CLASS 10 READING NOTES

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DEBUGGING

• JavaScript can be hard to learn and everyone makes mistakes when writing it. This chapter will help you learn how to find the errors in your code. It will also teach you how to write scripts that deal with potential errors gracefully.

• When you are writing JavaScript, do not expect to write it perfectly the first time. Programming is like problem solving: you are given a puzzle and not only do you have to solve it, but you also need to create the instructions that allow the computer to solve it. too.

• When writing a long script, nobody gets everything right in their first attempt. The error messages that a browser gives look cryptic at first, but they can help you determine what went wrong in your JavaScript and how to fix it. In this chapter you will learn about:

• THE CONSOLE & DEV TOOLS Tools built into the browser that help you hunt for errors.

• COMMON PROBLEMS Common sources of errors, and how to solve them.

• HANDLING ERRORS How code can deal with potential errors gracefully.

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 cannot complete until another statement or function has been run.

EXECUTION 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.

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

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.

HOW TO DEAL WITH ERRORS

Now that you know what an error is and how the browser treats them, there are two things you can do with the errors.

• DEBUG THE SCRIPT TO FIX ERRORS If you come across an error while writing a script (or when someone reports a bug), you will need to debug the code, track down the source of the error, and fix it.

• HANDLE ERRORS GRACEFULLY You can handle errors gracefully using try, catch, throw, and finally statements.

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.

WRITING FROM THE SCRIPT TO THE CONSOLE

Browsers that have a console have a console object, which has several methods that your script can use to display data in the console. The object is documented in the Console API.

## 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.

CONDITIONAL BREAKPOINTS

You can indicate that a breakpoint should be triggered only if a condition that you specify is met. The condition can use existing variables.

DEBUGGING TIPS

• ANOTHER BROWSER Some problems are browserspecific. Try the code in another browser to see which ones are causing a problem.
• ADD NUMBERS Write numbers to the console so you can see which the items get logged. It shows how far your code runs before errors stop it.
• STRIP IT BACK Remove parts of code, and strip it down to the minimum you need. You can do this either by removing the code altogether, or by just commenting it out using
• EXPLAINING THE CODE Programmers often report finding a solution to a problem while explaining the code to someone else.
• SEARCH Stack Overflow is a Q+A site for programmers. Or use a traditional search engine such as Google, Bing, or DuckDuckGo.
• CODE PLAYGROUNDS If you want to ask about problematic code on a forum, in addition to pasting the code into a post, you could add it to a code playground site (such as JSBin.com, JSFiddle. com, or Dabbl et. corn) and then post a link to it from the forum.

• VALIDATION TOOLS There are a number of on line validation tools that can help you try to find errors in your code:

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