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E nterpris e JavaS cript E rror
           Handling
            Nicholas C . Zakas
 P rincipal Front E nd E ngineer, Yahoo!
Who's this g uy?
• P rincipal Front E nd E ngineer, Yahoo!
  Front P age
• YUI C ontributor
• Author
Ques tion:
What do us ers s ee when
there's a JavaS cript error on
a web pag e?
Ans wer:
Nothing !*
“If an error is possible, someone will make it. The
   designer must assume that all possible errors
   will occur and des...
R ule #1:
As s ume your code will fail
My code is beautiful
  and never fails
As s umption?




• What if destination is null?
• What if source is null?
R ule #2:
Log errors to the s erver
S imple Log g ing
R ule #3:
You, not the brows er, handle
errors
try-catch




• Thrown errors contain extra information
• E rrors that are caught are considered to
  have been handled
window.onerror




• Last stop before browser responds
• R eturn true to indicate not to respond
• O nly supported in Inte...
E rror Lifecycle

   Browser Error


   window.onerror


      try-catch


        Error
R ule #4:
Identify where errors mig ht
occur
Types of E rrors
• Type coercion errors
Type C oercion E rrors
Types of E rrors
• Type coercion errors
• D ata type errors
Data Type E rrors
• O ften occurs with function arguments
• Typically a symptom of insufficient value
  checking
Data Type E rrors
Data Type E rrors
Data Type E rrors - Fixed
Data Type E rrors - Fixed
Types of E rrors
• Type coercion errors
• D ata type errors
• C ommunication errors
C ommunication E rrors
•   Invalid UR L/post data
•   S erver response status
•   No network connection
•   S erver respon...
Invalid UR L/Pos t Data
• Typically long string concatenations
• D on't forget to use
  encodeUR IC omponent() on each par...
S erver R es pons e S tatus
• 200 is not the only valid status that may
  be returned
  – Beware of 304
• Any other status...
S erver R es pons e S tatus
No Network C onnection
• Internet E xplorer throws an error when
  calling open() but then goes through
  normal lifecycle...
No Network C onnection
S erver R es pons e C ontent
• A status of 200/  304 is not enough
• S erver errors often return HTM L
• If possible, set ...
R ule #5:
Throw your own errors
Throw Your Own E rrors
B rows er R es ponds
Throw or Try-C atch?
• E rrors should be thrown in the low-level
  parts of the application
  – Utilities, core libraries,...
R ule #6:
Dis ting uis h fatal vers us non-
fatal
Non-Fatal E rrors
• Won't interfere with user's main tasks
• Affects only a portion of the page
  – E asily disabled/ignor...
Fatal E rrors
• The application absolutely cannot
  continue
• S ignificantly interferes with user's ability
  to be produ...
Fatal or Non-Fatal?
• D on't allow your code to determine what
  is and is not fatal
  – Watch out for loops
• The user's ...
Fatal or Non-Fatal?
R ule #7:
Provide a debug mode
Debug Mode
• Assign a variable that is globally
  available
• try-catch should re-throw the error
• window.onerror should ...
Debug Mode
Debug Mode
S ummary
R ules
1.Assume your code will fail
2.Log errors to the server
3.You, not the browser, handle errors
4.Identify where erro...
Ques tions ?
E tcetera
• M y blog:    www.nczonline.net
• M y email:   nzakas@ yahoo-inc.com
Happy debug g ing !
C reative C ommons Imag es
                     Us edcrazytales562/25148
• http:/www.flickr.com/
        /              ph...
Data Type E rrors •
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Data Type E rrors
• O ften occurs with function arguments
• Typically a symptom of insufficient value
checking

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