RFC: (WIP) Improve error marshaling from JS to Ruby

.travis.yml

@@ -5,7 +5,9 @@ sudo: false
 before_install:
   - if [ "$EXECJS_RUNTIME" == "V8" ]; then brew update; fi
   - if [ "$EXECJS_RUNTIME" == "V8" ]; then brew install v8; fi
-script: bundle exec ruby test/test_execjs.rb
+script:
+  - node --version
+  - bundle exec ruby test/test_execjs.rb
 
 matrix:
   include:

lib/execjs/duktape_runtime.rb

@@ -46,6 +46,7 @@ def wrap_error(e)
             re = / \(line (\d+)\)$/
             lineno = e.message[re, 1] || 1
             error = klass.new(e.message.sub(re, ""))
+            error.metadata = {} # FIXME: has to be available upstream
             error.set_backtrace(["(execjs):#{lineno}"] + e.backtrace)
             error
           else

lib/execjs/external_runtime.rb

@@ -65,11 +65,12 @@ def write_to_tempfile(contents)
         end
 
         def extract_result(output, filename)
-          status, value, stack = output.empty? ? [] : ::JSON.parse(output, create_additions: false)
+          status, value= output.empty? ? [] : ::JSON.parse(output, create_additions: false)
           if status == "ok"
             value
           else
-            stack ||= ""
+            demarshaled_err = value
+            stack = demarshaled_err['stack'] || ""
             real_filename = File.realpath(filename)
             stack = stack.split("\n").map do |line|
               line.sub(" at ", "")
@@ -79,8 +80,11 @@ def extract_result(output, filename)
             end
             stack.reject! { |line| ["eval code", "eval@[native code]"].include?(line) }
             stack.shift unless stack[0].to_s.include?("(execjs)")
-            error_class = value =~ /SyntaxError:/ ? RuntimeError : ProgramError
-            error = error_class.new(value)
+
+            error_class = demarshaled_err['name'].to_s =~ /SyntaxError/ ? RuntimeError : ProgramError
+
+            error = error_class.new(demarshaled_err.delete('message').to_s)
+            error.metadata = demarshaled_err
             error.set_backtrace(stack + caller)
             raise error
           end
@@ -156,6 +160,10 @@ def json2_source
         @json2_source ||= IO.read(ExecJS.root + "/support/json2.js")
       end
 
+      def encode_error_source
+        @encode_error_source ||= IO.read(ExecJS.root + "/support/encode_error.js")
+      end
+
       def encode_source(source)
         encoded_source = encode_unicode_codepoints(source)
         ::JSON.generate("(function(){ #{encoded_source} })()", quirks_mode: true)

lib/execjs/module.rb

@@ -3,8 +3,17 @@
 
 module ExecJS
   class Error           < ::StandardError; end
-  class RuntimeError              < Error; end
-  class ProgramError              < Error; end
+
+  class RuntimeError < Error
+    # Stores the unmarshaled JavaScript error information if it was available (type, message etc.)
+    attr_accessor :metadata
+  end
+
+  class ProgramError < Error
+    # Stores the unmarshaled JavaScript error information if it was available (type, message etc.)
+    attr_accessor :metadata
+  end
+
   class RuntimeUnavailable < RuntimeError; end
 
   class << self

lib/execjs/support/encode_error.js

@@ -0,0 +1,12 @@
+function encodeError(err) {
+  var errMeta = {type: typeof(err), name: null, message: '' + err, stack: null};
+  if (typeof(err) === 'object') {
+    // Copy "message" and "name" since those are standard attributes for an Error
+    if (typeof(err.name) !== 'undefined') errMeta.name = err.name;
+    if (typeof(err.message) !== 'undefined') errMeta.message = err.message;
+    if (typeof(err.stack) !== 'undefined') errMeta.stack = err.stack;
+    // Copy all of the other properties that are tacked on the Error itself
+    if (typeof(Object.assign) === 'function') Object.assign(errMeta, err);
+  }
+  return errMeta;
+};

