 			stmt.pos)
 	}
-	if c.pref.backend == .js {
+	if c.pref.backend.is_js() {
 		c.error('inline assembly is not supported in the js backend', stmt.pos)
 	}
 	if c.pref.backend == .c && c.pref.ccompiler_type == .msvc {
@@ -4901,7 +4901,7 @@ fn (mut c Checker) hash_stmt(mut node ast.HashStmt) {
 	if c.skip_flags {
 		return
 	}
-	if c.pref.backend == .js {
+	if c.pref.backend.is_js() {
 		if !c.file.path.ends_with('.js.v') {
 			c.error('hash statements are only allowed in backend specific files such "x.js.v"',
 				node.pos)
@@ -6771,7 +6771,7 @@ fn (mut c Checker) comp_if_branch(cond ast.Expr, pos token.Position) bool {
 				return false
 			} else if cname in checker.valid_comp_if_other {
 				match cname {
-					'js' { return c.pref.backend != .js }
+					'js' { return !c.pref.backend.is_js() }
 					'debug' { return !c.pref.is_debug }
 					'prod' { return !c.pref.is_prod }
 					'test' { return !c.pref.is_test }
diff --git a/vlib/v/gen/js/comptime.v b/vlib/v/gen/js/comptime.v
new file mode 100644
index 0000000000..d4c55ee1f5
--- /dev/null
+++ b/vlib/v/gen/js/comptime.v
@@ -0,0 +1,311 @@
+module js
+
+import v.ast
+import v.pref
+
+fn (mut g JsGen) comp_if(node ast.IfExpr) {
+	if !node.is_expr && !node.has_else && node.branches.len == 1 {
+		if node.branches[0].stmts.len == 0 {
+			// empty ifdef; result of target OS != conditional => skip
+			return
+		}
+	}
+
+	for i, branch in node.branches {
+		if i == node.branches.len - 1 && node.has_else {
+			g.writeln('else')
+		} else {
+			if i == 0 {
+				g.write('if (')
+			} else {
+				g.write('else if (')
+			}
+			g.comp_if_cond(branch.cond, branch.pkg_exist)
+			g.writeln(')')
+		}
+
+		if node.is_expr {
+			print('$branch.stmts')
+			len := branch.stmts.len
+			if len > 0 {
+				last := branch.stmts[len - 1] as ast.ExprStmt
+				if len > 1 {
+					tmp := g.new_tmp_var()
+					g.inc_indent()
+					g.writeln('let $tmp;')
+					g.writeln('{')
+					g.stmts(branch.stmts[0..len - 1])
+					g.write('\t$tmp = ')
+					g.stmt(last)
+					g.writeln('}')
+					g.dec_indent()
+					g.writeln('$tmp;')
+				} else {
+					g.stmt(last)
+				}
+			}
+		} else {
+			g.writeln('{')
+			g.stmts(branch.stmts)
+			g.writeln('}')
+		}
+	}
+}
+
+/*
+// returning `false` means the statements inside the $if can be skipped
+*/
+// returns the value of the bool comptime expression
+fn (mut g JsGen) comp_if_cond(cond ast.Expr, pkg_exist bool) bool {
+	match cond {
+		ast.BoolLiteral {
+			g.expr(cond)
+			return true
+		}
+		ast.ParExpr {
+			g.write('(')
+			is_cond_true := g.comp_if_cond(cond.expr, pkg_exist)
+			g.write(')')
+			return is_cond_true
+		}
+		ast.PrefixExpr {
+			g.write(cond.op.str())
+			return g.comp_if_cond(cond.right, pkg_exist)
+		}
+		ast.PostfixExpr {
+			ifdef := g.comp_if_to_ifdef((cond.expr as ast.Ident).name, true) or {
+				verror(err.msg)
+				return false
+			}
+			g.write('$ifdef')
+			return true
+		}
+		ast.InfixExpr {
+			match cond.op {
+				.and, .logical_or {
+					l := g.comp_if_cond(cond.left, pkg_exist)
+					g.write(' $cond.op ')
+					r := g.comp_if_cond(cond.right, pkg_exist)
+					return if cond.op == .and { l && r } else { l || r }
+				}
+				.key_is, .not_is {
+					left := cond.left
+					mut name := ''
+					mut exp_type := ast.Type(0)
+					got_type := (cond.right as ast.TypeNode).typ
+					// Handle `$if x is Interface {`
