Got confused by this behaviour. I thought doing something like:
if (variable name)
{
... do this...
}
should work. However if the variable is not defined, I just get 'ReferenceError: Can't find variable: "variable name, and the else block won't even be executed. For example the following snippet that I got off another StackOverflow question doesn't work when I test it. Any suggestions?
if(persons_name)
{
var name = persons_name;
}
else
{
var name = "John Doe";
}
if (typeof persons_name !== 'undefined') {
...
} else {
...
}
Note that you don't need braces with typeof.
false values, but talked about variable existence. If you want to check if a variable is defined, my code is right.
if (variable name) is a test for no "falsey" values, not merely a test for existence or not undefined. The typeof persons_name !== 'undefined' doesn't deal with the fact that you're trying to incorrectly reference an undeclared variable. It skirts the issue with a hack. A better solution is to heed the information in the error, and declare the variable properly.
Commented
Aug 15, 2011 at 21:48
Unlike a property, an unqualified name has to be defined before you can use it. So you if you were looking for a global variable persons_name you could write
if (window.persons_name)
and it would evaluate to undefined if persons_name didn't exist. Alternatively, you can simply declare persons_name if you expect it to exist.
var persons_name;
This won't change the value of persons_name if it already exists.
var name = (typeof persons_name !== 'undefined' || !(!!persons_name)) ?
persons_name :
'John Doe';
by saying var name; you define the variable; and by saying var name = "john doe"; you assign it a value.
