var str="abcdafgahiAbcAfgAhi";

var newstr = str.replace("a","o");  //obcdafgahiAbcAfgAhi (只替换一次)

newstr = str.replace(/a/g,"o");  //obcdofgohiAbcAfgAhi (g代表全局)

newstr = str.replace(/a/gi,"o");  //obcdofgohiobcofgohi (i代表忽略大小写)

js replace