+					// mut matches_interface := 'false'
+					if left is ast.TypeNode && cond.right is ast.TypeNode
+						&& g.table.get_type_symbol(got_type).kind == .interface_ {
+						// `$if Foo is Interface {`
+						interface_sym := g.table.get_type_symbol(got_type)
+						if interface_sym.info is ast.Interface {
+							// q := g.table.get_type_symbol(interface_sym.info.types[0])
+							checked_type := g.unwrap_generic(left.typ)
+							// TODO PERF this check is run twice (also in the checker)
+							// store the result in a field
+							is_true := g.table.does_type_implement_interface(checked_type,
+								got_type)
+							// true // exp_type in interface_sym.info.types
+							if cond.op == .key_is {
+								if is_true {
+									g.write('1')
+								} else {
+									g.write('0')
+								}
+								return is_true
+							} else if cond.op == .not_is {
+								if is_true {
+									g.write('0')
+								} else {
+									g.write('1')
+								}
+								return !is_true
+							}
+							// matches_interface = '/*iface:$got_type $exp_type*/ true'
+							//}
+						}
+					} else if left is ast.SelectorExpr {
+						name = '${left.expr}.$left.field_name'
+						exp_type = g.comptime_var_type_map[name]
+					} else if left is ast.TypeNode {
+						name = left.str()
+						// this is only allowed for generics currently, otherwise blocked by checker
+						exp_type = g.unwrap_generic(left.typ)
+					}
+
+					if cond.op == .key_is {
+						g.write('$exp_type == $got_type')
+						return exp_type == got_type
+					} else {
+						g.write('$exp_type != $got_type')
+						return exp_type != got_type
+					}
+				}
+				.eq, .ne {
+					// TODO Implement `$if method.args.len == 1`
+					g.write('1')
+					return true
+				}
+				else {
+					return true
+				}
+			}
+		}
+		ast.Ident {
+			ifdef := g.comp_if_to_ifdef(cond.name, false) or { 'true' } // handled in checker
+			g.write('$ifdef')
+			return true
+		}
+		ast.ComptimeCall {
+			g.write('$pkg_exist')
+			return true
+		}
+		else {
+			// should be unreachable, but just in case
+			g.write('1')
+			return true
+		}
+	}
+}
+
+fn (mut g JsGen) comp_if_to_ifdef(name string, is_comptime_optional bool) ?string {
+	match name {
+		// platforms/os-es:
+		'windows' {
+			return '(\$process.platform == "windows")'
+		}
+		'ios' {
+			return '(\$process.platform == "darwin")'
+		}
+		'macos' {
+			return '(\$process.platform == "darwin")'
+		}
+		'mach' {
+			return '(\$process.platform == "darwin")'
+		}
+		'darwin' {
+			return '(\$process.platform == "darwin")'
+		}
+		'linux' {
+			return '(\$process.platform == "linux")'
+		}
+		'freebsd' {
+			return '(\$process.platform == "freebsd")'
+		}
+		'openbsd' {
+			return '(\$process.platform == "openbsd")'
+		}
+		'bsd' {
+			return '(\$process.platform == "freebsd" || (\$process.platform == "openbsd"))'
+		}
+		'android' {
+			return '(\$process.platform == "android")'
+		}
+		'solaris' {
+			return '(\$process.platform == "sunos")'
+		}
+		'js_node' {
+			if g.pref.backend == .js_node {
+				return 'true'
+			} else {
+				return 'false'
+			}
+		}
+		'js_freestanding' {
+			if g.pref.backend == .js_freestanding {
+				return 'true'
+			} else {
+				return 'false'
+			}
+		}
+		'js_browser' {
+			if g.pref.backend == .js_browser {
+				return 'true'
+			} else {
