JQuery datepicker defaultdate from another datepicker

I have several jquery datepickers on my asp page and I want to set the defaultdate of date picker to be the same as the previous datepicker.

I am currently using

defaultDate: document.getElementById('<%=txtFirst开发者_运维知识库Date.ClientID%>').value

This works okay, but obviously errors if firstDate is null. Should I be using OnSelect function instead?

Appreciate any help

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var defaultDate = document.getElementById('<%=txtFirstDate.ClientID%>').value;

defaultDate: defaultDate || 'default-value'