Comparing to undefined not working every time

Question

When I try to check for a function parameter if it is defined, and use

 if (variablename === undefined) { do something };

sometimes it works as intendend, but sometimes(mainly very early on, when some scripts are possibly still loading) I get a javascript error, that states that

variablename is undefined

If I use

if (typeof variablename == 'undefined')

it works everytime.

Is the === undefined has some prerequisites, or what could be the problem?

`console.log(variablename)` will result in *variablename is undefined*. However, `var variablename = undefined; console.log(variablename);` will just print undefined — CodingIntrigue, Jun 17 '15 at 12:27
if a name is not defined it will throw an error, if the value is undefined it works. — Hacketo, Jun 17 '15 at 12:27
You are doing something wrong if you don't know whether your variables are declared or not. — Oriol, Jun 17 '15 at 12:34

score 1 · Accepted Answer · answered Jun 17 '15 at 12:28

The first thing that variablename === undefined does is to read the value of variablename.

It could be a variable which has the value undefined (which will work fine) or it could be a variable that isn't defined at all … which will throw a reference error.

Using typeof won't throw a reference error.

Comparing to undefined not working every time

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