reading-notes

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JS Debugging

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

Things to consider

• Order of excution
• Execution contexts
• The stack
• Execution context (2 phases)
  1. Prepare - new scope is created. variables, functions, and arguments are create
  2. Execute - Now it can assign values to variables, reference function and there code, execute statements.
• Hoisting

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 can help you find where your mistakes are and browsers have tool to help you read them.

Property	Descriptions
name	Type of error
message	description
fileNumber	Name of the javascript file
lineNumber	Line number of error

7 types of error objects

Property	Descriptions
Error	Generic error - the other errors are all based upon this error
Syntax error	Syntax has not been followed
Reference Error	Tried to reference a variable that is not declared
TypeError	an unexpected data type that cannot be coerced
RangedError	numbers not in acceptable range
URIError	encodeURI(), decodeURI(), and similar methods used incorrectly
evalError	eval() functioned incorrectly

You can handle errors graceully using try, catch, throw and finally statements.

try {
  // try to execute this code
} catch (exception) {
  // if there is an exception, run this code
} finally {
  // this always gets executed
}

Identify:

1. Where is the problem?
2. what exactly is the problem?

Tools:

1. Console Table() method:
   • objects
   • arrays that contain other objects

var contacts = {
  "london": {
    "tel": "+44 (0)200 948 0128:,
    "country"= "UK"},
  "New Youk": {
    "Tel": "+1 (0)1 555 2104
    "country": "USA"}
}

console.table(contacts);

var city, contactDetails;
contactDetails = '';

$.each(contacts, function(city, contacts){
  contactDetails += city + ': ' + contacts.Tel + '<br />;
});
$('h2').after('<p>' + contactDetails  + '</p>');

Learn breakpoints –> p476 & p477

Setting multiple breakpoints allows you to step through code.

You can add conditional breakpoints.

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')

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