you might notice something strange in the title. It's doesn't make sense,right? undefined is undefined. What does it have to with scope? In case you don't know scope, check out this link where I've discussed about scope.We all know what undefined is. It is a primitive value which means not defined. Say, if a variable is declared and it is not initialized , the variable will print undefined in the console.Let's declare one and see for yourself in the console.

  var fruit

console.log(fruit) // prints undefined

undefined is expected to generate a falsy value when used as a test case in if-else block. If a variable needs to be checked whether or not it is defined in a if-else block ,undefined is our friend in need.Let's check out this one

  var fruit

if(fruit == undefined){
    console.log("fruit is not defined")
}else{
    console.log("fruit is defined")
}

undefined is a property of global object.As it is a property of global object, question may arise whether or not we can create a variable called undefined and assign value to it.Let's try

  var undefined = true

if(undefined){
    console.log("undefined is set to true")
}else{
    console.log("undefined is still undefined")
}

It will print the log statement inside else case.So setting undefined to true will not work in global scope. Creating a variable named undefined and setting a truthy value to it will not pass the test of if-else block.The undefined property of global object will prevail in global scope.

But scenario might change with the change of scope and pre-assumed notions can change with it...

In javascript every function ships with it's own private scope. It is called local scope.When a function invocation occurs, javascript execution enters into a local scope. Now what undefined have to do with it. We've already seen that we can not change the default behaviour of undefined. As MDN page say

undefined is a property of the global object; i.e., it is a variable in global scope....

Can you identify the subtle difference? It doesn't say anything about local scope. That means we can create a variable called undefined in the local scope,doesn't it? Let's give it a try

  function myFunc(){

    var undefined = true

    if(undefined){
        console.log("undefined is set to true")
    }else{
        console.log("undefined is still undefined")
    }

}

// now invoke it 

myFunc() // prints "undefined is set to true

A variable called undefined is created inside myFunc function's local scope and it's value is set to true. Will it pass the test this time? Surprisingly it does pass the test. The if statement proves to be truthy and code inside the if statement gets executed.

undefined is a valid variable name in local scope....

That's interesting but as well as problematic. How we can check for undefined value in if-else statement now. A solution might be not using undefined for checking test cases at all.Then what's is the alternative? you might ask.the answer is void function. void takes an expression and returns a undefined value.

void always returns undefined,it doesn't care about the scope....

  var three = 3

function aFunction (){

   return 3

}
console.log(void(0))  // undefined

console.log(void(aFunction())) // invoking a function also return undefined, doesn't matter what value the function generates

void will always return undefined. So an if-else statement can be written using void statement to check for undefined value

  
function myFunc(){

    var x

    if(x === void(0)){
        console.log("x is undefined")
    }else{
        console.log("no,it's not")
    }

}

myFunc() // prints "x is undefined"

It is safer to use void instead of using undefined directly when multiple people working on a same file.Though it is a rare mistake. In practical no one with proper knowledge of javascript will ever declare a variable called undefined in their script. But it's good to to know that there is a workaround for using undefined.

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