lib/execjs/support/jsc_runner.js

@@ -1,5 +1,6 @@
 (function(program, execJS) { execJS(program) })(function() { #{source}
 }, function(program) {
+  #{encode_error_source}
   var output;
   try {
     result = program();
@@ -9,10 +10,10 @@
       try {
         print(JSON.stringify(['ok', result]));
       } catch (err) {
-        print(JSON.stringify(['err', '' + err, err.stack]));
+        print(JSON.stringify(['err', encodeError(err)]));
       }
     }
   } catch (err) {
-    print(JSON.stringify(['err', '' + err, err.stack]));
+    print(JSON.stringify(['err', encodeError(err)]));
   }
 });

lib/execjs/support/jscript_runner.js

@@ -2,6 +2,7 @@
   return eval(#{encode_source(source)});
 }, function(program) {
   #{json2_source}
+  #{encode_error_source}
   var output, print = function(string) {
     WScript.Echo(string);
   };
@@ -13,10 +14,10 @@
       try {
         print(JSON.stringify(['ok', result]));
       } catch (err) {
-        print(JSON.stringify(['err', err.name + ': ' + err.message, err.stack]));
+        print(JSON.stringify(['err', encodeError(err)]));
       }
     }
   } catch (err) {
-    print(JSON.stringify(['err', err.name + ': ' + err.message, err.stack]));
+    print(JSON.stringify(['err', encodeError(err)]));
   }
 });

lib/execjs/support/node_runner.js

@@ -3,6 +3,7 @@
   var output, print = function(string) {
     process.stdout.write('' + string);
   };
+  #{encode_error_source}
   try {
     result = program();
     if (typeof result == 'undefined' && result !== null) {
@@ -11,10 +12,10 @@
       try {
         print(JSON.stringify(['ok', result]));
       } catch (err) {
-        print(JSON.stringify(['err', '' + err, err.stack]));
+        print(JSON.stringify(['err', encodeError(err)]));
       }
     }
   } catch (err) {
-    print(JSON.stringify(['err', '' + err, err.stack]));
+    print(JSON.stringify(['err', encodeError(err)]));
   }
 });

lib/execjs/support/spidermonkey_runner.js

@@ -1,5 +1,6 @@
 (function(program, execJS) { execJS(program) })(function() { #{source}
 }, function(program) {
+  #{encode_error_source}
   var output;
   try {
     result = program();
@@ -9,10 +10,10 @@
       try {
         print(JSON.stringify(['ok', result]));
       } catch (err) {
-        print(JSON.stringify(['err', '' + err, err.stack]));
+        print(JSON.stringify(['err', encodeError(err)]));
       }
     }
   } catch (err) {
-    print(JSON.stringify(['err', '' + err, err.stack]));
+    print(JSON.stringify(['err', encodeError(err)]));
   }
 });

lib/execjs/support/v8_runner.js

@@ -1,5 +1,6 @@
 (function(program, execJS) { execJS(program) })(function() { #{source}
 }, function(program) {
+  #{encode_error_source}
   var output;
   try {
     result = program();
@@ -9,10 +10,10 @@
       try {
         print(JSON.stringify(['ok', result]));
       } catch (err) {
-        print(JSON.stringify(['err', '' + err, err.stack]));
+        print(JSON.stringify(['err', encodeError(err)]));
       }
     }
   } catch (err) {
-    print(JSON.stringify(['err', '' + err, err.stack]));
+    print(JSON.stringify(['err', encodeError(err)]));
   }
 });

test/test_execjs.rb

@@ -271,6 +271,30 @@ def test_exec_syntax_error
     end
   end
 
+  def test_exec_custom_js_error
+    begin
+      ExecJS.exec('CustomError = function(message, line, column) {
+          this.name = "CustomError";
+          this.message = message;
+          this.line = line;
+          this.column = column;
+          this.specialProperty = 123;
+        };
+        CustomError.prototype = Error.prototype;
+        throw new CustomError("this has failed", 10, 15)
+     ')
+      flunk
+    rescue ExecJS::ProgramError => e
+      assert e
+      assert_match /this has failed/, e.message
+
+      meta = e.metadata
+      assert_equal 'object', meta['type']
+      assert_equal 'CustomError', meta['name']
+      assert_equal 123, meta['specialProperty']
+    end
+  end
+
   def test_eval_syntax_error
     begin
       ExecJS.eval(")")