Error Handling & Debugging

• JavaScript can be hard to learn and everyone makes mistakes when writing it.

• To find the source of an error, it helps to know how scripts are processed.

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

EXECUTION CONTEXT : Every statement in a script lives in one of three execution contexts:

• GLOBAL CONTEXT : Code that is in the script, but not in a function , There is only one global context in any page.
• FUNCTION CONTEXT : Code that is being run within a function, Each function has its own function context.
• EVAL CONTEXT (NOT SHOWN): Text is executed like code in an internal function called eva.

VARIABLE SCOPE : The first two execution contexts correspond with the notion of scope :

• GLOBAL SCOPE: If a variable is declared outside a function, it can be used anywhere because it has global scope.
• FUNCTION-LEVEL SCOPE: When a variable is declared within a function, it can only be used within that function.

EXECUTION CONTEXT & HOISTING

• Each time a script enters a new execution context, there are two phases of activity:
• PREPARE:
  • The new scope is created.
  • Variables, functions, and arguments are created.
  • The value of the this keyword is determined.
• EXECUTE:
• Now it can assign values to variables.
• Reference functions and run their code.

• Execute statements.

• If a JavaScript statement generates an error, then it throws an exception , the interpreter stops and looks for exception-handl ing code.

• The console helps narrow down the area in which the error is located, so you can try to find the exact error.
• There are seven types of built-in error objects in JavaScript :

• If you know that you may get an error, you can handle it gracefully using the try, catch, finally statements.

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