+				return 'false'
+			}
+		}
+		//
+		'js' {
+			return 'true'
+		}
+		// compilers:
+		'gcc' {
+			return 'false'
+		}
+		'tinyc' {
+			return 'false'
+		}
+		'clang' {
+			return 'false'
+		}
+		'mingw' {
+			return 'false'
+		}
+		'msvc' {
+			return 'false'
+		}
+		'cplusplus' {
+			return 'false'
+		}
+		// other:
+		'threads' {
+			return 'false'
+		}
+		'gcboehm' {
+			return 'false'
+		}
+		// todo(playX): these should return true or false depending on CLI options
+		'debug' {
+			return 'false'
+		}
+		'prod' {
+			return 'false'
+		}
+		'test' {
+			return 'false'
+		}
+		'glibc' {
+			return 'false'
+		}
+		'prealloc' {
+			return 'false'
+		}
+		'no_bounds_checking' {
+			return 'CUSTOM_DEFINE_no_bounds_checking'
+		}
+		'freestanding' {
+			return '_VFREESTANDING'
+		}
+		// architectures:
+		'amd64' {
+			return '(\$process.arch == "x64")'
+		}
+		'aarch64', 'arm64' {
+			return '(\$process.arch == "arm64)'
+		}
+		// bitness:
+		'x64' {
+			return '(\$process.arch == "x64")'
+		}
+		'x32' {
+			return '(\$process.arch == "x32")'
+		}
+		// endianness:
+		'little_endian' {
+			return '(\$os.endianess == "LE")'
+		}
+		'big_endian' {
+			return '(\$os.endianess == "BE")'
+		}
+		else {
+			if is_comptime_optional
+				|| (g.pref.compile_defines_all.len > 0 && name in g.pref.compile_defines_all) {
+				return 'CUSTOM_DEFINE_$name'
+			}
+			return error('bad os ifdef name "$name"') // should never happen, caught in the checker
+		}
+	}
+	return none
+}
diff --git a/vlib/v/gen/js/js.v b/vlib/v/gen/js/js.v
index 503945634a..a81313f52e 100644
--- a/vlib/v/gen/js/js.v
+++ b/vlib/v/gen/js/js.v
@@ -48,34 +48,36 @@ mut:
 
 [heap]
 struct JsGen {
-	table &ast.Table
-	pref  &pref.Preferences
+	pref &pref.Preferences
 mut:
-	definitions         strings.Builder
-	ns                  &Namespace
-	namespaces          map[string]&Namespace
-	doc                 &JsDoc
-	enable_doc          bool
-	file                &ast.File
-	tmp_count           int
-	inside_ternary      bool
-	inside_loop         bool
-	inside_map_set      bool // map.set(key, value)
-	inside_builtin      bool
-	generated_builtin   bool
-	inside_def_typ_decl bool
-	is_test             bool
-	stmt_start_pos      int
-	defer_stmts         []ast.DeferStmt
-	fn_decl             &ast.FnDecl // pointer to the FnDecl we are currently inside otherwise 0
-	str_types           []string    // types that need automatic str() generation
-	method_fn_decls     map[string][]ast.FnDecl
-	builtin_fns         []string // Functions defined in `builtin`
-	empty_line          bool
-	cast_stack          []ast.Type
-	call_stack          []ast.CallExpr
-	is_vlines_enabled   bool // is it safe to generate #line directives when -g is passed
-	sourcemap           sourcemap.SourceMap // maps lines in generated javascrip file to original source files and line
+	table                 &ast.Table
+	definitions           strings.Builder
+	ns                    &Namespace
+	namespaces            map[string]&Namespace
+	doc                   &JsDoc
+	enable_doc            bool
+	file                  &ast.File
+	tmp_count             int
+	inside_ternary        bool
+	inside_loop           bool
+	inside_map_set        bool // map.set(key, value)
+	inside_builtin        bool
+	generated_builtin     bool
+	inside_def_typ_decl   bool
+	is_test               bool
