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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...
R ules 1.Assume your code
R ules 1.Assume your code
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R ules
1.Assume your code will fail
2.Log errors to the server
3.You, not the browser, handle errors
4.Identify where errors might occur
5.Throw your own errors
6.D istinguish fatal versus non-fatal
7.P rovide a debug mode

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