Tuesday, 10 September 2013

Handling runtime errors in JavaScript using try/catch/finally



                                    Error handling, like many aspects of JavaScript, has been maturing since the dark ages of Netscape and IE4. No longer are you forced to settle for what the browser throws in your face in an event of a JavaScript error, but instead can take the matter into your own hands. The try/catch/finally statement of JavaScript lets you dip your toes into error prune territory and "reroute" when a JavaScript "exception" is encountered. Along with other defensive coding techniques such as Object detection and the onError event, try/catch/finally adds the ability to navigate around certain errors that in the past would have instantly stopped your script at its tracks. No more!

try/catch/finally

try/catch/finally are so called exception handling statements in JavaScript. An exception is an error that occurs at runtime due to an illegal operation during execution. Examples of exceptions include trying to reference an undefined variable, or calling a non existent method. This versus syntax errors, which are errors that occur when there is a problem with your JavaScript syntax. Consider the following examples of syntax errors versus exceptions:
  • alert("I am missing a closing parenthesis //syntax error
  • alert(x) //exception assuming "x" isn't defined yet
  • undefinedfunction() //exception
try/catch/finally lets you deal with exceptions gracefully. It does not catch syntax errors, however (for those, you need to use the onerror event). Normally whenever the browser runs into an exception somewhere in a JavaScript code, it displays an error message to the user while aborting the execution of the remaining code. You can put a lid on this behaviour and handle the error the way you see fit using try/catch/finally. At its simplest you'd just use try/catch to try and run some code, and in the event of any exceptions, suppress them:

try{
 undefinedfunction()
}
catch(e){
 //catch and just suppress error
}

Assuming undefinedfunction() is undefined, when the browser runs the above, no errors will be shown. The syntax for try/catch/finally is a try clause followed by either a catch or finally clause (at least one or both of them). The catch clause if defined traps any errors that has occurred from try, and is indirectly passed the error object that contains additional info about the error. Lets see a slightly more complex example now:

try{
 undefinedfunction()
 alert('I guess you do exist')
}
catch(e){
 alert('An error has occurred: '+e.message)
}

 
Demo:

Click on the above button, and notice how only "An Error has occurred" alert pops up, but not "I guess you do exist". This tells us that when try encounters an error, it immediately skips any remaining code inside it and goes straight to catch. The default error message is obviously suppressed, though you can still retrieve this information by accessing the Error object that gets indirectly passed into catch. We'll look at the Error object in detail on the next page.

There's another clause, finally, that if defined will be executed regardless of whether an error occurs in the try clause proceeding it:

try{
 undefinedfunction()
 alert('I guess you do exist')
}
catch(e){
 alert('An error has occurred: '+e.message)
}
finally{
 alert('I am alerted regardless of the outcome above')
}

finally can be useful when you need to "clean up" after some code inside try. While it's true finally will always be executed if defined, certain statements inside try such as continue, break, return, or when an error has occurred and there is no catch clause will all cause finally to be executed immediately thereafter. In the following example, the value "5" is alerted, since control is handed over to finally when i reaches 5 inside try:

try{
 for (var i=0; i<10; i++){
  if (i==5)
   break
  x=i
 }
}
finally{
 alert(i) //alerts 5
}

Nested try/catch/finally statements

As a reminder, try should never be defined just by itself, but always followed by either catch, finally, or both. Within each clause, you can define additional try/catch/finally statements following the same aforementioned rule. Take the instance where an error has occurred within the catch clause- defining an additional try/catch statement inside it takes care of it:
var ajaxrequest=null
if (window.ActiveXObject){ //Test for support for different versions of ActiveXObject in IE
 try {
  ajaxrequest=new ActiveXObject("Msxml2.XMLHTTP")
 }
 catch (e){
  try{
   ajaxrequest=new ActiveXObject("Microsoft.XMLHTTP")
  } //end inner try
  catch (e){
   alert("I give up. Your IE doesn't support Ajax!")
  } //end inner catch

 } //end outer catch
}
else if (window.XMLHttpRequest) // if Mozilla, Safari etc
 ajaxrequest=new XMLHttpRequest()

ajaxrequest.open('GET', 'process.php', true) //do something with request

Here I'm using a nested try/catch statement to try and determine in IE which version of the ActiveX Object it supports that's needed to initialize an Ajax request. Using object detection won't work here, since the issue isn't whether the browser supports ActiveXObject here, but which version.

