Duckett Ch. 10 Error Handling and Debugging

Order of Execution

• understanding the order in whoch things are processed

Execution Context

• Every statemnet in a script live in one of three places
  1. Global context
  2. Function Context
  3. Eval Context

Error Objects

• Error : Generic error
• SyntaxError: Syntax has not been followed
• ReferenceError: Tried to reference a variable that is not declared/within scope
• TypeError: An unexpected data type that can not be coerced

How to Deal With Errors

1. Debug the script to fix errors
   • use developer tools
2. Handle Errors Gracefully
   • using try, catch, throw and finally statements

Debugging Workflow

• Where is the problem?
  1. Look at the error message, it tells you
  2. Check how far the script is running
  3. The type of error
• What exactly is the problem?
  1. When you have set breakpoints, you can see if the variables arround them have the values you would expect them to.
  2. Break down/break out parts of the code to test
  3. Check the numbe of parameters for a function or the number of items for an array

Summary

• If you understand execution contexts (which have twostages) and stacks, you are more likely to find the error in your code.

• Debugging is the process of finding errors. It involves a process of deduction.

• The console helps narrow down the area in which the error is located, so you can try to find the exact error.

• JavaScript has 7 different types of errors. Each creates its own error object, which can tell you its line number and gives a description of the error.

• If you know that you may get an error, you can handle it gracefully using the try, catch, finally statements. Use them to give your users helpful feedback.

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