Which Type Is Assigned to Variables With Null Type

null and undefined in JavaScript

As we have seen in the variable section that we can assign any primitive or non-primitive type of value to a variable. JavaScript includes two additional primitive type values - null and undefined, that can be assigned to a variable that has special meaning.

null

You can assign null to a variable to denote that currently that variable does not have any value but it will have later on. A null means absence of a value.

                          var              myVar =              null;  alert(myVar);              // null                                    

In the above example, null is assigned to a variable myVar. It means we have defined a variable but have not assigned any value yet, so value is absence.

null is of object type e.g. typeof null will return "object"

If you try to find DOM element using document.getElelementByID for example, and if element is found then it will return null. So it is recommended to check for null before doing something with that element.

                          var              saveButton = document.getElementById("save");              if              (saveButton !== null)         saveButton.submit();                      

A null value evaluates to false in conditional expression. So you don't have to use comparison operators like === or !== to check for null values.

                          var              myVar =              null;              if              (myVar)     alert("myVar is not null');              else              alert("myVar is null"              );                      

undefined

Undefined is also a primitive value in JavaScript. A variable or an object has an undefined value when no value is assigned before using it. So you can say that undefined means lack of value or unknown value.

                          var              myVar;  alert(myVar);              // undefined                                    

undefined is a token. typeof undefined will return undefined not an object.

In the above example, we have not assigned any value to a variable named 'myVar'. A variable 'myVar' lacks a value. So it is undefined.

You will get undefined value when you call a non-existent property or method of an object.

                          function              Sum(val1, val2) {              var              result = val1 + val2; }              var              result = Sum(5, 5); alert(result);// undefined                                    

In the above example, a function Sum does not return any result but still we try to assign its resulted value to a variable. So in this case, result will be undefined.

If you pass less arguments in function call then, that parameter will have undefined value.

                          function              Sum(val1, val2) {              return              val1 + val2;                              // val2 is undefined                            }  Sum(5);                      

An undefined evaluates to false when used in conditional expression.

                          var              myVar;              if              (myVar)     alert("myVar evaluates to true");              else              alert("myVar evaluates to false");                      

null and undefined is one of the main reasons to produce a runtime error in the JavaScript application. This happens if you don't check the value of unknown return variables before using it. If you are not sure that a variable will always have some value, the best practice is to check the value of variables for null or undefined before using them.

1. null and undefined are primitive values in JavaScript.
2. A null value means absence.
3. An undefined value means lack of value.
4. A null or undefined value evalutes to false in conditional expression.

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Which Type Is Assigned to Variables With Null Type

Source: https://www.tutorialsteacher.com/javascript/javascript-null-and-undefined

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