javascript

Chapter 14: Error Handling and Debugging
Data Type Errors
Since JavaScript is loosely typed, variables and function arguments aren ’ t compared to ensure that the
correct type of data is being used. It is up to you, as the developer, to do an appropriate amount of data
type checking to ensure that an error will not occur. Data type errors most often occur as a result of
unexpected values being passed into a function.
In the previous example, the data type of the third argument is checked to ensure that it ’ s a string, but
the other two arguments aren ’ t checked at all. If the function must return a string, then passing in two
numbers and omitting the third argument easily breaks it. A similar situation is present in the following
function:
//unsafe function, any non-string value causes an error
function getQueryString(url){
var pos = url.indexOf(“?”);
if (pos > -1){
return url.substring(pos +1);
}
return “”;
}
The purpose of this function is to return the query string of a given URL. To do so, it first looks for a
question mark in the string using indexOf() and if found, returns everything after the question mark
using the substring() method. The two methods used in this example are specific to strings, so any
other data type that is passed in will cause an error. The following simple type check makes this function
less error prone:
function getQueryString(url){
if (typeof url == “string”){ //safer with type check
var pos = url.indexOf(“?”);
if (pos > -1){
return url.substring(pos +1);
}
}
return “”;
}
In this rewritten version of the function, the first step is to check that the value passed in is actually a
string. This ensures that the function will never cause an error because of a nonstring value.
As discussed in the previous section, using non - Boolean values as conditions for flow control statements
is a bad idea because of type coercion. This is also a bad practice that can cause data type errors.
Consider the following function:
//unsafe function, non-array values cause an error
function reverseSort(values){
if (values){ //avoid!!!
values.sort();
values.reverse();
}
}
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