New to javascript, how to write reverse iteration?

There's no need to split this string into an array. Just use the charAt() function and a simple for loop.

var name = window.prompt("Enter name:");
var reverse = "";
for (var i = name.length - 1; i >=0; i--) {
    reverse += name.charAt(i);
}
console.log(reverse)

Instead of converting the string to an array first, you're just reading the characters out of the string directly.

You can accomplish this by iterating only half the number of characters.

DEMO: http://jsfiddle.net/vgG2P/

CODE:

var name = "Bob Dylan".split("");

//                    The counters will meet in the middle.
//                    --------------+----------------------
//      first char   last char      |   inc  dec
// -------v-------------v-----------v----v----v
for(var i = 0, j = name.length-1; i < j; i++, j--) {

    var temp = name[i];  // Store the `i` char
    name[i] = name[j];   // Replace the `i` char with the `j` char
    name[j] = temp;      // Replace the `j` char with the `i` char we stored

}

console.log(name.join(""));  "nalyD boB"

EXPLANATION:

What we did was split the characters into an Array, and maintain two counters, one that increments from the first character at 0, and the other that decrements from the last character at .length - 1. Then simply swap the characters.

The iteration continues while your incrementing counter is less than your decrementing counter. Since they will meet in the middle, you'll end up incrementing only half the total length.

We can also build the halves of the result without using an Array:

DEMO: http://jsfiddle.net/vgG2P/1/

var name = "Bob Dylan";

var start = "", end = ""

for(var i = 0, j = name.length-1; i < j; i++, j--) {
    end = name.charAt(i) + end
    start += name.charAt(j)
}

if (i === j)
    start += name.charAt(i)

console.log(start + end);  "nalyD boB"

I'm assuming that your professor would not be asking you how to reverse a string if he hasn't yet introduced you to the concept of arrays and loops. Basically, a string like John Smith is just an array of characters like this:

0123456789
John Smith

Again, thinking in the sense that a string is just an array of characters, you have have 10 characters that need to be reversed. So how do you go about doing this? Well, you basically need to take the last character h from the "array" you're given and make it the first character in a new "array" you're going to create. Here's an example:

var known = 'John Smith';
var reversed = ''; // I'm making an empty string aka character array
var last = known.length - 1 // This is the index of the last character

for (var i = 0; i < known.length; i++)
{
  temp += known[last - i];
}

(You can see it working here)

So what's happening?

• We're looping over known starting at 0 and ending at 9 (from the first character to the last)
• During each iteration, i is incrementing from 0 - 9
• last always has a value of 9
• last - i will give us indexes in reverse order (9, 8, 7, ..., 0)
• So, when i is 0, last - i is 9 and known[9] is "h"; repeat this process and you get the reversed string

Hopefully this helps explain a little better what's happening when you call reverse() on an array.

(1) A more straight forward way without built-in functions:

function reverse(str) {
    let reversed_string = "";
    for (let i = str.length - 1; i >= 0; i--) {
        reversed_string += str[i];
    }
    return reversed_string;
}

(2) Using ES2015 'for' helper function:

function reverse(str) {
    let reversed_string = "";
    for (let character of str) {
        reversed_string = character + reversed_string;
    }
    return reversed_string;
}

(3) Using ES6 syntax and ES5.1 reduce():

function reverse(str) {
    return str.split('').reduce((reversed, char) => char + reversed, '');
}

// reduce takes in 2 arguments. (1) arrow function, and (2) empty string.

Chances are, for an interview, that you will not able to use the built-in functions, especially for "reverse()".