Code 201: Class 10 - Error Handling and Debugging

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Read: JS Chapter 10

Execution Contexts

• Global Context code in the script, but not in a function
• Function Context code run within a function
• Global Scope variable declared outside a function, can be used anywhere
• Function-Level Scope variable declared within a function can only b used within that function

Error Objects

• Property Description
  • name type of error
  • message description
  • fileNumber name of the JS file
  • lineNumber line number error
• Object Description
  • Error generic error
  • SyntaxError syntax has not been followed
  • ReferenceError tried to reference a variable that is not declared or outside the scope
  • TypeError unexpected data type that cannot be coerced
  • RangeError numbers not in acceptable range
  • URIError encodeURI(), decodeURI(), adn similar methods not used correctly
  • EvalError eval() function used incorrectly

List of Chrome Browser Errors and their definitions on page 460

Debugging Workflow

• Where is the problem start at the point of the error and work backwards to narrow down the cause
• what exactly is the problem

Typing into the Chrome console

• can type code directly into the console
• variables are stored until the console is cleared
• the “no entry” sign clears the console

Console Methods

• console.log('Example'); Prints “Example” to the console without any symbols
• console.info('Example'); Prints “Example” to the console with an info symbol
• console.warn('Example'); Prints “Example” to the console with a warning symbol
• console.error('Example'); Prints “Example” to the console with an error symbol
• console.table(); Output tables showing objects, and nested arrays
• console.assert(); test if a condition is met and write to the console only if expression evaluates to false

Breakpoints

• Sources tab, click a line to set a break point the refresh page to run code up to that point, hovering over a variable in the code will display its current value at the break point

Debugger Keyword

• debugger keyword that when placed in code will stop the interpreter at that line of code if the developer tools are open, can be placed inside a conditional statement so that code is only paused if certain situation is present

Handling Exceptions

• Try first try specific code
• Catch if try code block fails it is passed to the catch code block and attempts another option or indicates the error to the user
• Finally if neither of the first two options succeed, run this code which is intended to work in all situations

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