In JavaScript, you can test if a variable is defined in local storage by using the localStorage.getItem() method. This method retrieves the value of the specified key from local storage. If the key does not exist, it will return null.

Here's an example of how you can check if a variable exists in local storage:

// Check if the variable exists in local storage
if (localStorage.getItem('yourVariableName') !== null) {
    // Variable is defined in local storage
    console.log('Variable is defined in local storage');
} else {
    // Variable is not defined in local storage
    console.log('Variable is not defined in local storage');
}

In this example, replace yourVariableName with the name of the variable you want to check. If the variable exists in local storage, this condition will be true, otherwise, it will be false.