How to determine if a variable is undefined

In JavaScript, you can check for undefined using various methods. Here are the details along with examples:

Using the typeof Operator

The typeof operator can be used to check if a variable or expression is undefined.

let myVariable; if (typeof myVariable === 'undefined') { console.log("myVariable is undefined"); }

Direct Comparison

You can directly compare a variable to undefined to check if it's undefined.

let myVariable; if (myVariable === undefined) { console.log("myVariable is undefined"); }

Using the === Operator

The === operator performs strict equality comparison, which can be used to check if a value is exactly undefined.

let myVariable; if (myVariable === undefined) { console.log("myVariable is undefined"); }

Function Parameters

If a function parameter is not provided, it's set to undefined. You can check this way:

function checkParam(param) { if (param === undefined) { console.log("Parameter is undefined"); } } checkParam(); // Output: Parameter is undefined

Property Existence

You can also check if a property of an object is undefined.

let myObject = { prop: undefined }; if (myObject.prop === undefined) { console.log("Property is undefined"); }

JavaScript undefined

JavaScript: Check if Variable is undefined or null

The term "undefined" is used to indicate that a variable has not been assigned a value or hasn't been declared at all. It's a property associated with the global object and can be considered a variable in the global scope. As a primitive type in JavaScript, undefined represents a particular type as well. When you define a variable but don't provide it with a value, the variable holds the value undefined, indicating the absence of a meaningful value.

let myVal; console.log(typeof myVal); //output: undefined


let myVal; console.log(myVal); //output: undefined


Remember that undefined is a built-in value in JavaScript. Be careful not to accidentally create variables with the value of undefined, as it might lead to confusion. To check if a variable exists (has been declared) and is currently without a value, use the typeof or direct comparison methods.