Question

There are two ways I can check to see if a variable or property exists.

The first asserts that no falsy values are returned.

1.

var testVar;
if(!testVar){

    //use testVar becuase it exists 

}

The second asserts the two conditions explicitly.

2.

var testVar;

if(testVar !== undefined && testVar !== null){

    //use testVar becuase it exists 

}

With the goal of checking if the object/string/number/array exists, is there any difference between the two ?

Answer 1

They are different. If you try to put {} or 1 or true or "nyan cat" … into testVar, then it returns false for !testVar:

var testVar = {};
if(!testVar){  //false
    //will not be executed, but testVar does exist
}

A better way to check if a variable exist:

typeof(randomVar); //undefined
typeof(window);    //object

---

PS

var testVar;
if(!testVar){
    //...
}

This actually does not check if it exists. Try

var testVar = "something";
if(!testVar){              //false
    console.log("Exist.");
}
//nothing happens.