JS Debugging
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In this blog I will give a summary for chapter 10 of the book: “Javascript and Jquery” :books:
- Chapter 10: Error Handling & Debugging ✔️
Chapter 10: Error Handling & Debugging
The JavaScript interpreter uses the concept of execution contexts. There is one global execution context; plus, each function creates a new new execution context. They correspond to variable scope.
EXECUTION CONTEXT
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GLOBAL CONTEXT Code that is in the script, but not in a function. There is only one global context in any page.
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FUNCTION CONTEXT Code that is being run within a function. Each function has its own function context.
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EVAL CONTEXT Text is executed like code in an internal function called eval ()
VARIABLE SCOPE
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GLOBAL SCOPE If a variable is declared outside a function, it can be used anywhere because it has global scope.
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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.
EXECUTION CONTEXT & HOISTING
Each time a script enters a new execution context, there are two phases of activity:
1: PREPARE • The new scope is created • Variables, functions, and arguments are created • The value of the this keyword is determined
2: EXECUTE • Now it can assign values to variables • Reference functions and run their code • Execute statements
UNDERSTANDING SCOPE
In the interpreter, each execution context has its own va ri ables object. It holds the variables, functions, and parameters available within it. Each execution context can also access its parent’s v a ri ables object.
Functions in JavaScript are said to have lexical scope. They are linked to the object they were defined within. So, for each execution context, the scope is the current execution context’s variables object, plus the variables object for each parent execution context.
UNDERSTANDING ERRORS
If a JavaScript statement generates an error, then it throws an exception. At that point, the interpreter stops and looks for exception-handling code.
ERROR OBJECTS
Syntax Error
(SYNTAX IS NOT CORRECT)
- MISMATCHING OR UNCLOSED QUOTES
- MISSING CLOSING BRACKET
- MISSING COMMA IN ARRAY
- MALFORMED PROPERTY NAME
Reference Error
(VARIABLE DOES NOT EXIST)
- VARIABLE IS UNDECLARED
- NAMED FUNCTION IS UNDEFINED
Eval Error
(INCORRECT USE OF eval() FUNCTION )
URI Error
(INCORRECT USE OF URI FUNCTIONS)
Type Error
(VALUE IS UNEXPECTED DATA TYPE)
RangeError
(NUMBER OUTSIDE OF RANGE)
Error
(GENERIC ERROR OBJECT)
NaN
(NOT AN ERROR)
HOW TO DEAL WITH ERRORS
1: DEBUG THE SCRIPT TO FIX ERRORS 2: **HANDLE ERRORS GRACEFULLY **
HANDLING EXCEPTIONS
Using try, catch, and finally.
try {
catch (exception) {
finally {
}
THROWING ERRORS
If you know something might cause a problem for your script, you can generate your own errors before the interpreter creates them.
throw new Error( 'message') ;
Key points:
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Debugging is the process of finding errors. It involves a process of deduction.
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The console helps narrow down the area in which the error is located.
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JavaScript has 7 different types of errors. Each creates its own error object, which can tell its line number and gives a description of the error.