Javascript: Add suppport for ES6 style exports (#4754)

* Add suppport for ES6 style exports

Adds support for ECMAScript 6 module exports during Javascript
generation. This is useful as many development projects are
switching to this new standard and away from custom module
solutions. By integrating support into flatbuffers, users
do not need to perform additional post-processing of generated
files in order to use flatbuffers output directly in their
codebases.

Reference to ECMAScript 6 modules:
https://www.ecma-international.org/ecma-262/6.0/#sec-exports

Changes:
* Added `--es6-js-export` option to cli parser tool
* Added conditional code to generate a ES6 style export
  line, replacing the normal NodeJS/RequireJS line.

* Fixed missing export statements

Added exports for definition and struct names that were not inside namespaces

* Updated Compiler.md with new generator option

Added entry to Compiler.md in docs for the `--es6-js-export` flag, including a brief description of the effects and usefulness.

2 files changed, 15 insertions, 1 deletions

diff --git a/src/flatc.cpp b/src/flatc.cpp
index c7e1b091..0487a451 100644
--- a/src/flatc.cpp
+++ b/src/flatc.cpp
@@ -97,6 +97,7 @@ std::string FlatCompiler::GetUsageString(const char *program_name) const {
     "                     Default value is \"T\"\n"
     "  --no-js-exports    Removes Node.js style export lines in JS.\n"
     "  --goog-js-export   Uses goog.exports* for closure compiler exporting in JS.\n"
+    "  --es6-js-export    Uses ECMAScript 6 export style lines in JS.\n"
     "  --go-namespace     Generate the overrided namespace in Golang.\n"
     "  --go-import        Generate the overrided import for flatbuffers in Golang.\n"
     "                     (default is \"github.com/google/flatbuffers/go\")\n"
@@ -191,6 +192,10 @@ int FlatCompiler::Compile(int argc, const char **argv) {
         opts.skip_js_exports = true;
       } else if (arg == "--goog-js-export") {
         opts.use_goog_js_export_format = true;
+        opts.use_ES6_js_export_format = false;
+      } else if (arg == "--es6-js-export") {
+        opts.use_goog_js_export_format = false;
+        opts.use_ES6_js_export_format = true;
       } else if (arg == "--go-namespace") {
         if (++argi >= argc) Error("missing golang namespace" + arg, true);
         opts.go_namespace = argv[argi];
diff --git a/src/idl_gen_js.cpp b/src/idl_gen_js.cpp
index 3e3730ac..184749a8 100644
--- a/src/idl_gen_js.cpp
+++ b/src/idl_gen_js.cpp
@@ -103,7 +103,10 @@ class JsGenerator : public BaseGenerator {
 
     if (lang_.language == IDLOptions::kJs && !exports_code.empty() &&
         !parser_.opts.skip_js_exports) {
-      code += "// Exports for Node.js and RequireJS\n";
+        if( parser_.opts.use_ES6_js_export_format )
+            code += "// Exports for ECMAScript6 Modules\n";
+        else
+            code += "// Exports for Node.js and RequireJS\n";
       code += exports_code;
     }
 
@@ -223,6 +226,8 @@ class JsGenerator : public BaseGenerator {
           code += "var ";
           if (parser_.opts.use_goog_js_export_format) {
             exports += "goog.exportSymbol('" + *it + "', " + *it + ");\n";
+          } else if( parser_.opts.use_ES6_js_export_format){
+            exports += "export {" + *it + "};\n";
           } else {
             exports += "this." + *it + " = " + *it + ";\n";
           }
@@ -293,6 +298,8 @@ class JsGenerator : public BaseGenerator {
         if (parser_.opts.use_goog_js_export_format) {
           exports += "goog.exportSymbol('" + enum_def.name + "', " +
                      enum_def.name + ");\n";
+        } else if (parser_.opts.use_ES6_js_export_format) {
+          exports += "export {" + enum_def.name + "};\n";   
         } else {
           exports += "this." + enum_def.name + " = " + enum_def.name + ";\n";
         }
@@ -561,6 +568,8 @@ class JsGenerator : public BaseGenerator {
         if (parser_.opts.use_goog_js_export_format) {
           exports += "goog.exportSymbol('" + struct_def.name + "', " +
                      struct_def.name + ");\n";
+        } else if (parser_.opts.use_ES6_js_export_format) {
+          exports += "export {" + struct_def.name + "};\n";
         } else {
           exports +=
               "this." + struct_def.name + " = " + struct_def.name + ";\n";