How to check for undefined in JavaScript

How to check for undefined in JavaScript

window.setTimeout = function () {      alert("Got you now!");  };  

Bottom line, the "it can be redefined" argument to not use a raw === undefined is bogus.

If you are still concerned, there are two ways to check if a value is undefined even if the global undefined has been overwritten.

You can use the void operator to obtain the value of undefined. This will work even if the global window.undefined value has been over-written:

if (name === void(0)) {...}  

The zero in this example doesn't have any special meaning, you could just as well use 1 or function(){}. void(anything) will always evaluate to undefined.

Alternatively, you can use the typeof operator to safely check if a value was assigned. Instead of comparing to the global undefined value you check if the value's type is "undefined":

if (typeof name === "undefined") {...}  

Note that this is slightly different from the previous options. Even if name wasn't declared typeof would still say it's undefined. If you compared an undeclared variable to undefined or void(0) you would instead get a ReferenceError.

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