syntax for javascript reverse function

Those functions don't modify the string; strings are immutable. The functions return new values.

So, a statement like

str.split('');

is a valid statement, but it has no net effect because the returned array is ignored. In the first example you quoted, the returned values are used as object contexts from which the subsequent functions are accessed and called. The return statement there returns the result of the last function call in the chain (the .join() call).

Try using var , if expected result is to re-define str , set using str = /*new value */

function reverseString(str) {
  var copy = str.split("");
  copy.reverse();
  str = copy.join("");
  return str;
}

console.log(reverseString("hello"))

Firstly, strings are immutable, you can create new required string, using methods like the first one to return value by operating/invoking methods.

The first function calls methods in chain. Meaning, the return value of one function (object/reference types only) to invoke its method to compute new result.

str.split('').reverse().join('');

Here, split returns array and array's reverse method reverses contents in array (index) and finally join method joins the array elements with ' ' as separator.

whereas in second function its just a sequential call of statements. I guess in str.join(' '); there is no function called join in string prototype.

The method calls are chained so that each method uses the return value from the previous. You can do the same in separate statements if you keep the return value so that you can use it in the next statement:

function reverseString(str) {
  var arr = str.split('');
  arr = arr.reverse();
  var result = arr.join('');
  return result;
}

console.log(reverseString("hello"));

Note also that there is a difference between split('') and split(' '). The first one splits between each character while the second one splits at space characters.