javascript

Chapter 14: Error Handling and Debugging
The reverseSort() function sorts an array in reverse order, using both the sort() and reverse()
methods. Because of the control condition in the if statement, any non - array value that converts to true
will cause an error. Another common mistake is to compare the argument against null as in this example:
//still unsafe, non-array values cause an error
function reverseSort(values){
if (values != null){ //avoid!!!
values.sort();
values.reverse();
}
}
Comparing a value against null only protects the code from two values: null and undefined (which
are equivalent to using the equal and not equal operators). A null comparison doesn ’ t do enough to
ensure that the value is appropriate; therefore, this technique should be avoided. It ’ s also recommended
that you don’t compare a value against undefined , for the same reason.
Another poor choice is to use feature detection for only one of the features being used. Here is an
example:
//still unsafe, non-array values cause an error
function reverseSort(values){
if (typeof values.sort == “function”){ //avoid!!!
values.sort();
values.reverse();
}
}
In this example, the code checks for the existence of a sort() method on the argument. This leaves open
the possibility that an object may be passed in with a sort() function that is not an array, in which case the
call to reverse() causes an error. When you know the exact type of object that is expected, it ’ s best to use
instanceof as shown in the following example to determine that the value is of the right type:
//safe, non-array values are ignored
function reverseSort(values){
if (values instanceof Array){ //fixed
values.sort();
values.reverse();
}
}
This last version of reverseSort() is safe — it tests the values argument to see if it ’ s an instance of
Array . In this way, the function is assured that any non - array values are ignored.
Generally speaking, values that should be primitive types should be checked using typeof , and values
that should be objects should be checked using instanceof . Depending on how a function is being
used, it may not be necessary to check the data type of every argument, but any public - facing APIs
should definitely perform type checking to ensure proper execution.
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