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.
To complete step one, the interpreter needs the results of the functions in steps two and three (because the message contains values returned by those functions). The order of execution is more like this: 1, 2, 3, 2, 1, 4.
1. The greeting variable gets its value from the greetUser() function.
2. greetUser() creates the message by combining the string 'He 11 o ' with the result of getName ().
3. getName () returns the name to greetUser().
2. greetUser() now knows the name, and combines it with the string. It then returns the message to the statement that called it in step 1.
1. The value of the greeting is stored in memory.
4. This greeting variable is written to an alert box
There is one global execution context; plus, each function creates a new execution context. They correspond to variable scope
Every statement in a script lives in one of three
execution contexts:
Q GLOBAL CONTEXT
Code that is in the script, but not in a function.
There is only one global context in any page.
FUNCTION CONTEXT
Code that is being run within a function.
Each function has its own function context.
Q EVAL CONTEXT (NOT SHOWN)
Text is executed like code in an internal function
called eval {) (which is not covered in this book).
The first two execution contexts correspond with the
notion of scope (which you met on p98):
Q GLOBAL SCOPE
If a variable is declared outside a function, it can
be used anywhere because it has global scope.
If you do not use the var keyword when creating
a variable, it is placed in global scope.
FUNCTION-LEVEL SCOPE
When a variable is declared within a function,
it can only be used within that function. This is
because it has function-level scope.
Error objects can help you find where your mistakes are and browsers have tools to help you read them.
This is caused by incorrect use of the rules of the language. It is often the result of a simple typo.
document .write ("Howdyl );
document .getElementByid('page' I
SyntaxErr or : Expected token ' ) '
MISSING COMMA IN ARRAY
Would be same for missing] at the end
var l ist = ['Item 1', 'Item 2 ' l 'rtem 3'];
SyntaxError: Expected token ']'
MALFORMED PROPERTY NAME
It has a space but is not surrounded by quote marks
user = {f i rstl name: "Ben", lastName: "Lee"};
Synt axError: Expected an identifier but
found 'name ' instead
VARIABLE DOES NOT EXIST
This is caused by a variable that is not declared or is
out of scope.
VARIABLE IS UNDECLARED
var width = 12 ;
var area = width * llt!ftNU! ;
ReferenceError: Can ' t find vari able:
height
NAMED FUNCTION IS UNDEFINED
document.write ( randomFunction() ) ;
ReferenceError: Can't find variable :
randomFunctio