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Read: 10 - JS Debugging

EXECUTION CONTEXT

  • 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 l {) (which is not covered in this book). VARIABLE SCOPE

  • VARIABLE 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. This is because it has function-level scope. The stack: javascript interpreter procced one line of code at a time,when statement need another data from a function it stacks or piles

  • Each time a script enters a new execution context, there are two phases of activity: 1.PREPARE 2.EXECUTE

  • hoisting

    • Call functions before they have been declared
    • Assign a value to a va ria ble that has not yet been declared

  • The preparation phase is often described as taking all of the variables and functions and hoisting them to the top of the execution context.

  • Each execution context also creates its own variab1es object. This object contains details of all of the variables, functions, and parameters for that execution context.

  • If a JavaScript statement generates an error, then it throws an exception. At that point, the interpreter stops and looks for exception-handling code.

  • When an Er ror object is created, it will contain the following properties:

    • name : Type of execution
    • message :Description
    • fileNumber :Name of the JavaScript
    • fillineNumber :Line number of error

  • There are seven types of built-in error objects in JavaScript.

    1. Error :Generic error - the other errors are all based upon this error
    2. Syntax Error :Syntax has not been followed
    3. ReferenceError:Tried to reference a variable that is not declared/within scope
    4. TypeError :An unexpected data type that cannot be coerced
    5. Range Error: Numbers not in acceptable range
    6. URI Error: encodeURI ().decodeURI(),and similar methods used incorrectly
    7. EvalError: eval () function used incorrectly

HOW TO DEAL WITH ERRORS 1: DEBUG THE SCRIPT TO FIX ERRORS 2: HANDLE ERRORS GRACEFULLY

Resources:

JavaScript book, Ch. 10, “Error Handling & Debugging”