JavaScript Check if Variable is Defined and Not Empty

JavaScript is a versatile and dynamic programming language that allows you to work with variables of different types. However, when dealing with variables, it’s essential to ensure that they are both defined and not empty before using them to avoid unexpected errors or issues in your code. In this blog post, we’ll explore various techniques to check if a variable is defined and not empty in JavaScript.

Understanding the Importance of Variable Validation

Before diving into the techniques for checking if a variable is defined and not empty, let’s understand why this is crucial:

  1. Error Prevention: Attempting to use an undefined variable or an empty value can lead to runtime errors, which can be challenging to debug.
  2. Data Quality: Ensuring that your variables are correctly defined and not empty helps maintain the quality and integrity of your data.
  3. User Experience: Validating input from users or external sources is critical to provide a smooth user experience and prevent unexpected crashes or issues.

Now, let’s look at various methods to achieve this validation in JavaScript.

1. Using typeof

The typeof operator is a simple way to check if a variable is defined:

if (typeof myVar !== 'undefined') { // myVar is defined } 

However, this method does not account for empty values, so you might need additional checks to ensure the variable is not empty.

2. Using typeof and Value Check

To check if a variable is both defined and not empty, you can combine typeof with additional checks:

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if (typeof myVar !== 'undefined' && myVar !== null) { // myVar is defined and not null }

This method works well for variables that are expected to contain primitive values, but it may not be suitable for more complex data structures like arrays and objects.

3. Using Truthy/Falsy Evaluation

if (Array.isArray(myArray) && myArray.length > 0) { // myArray is an array and not empty }

In JavaScript, you can use the truthy/falsy values to check if a variable is defined and not empty:

if (myVar) { // myVar is defined and not empty (if it's a falsy value, it's empty) }

This approach is concise and works for many cases. However, it can give false positives for variables that contain the number 0, an empty string "", or false.

if (Array.isArray(myArray) && myArray.length > 0) { // myArray is an array and not empty }

This method is specific to arrays and may not work for other data types.

5. Using Object Key Check

To check if an object is not empty, you can verify if it has any keys:

if (Object.keys(myObject).length > 0) { // myObject is an object and not empty }

This method is suitable for objects and works even if they contain nested properties.

6. Using Custom Validation Functions

For more complex cases or when you need specific validation criteria, you can create custom validation functions. These functions can be tailored to your requirements and provide a more precise check for your variables.

function isDefinedAndNotEmpty(value) { return typeof value !== 'undefined' && value !== null && value !== ''; } if (isDefinedAndNotEmpty(myVar)) { // myVar is defined and not empty }

Custom validation functions are versatile and can be reused across your codebase.


In JavaScript, checking if a variable is defined and not empty is a crucial aspect of writing robust and error-free code. Depending on the variable type and your specific use case, you can choose from the methods mentioned above. Whether you use typeof, truthy/falsy checks, specific checks for arrays and objects, or custom validation functions, ensuring your variables meet the criteria for being defined and not empty will help you write more reliable JavaScript code.

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