The Error object and throwing your own errors

The Error Object

As promised, we're going to take a closer look at the Error object that gets passed into the catch clause to see just what we can extract from it in an event of an error. The Error object in all browsers support the following two properties:
  1. name: The name of the error, or more specifically, the name of the constructor function the error belongs to.
  2. message: A description of the error, with this description varying depending on the browser.
try{
 document.body.filters[0].apply()
}
catch(e){
 alert(e.name + "\n" + e.message)
}
Demo:
Six possible values can be returned by the name property, which as mentioned correspond to the names of the error's constructors. They are:
Error Name Description
EvalError An error in the eval() function has occurred.
RangeError Out of range number value has occurred.
ReferenceError An illegal reference has occurred.
SyntaxError A syntax error within code inside the eval() function has occurred. All other syntax errors are not caught by try/catch/finally, and will trigger the default browser error message associated with the error. To catch actual syntax errors, you may use the onerror event.
TypeError An error in the expected variable type has occurred.
URIError An error when encoding or decoding the URI has occurred (ie: when calling encodeURI()).

This level of detail may be useful when you wish to sniff out a specific type of error in your catch clause. In the below, no DIV on the page exists with ID="mydiv". When trying to set its .innerHTML property, a TypeError occurs, since we're trying to assign the .innerHTML property to a null object:
try{
 document.getElementById("mydiv").innerHTML='Success' //assuming "mydiv" is undefined
}
catch(e){
 if (e.name.toString()=="TypeError") //evals to true in this case
  //do something
}

Ok, so maybe it's not that useful most of the time, but you just never know.

Throwing your own errors (exceptions)

Instead of waiting for one of the 6 types of errors above to occur before control is automatically transferred from the try block to the catch block, you can also explicitly throw your own exceptions to force that to happen on demand. This is great for creating your own definitions of what an error is and when control should be transferred to catch.
To throw an error, invoke, well, the throw statement inside your try/catch/finally blocks. The syntax is:
throw myerrorobject
Where myerrorobject can in fact be anything from a string, number, Boolean, to a new or one of the 6 default Error Constructor functions. What myerrorobject is set to mainly just affects what error.name and error.message returns in your catch clause. Most commonly you would just throw a new Error object:
  • throw new Error("Oh oh, an error has occured")
Lets see a meaningful example of throw in action:
function entrycheck(){
 try{
  var agecheck=prompt("This movie is rated PG-13. Please enter your age before continuing:")
  if (isNaN(parseInt(agecheck)))
   throw new Error("Please enter a valid age")
  else if (agecheck<13)
   throw new Error("Sorry, but you are too young for this movie")
  alert("Enjoy the movie!")
 }
 catch(e){
  alert(e.name+" "+e.message)
 }
}

Demo:
Try entering a none numeric value (ie: "haha") or a number less than 13 (ie: 11). In both cases, by using throw, control is instantly transferred to catch, with e.message displaying a different message. Technically entering a string or number less than 13 certainly doesn't constitute an exception in JavaScript, though for our purpose here, they should. That's how throw can be useful- when you need to specify your own parameters of what an error is inside try/catch/finally.
As mentioned, there are a number of other things apart from new Error() you can throw, which changes the contents of the error object passed into catch. The following are all valid throws:
  • throw "An error has occurred"
  • throw true
  • throw new Error("I detect an error!")
  • throw new SyntaxError("Your syntax is no good")
In the last instance, you can substitute SyntaxError with one of the 6 Error constructor function names to throw a specific type of error. In our age check example above, we could have thrown a SyntaxError when the value entered was a string, and a RangeError when the value was less than 13:
function entrycheck(){
 try{
  var agecheck=prompt("This movie is rated PG-13. Please enter your age before continuing:")
  if (isNaN(parseInt(agecheck)))
   throw new SyntaxError("Please enter a valid age")
  else if (agecheck<13)
   throw new RangeError("Sorry, but you are too young for this movie")
  alert("Enjoy the movie!")
 }
 catch(e){
  alert(e.name+" "+e.message)
 }
}
This has the effect of changing what e.name returns- SyntaxError and RangeError, respectively. If that's not enough, you can even throw a generic Error object with custom name and message properties:
throw{
 name: "JavaScriptKit Error",
 message: "Error detected. Please contact webmaster"
}

And with that we throw in the towel!
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