Error Handling & Debugging

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 staement or function has been run.

Execution Context

• The JavaScript interpreter uses the concept of execution contexts.
• There is one global excution context; plus, each function creates a new execution context. They correspond to variable scope.
• Execution Context
  • Global Context
  • Function Context
  • Eval Context
• Variable Scope
  • Global Scope
  • Function-Level Scope

The Stack

• The JavaScript interpreter processes one line opf code at a time.
• When a statement needs data from another function, it stacks the new funmction on top of the current task.

Execution Context & Hoisting

• Each time a script enters a new execution context, there are two phases of activity;
• i. Prepare
  • The new scope is created
  • Variable, functions, and arguments are created
• ii. 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 variables object.
• It holds the variables, functions, and parameters available within it.
• Each execution context can also access its parent’s variable object.

Understading Errors

• If a JavaScript statement generates an error, then it throws an exception.
• At that point, the interpreter stops and looks for excception-handling code.
• Error objects can help you find where you mistakes are and browsers have tools to help you read them.
  • SyntaxError
  • ReferenceError
  • EvalError
  • URIError
  • TypeError
  • RangeError
  • Error
  • NaN

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.
• i. Debug the script to fix errors
• ii. Handle errors gracefully