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

Make sure you understand the order of how scripts are processed and how statement are executed to be able to better pinpoint problems. remember that JS processes code line by line

Execution Contexts:

statements in a script exist in either,

global context ( code in the script yet not a function-only ONE)
function context (code within function)
eval context

These contexts have a scope, either a global scope(a variable outside of a function or a variable not lead with “var”) or function-level(variable only working within function). As script enters a new context two things happen.

prepare- new scope, variables, functions and arguments are created execute - script can now assign values to variable, call functions to run, and execute statements

To understand scope know that each execution context posesses it’s own variables objection holding functions parameters and variables withing it and parent variables object is accessible as well.

To understand errors know that JS will deploy an exception which the interpreter stops to look for a handling code for.

Dealing With Errors

When dealing with an error you can find it written in the script. Handle the error gracefully using statements:

try
catch
throw
finally

Dubugging Workflow

what does the error message tell you?
What part of the script is the problem?
What line number in the code can it be found in
what kind of error is it?

insert “dubugg” in script to run up to a certain point to try and narrow it down
use breakpoint

what is the problem

check the variables, try commenting out parts of code to test specific areas.
check the parameter for a function or number of items in array

CHROME DEV TOOLS

open dev tools/console
look at errors and line info (a specified line does not always mean that is where the problem is, it can be where the browser first noticed the error.)
use try,catch,finally blocks with specific code in question inside example pp.480