str.replace()不改变原str，例二为何加了return 就改变了原str

Question

例一function titleCase(str) {    var newArr = str.toLowerCase().split(" ");    newArr = newArr.map(function(val) {        val.replace(val[0],val[0].toUpperCase());        return val;    });        return newArr.join(" ");}titleCase("I'm a little tea pot");  //i'm a little tea pot例二function titleCase(str) {    var newArr = str.toLowerCase().split(" ");    newArr = newArr.map(function(val) {               return val.replace(val[0],val[0].toUpperCase());;    });        return newArr.join(" ");}titleCase("I'm a little tea pot");  //I`m A Little Tea Pot

Answer 1

return val.replace(val[0],val[0].toUpperCase());;

等于

var tmp = val.replace(val[0],val[0].toUpperCase());

return tmp;

并没有改变原来的str 只是将返回的字符串直接return了