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