We are frequently using the following code pattern in our JavaScript code:

if (typeof(some_variable) != 'undefined' && some_variable != null)
{
    // Do something with some_variable
}

Is there a less verbose way of checking that has the same effect?

According to some forums, and literature saying, simply the following should have the same effect.

if (some_variable)
{
    // Do something with some_variable
}

Unfortunately, Firebug evaluates such a statement as error at runtime when some_variable is undefined, whereas the first one is just fine for it. Is this only an (unwanted) behavior of Firebug or is there really some difference between those two ways?

javascript null undefined
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