JS Debugging

• If you understand execution contexts (which have two stages) 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.

ORDER OF EXECUTION

To find the source of an error, it helps to know how scripts are processed. The order in which statements are executed can be complex; some tasks

EXECUT.ION CONTEXTS

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.

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.

ERROR OBJECTS

Error objects can help you find where your mistakes are and browsers have tools to help you read them.

• HOW TO DEAL WITH ERRORS

DEBUG THE SCRIPT TO FIX ERRORS HANDLE ERRORS GRACEFULLY

#### A DEBUGGING WORKFLOW Debugging is about deduction: eliminating potential causes of an error. Here is a workflow for techniques you will meet over the next 20 pages. Try to narrow down where the problem might be, then look for clues.

TYPING IN THE CONSOLE IN CHROME

You can also just type code into the console and it will show you a result.

BREAKPOINTS

You can pause the execution of a script on any line using breakpoints. Then you can check the values stored in variables at that point in time.

TRY, CATCH, FINALLY

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