js: os now compiles to the JS backend, more builtins & minor codegen fixes (#11302)

vlib/builtin/js/byte.js.v

@@ -2,7 +2,15 @@ module builtin
 
 pub fn (b byte) is_space() bool {
 	mut result := false
-	#result = /^\s*$/.test(String.fromCharCode(b))
+	#result.val = /^\s*$/.test(String.fromCharCode(b))
+
+	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
 }