Allow JavaScript errors to be raised as exceptions (#988)

When configured, all driver commands (except reading console messages) will
check for JavaScript errors. 

The ConsoleMessages command is excluded from the check to prevent recursion, and
maintain the ease of debugging JavaScript errors interactively.

README.md

@@ -129,6 +129,9 @@ Capybara::Webkit.configure do |config|
     user: "proxy",
     pass: "secret"
   )
+
+  # Raise JavaScript errors as exceptions
+  config.raise_javascript_errors = true
 end
 ```
 

lib/capybara/webkit/browser.rb

@@ -5,6 +5,7 @@ module Capybara::Webkit
   class Browser
     def initialize(connection)
       @connection = connection
+      apply_defaults
     end
 
     def authenticate(username, password)
@@ -36,6 +37,7 @@ module Capybara::Webkit
     end
 
     def reset!
+      apply_defaults
       command("Reset")
     end
 
@@ -99,6 +101,14 @@ module Capybara::Webkit
       command("SetSkipImageLoading", skip_image_loading)
     end
 
+    def set_raise_javascript_errors(is_enabled)
+      @raise_javascript_errors = is_enabled
+    end
+
+    def raise_javascript_errors?
+      @raise_javascript_errors
+    end
+
     def window_focus(selector)
       command("WindowFocus", selector)
     end
@@ -209,7 +219,9 @@ module Capybara::Webkit
         @connection.print arg.to_s
       end
       check
-      read_response
+      response = read_response
+      check_javascript_errors(name)
+      response
     rescue SystemCallError => exception
       @connection.restart
       raise(Capybara::Webkit::CrashError, <<-MESSAGE.strip)
@@ -295,6 +307,10 @@ https://github.com/thoughtbot/capybara-webkit/wiki/Reporting-Crashes
 
     private
 
+    def apply_defaults
+      @raise_javascript_errors = false
+    end
+
     def check
       result = @connection.gets
       result.strip! if result
@@ -308,6 +324,14 @@ https://github.com/thoughtbot/capybara-webkit/wiki/Reporting-Crashes
       result
     end
 
+    def check_javascript_errors(command_name)
+      return if command_name == "ConsoleMessages"
+
+      if raise_javascript_errors? && error_messages.any?
+        raise JavaScriptError, error_messages
+      end
+    end
+
     def read_response
       response_length = @connection.gets.to_i
       if response_length > 0

lib/capybara/webkit/configuration.rb

@@ -30,6 +30,7 @@ module Capybara
       attr_accessor :stderr
       attr_accessor :timeout
       attr_writer :skip_image_loading
+      attr_accessor :raise_javascript_errors
 
       def initialize
         @allowed_urls = []
@@ -41,6 +42,7 @@ module Capybara
         @skip_image_loading = false
         @stderr = $stderr
         @timeout = -1
+        @raise_javascript_errors = false
       end
 
       def allow_url(url)
@@ -93,7 +95,8 @@ module Capybara
           proxy: proxy,
           skip_image_loading: skip_image_loading?,
           stderr: stderr,
-          timeout: timeout
+          timeout: timeout,
+          raise_javascript_errors: raise_javascript_errors,
         }
       end
     end

lib/capybara/webkit/driver.rb

@@ -398,6 +398,10 @@ module Capybara::Webkit
         @browser.timeout = @options[:timeout]
       end
 
+      if @options.has_key? :raise_javascript_errors
+        @browser.set_raise_javascript_errors(@options[:raise_javascript_errors])
+      end
+
       Array(@options[:allowed_urls]).each { |url| @browser.allow_url(url) }
       Array(@options[:blocked_urls]).each { |url| @browser.block_url(url) }
     end

lib/capybara/webkit/errors.rb

@@ -44,4 +44,13 @@ module Capybara::Webkit
       Capybara::Webkit.const_get @class_name
     end
   end
+
+  class JavaScriptError < StandardError
+    def initialize(errors)
+      @javascript_errors = errors
+      super(errors.join(","))
+    end
+
+    attr_reader :javascript_errors
+  end
 end

spec/browser_spec.rb

@@ -43,4 +43,20 @@ describe Capybara::Webkit::Browser do
       end
     end
   end
+
+  describe "#reset!" do
+    it "resets to the default state" do
+      connection = double("connection", puts: nil, print: nil)
+      allow(connection).to receive(:gets).and_return("ok\n", "{}\n")
+
+      browser = Capybara::Webkit::Browser.new(connection)
+      browser.set_raise_javascript_errors(true)
+
+      expect(browser.raise_javascript_errors?).to be true
+
+      browser.reset!
+
+      expect(browser.raise_javascript_errors?).to be false
+    end
+  end
 end

spec/driver_spec.rb

@@ -3440,4 +3440,50 @@ CACHE MANIFEST
   def driver_url(driver, path)
     URI.parse(driver.current_url).merge(path).to_s
   end
+
+  context "page with JavaScript errors" do
+    let(:driver) do
+      driver_for_app do
+        get "/" do
+          <<-HTML
+            <!DOCTYPE html>
+            <html>
+            <body>
+              <script type="text/javascript">
+                undefinedFunc();
+              </script>
+            </body>
+            </html>
+          HTML
+        end
+      end
+    end
+
+    it "raises errors as an exception, when configured" do
+      configure do |config|
+        config.raise_javascript_errors = true
+      end
+
+      expected_error = Capybara::Webkit::JavaScriptError
+      expected_message = "ReferenceError: Can't find variable: undefinedFunc"
+
+      expect { visit("/") }.to raise_error(expected_error) do |error|
+        expect(error.javascript_errors.first[:message]).to eq expected_message
+      end
+      expect { driver.find_css("h1") }.to raise_error(expected_error)
+    end
+
+    it "does not raise an exception when fetching the error messages" do
+      configure do |config|
+        config.raise_javascript_errors = true
+      end
+
+      expect { driver.error_messages }.to_not raise_error
+    end
+
+    it "does not raise errors as an exception by default" do
+      expect { visit("/") }.to_not raise_error
+      expect(driver.error_messages).to_not be_empty
+    end
+  end
 end

spec/support/app_runner.rb

@@ -21,6 +21,7 @@ module AppRunner
     end
 
     self.browser = $webkit_browser
+    self.browser.reset!
 
     self.configuration = Capybara::Webkit::Configuration.new
   end