var str_word='麦当劳,可口可乐,百事可乐,可乐';
var str2_word;
str2_word=str_word.split(",");
rnd.today=new Date(); 
rnd.seed=rnd.today.getTime(); 
function rnd()
{ 
rnd.seed = (rnd.seed*9301+49297) % 233280; 
return rnd.seed/(233280.0); 
}; 
function rand(number)
{ 
return Math.ceil(rnd()*number); 
}; 
var i_word=rand(str2_word.length);
var j_word=i_word-1;
document.getElementById("abc").value=str2_word[j_word];