JavaScript undefined Property

The JavaScript undefined property specifies if a variable has been declared or assigned a value yet.

Following is the code for the JavaScript undefined property −


 Live Demo

<!DOCTYPE html>
<html lang="en">
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
   body {
      font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
   .sample {
      font-size: 18px;
      font-weight: 500;
      color: red;
<h1>JavaScript undefined property</h1>
<div class="sample"></div>
<button class="Btn">CLICK HERE</button>
CLICK the above button to know if variable age has been defined or not
   let sampleEle = document.querySelector(".sample");
   let age;
   document.querySelector(".Btn").addEventListener("click", () => {
      if (age === undefined) {
         sampleEle.innerHTML ="The variable age has not been defined yet or no value is given to it";
      } else
      sampleEle.innerHTML ="The variable age is defined and its value = " + age;


On clicking the ‘CLICK HERE’ button −

Updated on: 12-May-2020


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