+	stmt_start_pos        int
+	defer_stmts           []ast.DeferStmt
+	fn_decl               &ast.FnDecl // pointer to the FnDecl we are currently inside otherwise 0
+	str_types             []string    // types that need automatic str() generation
+	method_fn_decls       map[string][]ast.FnDecl
+	builtin_fns           []string // Functions defined in `builtin`
+	empty_line            bool
+	cast_stack            []ast.Type
+	call_stack            []ast.CallExpr
+	is_vlines_enabled     bool // is it safe to generate #line directives when -g is passed
+	sourcemap             sourcemap.SourceMap // maps lines in generated javascrip file to original source files and line
+	comptime_var_type_map map[string]ast.Type
+	defer_ifdef           string
 }
 
 pub fn gen(files []&ast.File, table &ast.Table, pref &pref.Preferences) string {
@@ -303,6 +305,25 @@ pub fn (mut g JsGen) init() {
 	g.definitions.writeln('"use strict";')
 	g.definitions.writeln('')
 	g.definitions.writeln('var \$global = (new Function("return this"))();')
+
+	if g.pref.backend != .js_node {
+		g.definitions.writeln('const \$process = {')
+		g.definitions.writeln('  arch: "js",')
+		if g.pref.backend == .js_freestanding {
+			g.definitions.writeln('  platform: "freestanding"')
+		} else {
+			g.definitions.writeln('  platform: "browser"')
+		}
+		g.definitions.writeln('}')
+
+		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;')
+	}
 }
 
 pub fn (g JsGen) hashes() string {
@@ -1443,6 +1464,10 @@ fn (mut g JsGen) gen_lock_expr(node ast.LockExpr) {
 }
 
 fn (mut g JsGen) gen_if_expr(node ast.IfExpr) {
+	if node.is_comptime {
+		g.comp_if(node)
+		return
+	}
 	type_sym := g.table.get_type_symbol(node.typ)
 	// one line ?:
 	if node.is_expr && node.branches.len >= 2 && node.has_else && type_sym.kind != .void {
@@ -1936,3 +1961,14 @@ fn (mut g JsGen) gen_float_literal_expr(it ast.FloatLiteral) {
 
 	g.write('${g.typ(typ)}($it.val)')
 }
+
+fn (mut g JsGen) unwrap_generic(typ ast.Type) ast.Type {
+	if typ.has_flag(.generic) {
+		if t_typ := g.table.resolve_generic_to_concrete(typ, g.table.cur_fn.generic_names,
+			g.table.cur_concrete_types)
+		{
+			return t_typ
+		}
+	}
+	return typ
+}
diff --git a/vlib/v/gen/js/program_test.v b/vlib/v/gen/js/program_test.v
index e9439e21ea..8ba06ed125 100644
--- a/vlib/v/gen/js/program_test.v
+++ b/vlib/v/gen/js/program_test.v
@@ -60,7 +60,7 @@ fn check_path(dir string, tests []string) ?int {
 			os.write_file(program_out, '') ?
 		}
 		print(program + ' ')
-		res := os.execute('$vexe -b js run $program')
+		res := os.execute('$vexe -b js_node run $program')
 		if res.exit_code < 0 {
 			panic(res.output)
 		}
diff --git a/vlib/v/parser/fn.v b/vlib/v/parser/fn.v
index 0d67a1471d..3ecb0ca664 100644
--- a/vlib/v/parser/fn.v
+++ b/vlib/v/parser/fn.v
@@ -633,7 +633,7 @@ fn (mut p Parser) anon_fn() ast.AnonFn {
 	old_inside_defer := p.inside_defer
 	p.inside_defer = false
 	p.open_scope()
-	if p.pref.backend != .js {
+	if !p.pref.backend.is_js() {
 		p.scope.detached_from_parent = true
 	}
 	// TODO generics
diff --git a/vlib/v/pref/os.v b/vlib/v/pref/os.v
index bc1e560a60..578b00d00e 100644
--- a/vlib/v/pref/os.v
+++ b/vlib/v/pref/os.v
@@ -13,7 +13,9 @@ pub enum OS {
 	openbsd
 	netbsd
 	dragonfly
-	js // TODO
+	js_node
+	js_browser
+	js_freestanding
 	android
 	solaris
 	serenity
