jQuery - Create hidden form element on the fly

$('#myformelement').append('<input type="hidden" name="myfieldname" value="myvalue" />');

The same as David's, but without attr()

$('<input>', {
    type: 'hidden',
    id: 'foo',
    name: 'foo',
    value: 'bar'
}).appendTo('form');

if you want to add more attributes just do like:

$('<input>').attr('type','hidden').attr('name','foo[]').attr('value','bar').appendTo('form');

$('<input>').attr({
    type: 'hidden',
    id: 'foo',
    name: 'foo[]',
    value: 'bar'
}).appendTo('form');

$('<input>').attr('type','hidden').appendTo('form');

To answer your second question:

$('<input>').attr({
    type: 'hidden',
    id: 'foo',
    name: 'bar'
}).appendTo('form');

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