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 .
This script above creates a greeting message, then writes it to an alert box (see right-hand page). In order to create that greeting, two functions are used: greetUser() and getName() .
You might think that the order of execution (the order in which statements are processed) would be as numbered: one through to four. However, it is a little more complicated
The JavaScript interpreter uses the concept of execution contexts. There is one global execution context; plus, each function creates a new execution context. They correspond to variable scope.
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 tools to help you read them.
| PROPERTY | DESCRIPTION |
|---|---|
| name | Type of execution |
| message | Description |
| fileNumber | Name of the JavaScript file |
| lineNumber | Line number of error |
There are seven types of built-in error objects in JavaScript. You'll see them on the next two pages:
| Object | Description |
|---|---|
| Error | Generic error - the other errors are all based upon this error |
| SyntaxError | Syntax has not been followed |
| ReferenceError | Tried to reference a variable that is not declared/within scope |
| TypeError | An unexpected data type that cannot be coerced |
| RangeError | Numbers not in acceptable range |
| URIError | encodeURI ().decodeURI(),and similar methods used incorrectly |
| EvalError | eval () function used incorrectly |
- DEBUG THE SCRIPT TO FIX ERRORS
- HANDLE ERRORS GRACEFULLY