The undefined value is a condition where an expression does not have a correct value. JavaScript undefined property indicates that a variable has not been assigned a value, or not declared at all.
When you attempt to access an undefined property, JavaScript will return the value undefined. For example, consider the following code
let obj = { name: "John", age: 30 };
console.log(obj.address); // undefined
Example JavaScript undefined
A simple example code will get an undefined value when you call a non-existent property or method of an object.
Variable not declared:
<!DOCTYPE html>
<html>
<body>
<script>
if (typeof myVar === "undefined") {
console.log("myVar is undefined")
} else {
console.log("myVar is defined")
}
</script>
</body>
</html>
Output:
How can I check for “undefined” in JavaScript?
Answer: One reason to use typeof is that it does not throw an error if the variable has not been declared.
if (typeof myVar !== "undefined") {
doSomething();
}Or use
myVar === undefinedWarning: Please note that === is used over == and that myVar has been previously declared (not defined).
You can use the “in” operator to check if a property exists within an object. For example:
let obj = { name: "John", age: 30 };
if ("address" in obj) {
console.log(obj.address);
} else {
console.log("Address property does not exist");
}
Comment if you have any doubts or suggestions on this JS undefined variable topic.
Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.
OS: Windows 10
Code: HTML 5 Version