Question

By definition, there can only be one control on a page with a given ID.

This statement set the value of a single control:

document.getElementById("ctl00_phPageContent_dtmVisitChinaFrom_txtSkipValidation").value = "1";

Why is the “[0]” necessary?

$("#ctl00_phPageContent_dtmVisitChinaFrom_txtSkipValidation")[0].value = "1";

Answer 1

When you do this:

$("ctl00_phPageContent_dtmVisitChinaFrom_txtSkipValidation")[0]

Or

$("ctl00_phPageContent_dtmVisitChinaFrom_txtSkipValidation").get(0)

You are accessing actual DOM element which has value property available.

It isn’t necessary if you use val() method of jQuery instead:

$("#ctl00_phPageContent_dtmVisitChinaFrom_txtSkipValidation").val('1')

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So [index] part is necessary when you want to access native methods/properties of elements not otherwise.