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$('#ddlID').val('New_OptionValue').change()
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/*make sure that value is included in the options value of the dropdownlist
e.g.
(<select><option value='CA'>California</option><option value='AK'>Alaska</option> </select>)
*/
$('#mycontrolId').val(myvalue).attr("selected", "selected");
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$('select[name^="salesrep"] option[value="Bruce Jones"]').attr("selected","selected");
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$('select[name^="salesrep"] option:selected').attr("selected",null);
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var newValue = 'value to set in select input';
var dropDown = $('#id-of-select-input'); // or use class i.e, $('.class-of-select-input')
dropDown.val(newValue); // set the value of select input
dropDown.trigger('change'); // trigger to reflect change in select input
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$('#drpdownID').val('<option value="0">- Select -</option>').change().empty();