Why is this array.length return 1 for an empty array?

Using:

<input id="selectedRecords" name="selectedRecords" type="hidden" />

Executing this code:

var keyValues = $('#selectedRecords').val().split(",").map(item => item.trim());

console.log('$(#selectedRecords).val(): [' + $('#selectedRecords').val() + ']');
console.log('keyValues: [' + keyValues + ']');
console.log('keyValues.length: [' + keyValues.length + ']');

Gives this console readout (copy pasted from console window in Brave browser):

$(#selectedRecords).val(): []
keyValues: []
keyValues.length: [1]

Why is keyValues.length returning a count of 1 ?