@@ -34,7 +36,9 @@ pub fn os_from_string(os_str string) ?OS {
 		'openbsd' { return .openbsd }
 		'netbsd' { return .netbsd }
 		'dragonfly' { return .dragonfly }
-		'js' { return .js }
+		'js', 'js_node' { return .js_node }
+		'js_freestanding' { return .js_freestanding }
+		'js_browser' { return .js_browser }
 		'solaris' { return .solaris }
 		'serenity' { return .serenity }
 		'vinix' { return .vinix }
@@ -58,7 +62,9 @@ pub fn (o OS) str() string {
 		.openbsd { return 'OpenBSD' }
 		.netbsd { return 'NetBSD' }
 		.dragonfly { return 'Dragonfly' }
-		.js { return 'JavaScript' }
+		.js_node { return 'NodeJS' }
+		.js_freestanding { return 'JavaScript' }
+		.js_browser { return 'JavaScript(Browser)' }
 		.android { return 'Android' }
 		.solaris { return 'Solaris' }
 		.serenity { return 'SerenityOS' }
diff --git a/vlib/v/pref/pref.v b/vlib/v/pref/pref.v
index a86de7d7d3..8c8a425818 100644
--- a/vlib/v/pref/pref.v
+++ b/vlib/v/pref/pref.v
@@ -45,10 +45,23 @@ pub enum ColorOutput {
 
 pub enum Backend {
 	c // The (default) C backend
-	js // The JavaScript backend
+	js_node // The JavaScript NodeJS backend
+	js_browser // The JavaScript browser backend
+	js_freestanding // The JavaScript freestanding backend
 	native // The Native backend
 }
 
+pub fn (b Backend) is_js() bool {
+	match b {
+		.js_node, .js_browser, .js_freestanding {
+			return true
+		}
+		else {
+			return false
+		}
+	}
+}
+
 pub enum CompilerType {
 	gcc
 	tinyc
@@ -513,7 +526,7 @@ pub fn parse_args(known_external_commands []string, args []string) (&Preferences
 			'-o', '-output' {
 				res.out_name = cmdline.option(current_args, arg, '')
 				if res.out_name.ends_with('.js') {
-					res.backend = .js
+					res.backend = .js_node
 				}
 				if !os.is_abs_path(res.out_name) {
 					res.out_name = os.join_path(os.getwd(), res.out_name)
@@ -524,6 +537,9 @@ pub fn parse_args(known_external_commands []string, args []string) (&Preferences
 				sbackend := cmdline.option(current_args, arg, 'c')
 				res.build_options << '$arg $sbackend'
 				b := backend_from_string(sbackend) or { continue }
+				if b.is_js() {
+					res.output_cross_c = true
+				}
 				res.backend = b
 				i++
 			}
@@ -723,7 +739,10 @@ fn is_source_file(path string) bool {
 pub fn backend_from_string(s string) ?Backend {
 	match s {
 		'c' { return .c }
-		'js' { return .js }
+		'js' { return .js_node }
+		'js_node' { return .js_node }
+		'js_browser' { return .js_browser }
+		'js_freestanding' { return .js_freestanding }
 		'native' { return .native }
 		else { return error('Unknown backend type $s') }
 	}
diff --git a/vlib/v/pref/should_compile.v b/vlib/v/pref/should_compile.v
index 37a9ccdba0..5f69fa4fe6 100644
--- a/vlib/v/pref/should_compile.v
+++ b/vlib/v/pref/should_compile.v
@@ -18,13 +18,13 @@ pub fn (prefs &Preferences) should_compile_filtered_files(dir string, files_ []s
 		if prefs.backend == .c && !prefs.should_compile_c(file) {
 			continue
 		}
-		if prefs.backend == .js && !prefs.should_compile_js(file) {
+		if prefs.backend.is_js() && !prefs.should_compile_js(file) {
 			continue
 		}
 		if prefs.backend == .native && !prefs.should_compile_native(file) {
 			continue
 		}
-		if prefs.backend != .js && !prefs.should_compile_asm(file) {
+		if !prefs.backend.is_js() && !prefs.should_compile_asm(file) {
 			continue
 		}
 		if file.starts_with('.#') {