// for older jquery
var var_name = $("input[@name='radio_name']:checked").val();

// for jquery 1.3
var var_name = $("input[name='radio_name']:checked").val();

Both of these versions work for me:

var var_name = $("input[@name=radio_name]:checked").val();

var var_name = $("input[@name='radio_name']:checked").val();

var var_name = $("input[@name=radio_name]:checked").val();

Radio button values using jquery

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