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[javascript] How to check if a JavaScript variable is NOT undefined?

Things I’ve tried that don’t seem to work:

if(lastName != "undefined")

if(lastName != undefined)

if(undefined != lastName)

This question is related to javascript undefined

The answer is

var lastname = "Hi";

if(typeof lastname !== "undefined")
{
  alert("Hi. Variable is defined.");
}

Source: Stackoverflow.com

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