How to Check if Variable is Undefined or False in Javascript
In this tutorial, you will learn how to check if variable is undefined or false in javascript. Whenever you create a variable and do not assign any value to it, by default it is always undefined. It becomes defined only when you assign some value to it. A boolean is one of the primitive data types in javascript. It can have only two values, true or false.
There are numerous ways to check if a variable is undefined or false. We are going to use one of the easiest solutions which involve the usage of the typeof
and ternary (?
) operators. The typeof
operator returns the type of the variable. The ternary operator is also known as the conditional operator which acts similar to the if-else statement.
In the following example, we have one global variable and upon click of a button, we will check if the variable is undefined or false and display the result on the screen. Please have a look over the code example and the steps given below.
HTML & CSS
- We have 3 elements in the HTML file (
div
,button
, andh1
). Thediv
element is just a wrapper for the rest of the elements. - The
innerText
for thebutton
element is“Check”
and for theh1
element, it is“Result”
. - We have done some basic styling using CSS and added the link to our
style.css
stylesheet inside thehead
element. - We have also included our javascript file
script.js
with ascript
tag at the bottom.
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta http-equiv="X-UA-Compatible" content="ie=edge"> <link rel="stylesheet" href="style.css"> <title>Document</title> </head> <body> <div> <button>Check</button> <h1>Result</h1> </div> <script src="script.js"></script> </body> </html>
body { text-align: center; } div { display: inline-block; } button { display: inline-block; padding: 10px 20px; }
Javascript
- We have selected the
button
element andh1
element using thedocument.querySelector()
method and stored them inbtnCheck
andresult
variables respectively. - We have a global variable
myVar
which holds a boolean as its value. - We have attached a
click
event listener to thebutton
element. - In the event handler function, we are calling
checkVariable()
method and passing itmyVar
as a parameter. This method will verify whethermyVar
is undefined or false. - In the
checkVariable()
method, we are usingtypeof
and ternary (?
) operators as well asswitch
statement for verification. Depending upon the result of the check, this method will return“False”
,"Undefined"
or“Neither False nor Undefined”
. - We are displaying the returned value in the
h1
element using theinnerText
property.
let btnCheck = document.querySelector("button"); let result = document.querySelector("h1"); let myVar = false; btnCheck.addEventListener("click", () => { result.innerText = checkVariable(myVar); }); function checkVariable(value) { let output = typeof value == "undefined" ? 1 : 0; if (output == 0) output = value == false ? 2 : 0; switch (output) { case 1: return "Undefined"; case 2: return "False"; default: return "Neither False nor Undefined"; } }