Disabling a field of type Text Area

Hello Experts,

I need to disable a field of type Text Area. I am using the below code on form populate event

var input1 = getFieldByName("Test_Text_Area");
setTimeout(function() {
input1 .getElementsByTagName("input")[0].disabled = true;
}, 100);

When the field is of type Text, the above code works fine but when its of type Text Area , the field is not getting disabled.

Any help is highly appreciated.