为什么一种代码有效，而另一种无效？

Question

The question I am answering: Complete the solution so that it reverses the string value passed into it.

I answered this:

function solution(str){
 str.split("").reverse().join("");
 return(str);
}

but it didnt work

Instead, this was the correct solution:

function solution(str){
  return str.split("").reverse().join("");
}

why is it that the second solution works but not the first?

Answer 1

In the first one, the variable str doesn't get redefined, so you end up returning the original value. You would need to do this:

function solution(str){
 str = str.split("").reverse().join("");
 return(str);
}

Answer 2

Strings are immutable. Any method that "alters" a String is really returning the new version of the string for you to use.

In the first bit, split , reverse and join are all taking a string, and returning a new string . They don't modify str . You're having those functions do work, throwing away the result of the functions, then returning the original str .

The second version works because join returns a new joined String, then that new string is given to return to be returned.