Reversing a string in JavaScript

String.prototype.reverse = function () {
    return this.split("").reverse().join("");
}

Inspired by the first result I got when I did a Google for javascript string reverse.

reverse() is a method of array instances. It won't directly work on a string. You should first split the characters of the string into an array, reverse the array and then join back into a string:

var backway = oneway.split("").reverse().join("");

Update

The method above is only safe for "regular" strings. Please see comment by Mathias Bynens below and also his answer for a safe reverse method.

The following technique (or similar) is commonly used to reverse a string in JavaScript:

// Don’t use this!
var naiveReverse = function(string) {
    return string.split('').reverse().join('');
}

In fact, all the answers posted so far are a variation of this pattern. However, there are some problems with this solution. For example:

naiveReverse('foo 𝌆 bar');
// → 'rab �� oof'
// Where did the `𝌆` symbol go? Whoops!

If you’re wondering why this happens, read up on JavaScript’s internal character encoding. (TL;DR: 𝌆 is an astral symbol, and JavaScript exposes it as two separate code units.)

But there’s more:

// To see which symbols are being used here, check:
// http://mothereff.in/js-escapes#1ma%C3%B1ana%20man%CC%83ana
naiveReverse('mañana mañana');
// → 'anãnam anañam'
// Wait, so now the tilde is applied to the `a` instead of the `n`? WAT.

A good string to test string reverse implementations is the following:

'foo 𝌆 bar mañana mañana'

Why? Because it contains an astral symbol (𝌆) (which are represented by surrogate pairs in JavaScript) and a combining mark (the ñ in the last mañana actually consists of two symbols: U+006E LATIN SMALL LETTER N and U+0303 COMBINING TILDE).

The order in which surrogate pairs appear cannot be reversed, else the astral symbol won’t show up anymore in the ‘reversed’ string. That’s why you saw those �� marks in the output for the previous example.

Combining marks always get applied to the previous symbol, so you have to treat both the main symbol (U+006E LATIN SMALL LETTER N) as the combining mark (U+0303 COMBINING TILDE) as a whole. Reversing their order will cause the combining mark to be paired with another symbol in the string. That’s why the example output had ã instead of ñ.

Hopefully, this explains why all the answers posted so far are wrong.

---

To answer your initial question — how to [properly] reverse a string in JavaScript —, I’ve written a small JavaScript library that is capable of Unicode-aware string reversal. It doesn’t have any of the issues I just mentioned. The library is called Esrever; its code is on GitHub, and it works in pretty much any JavaScript environment. It comes with a shell utility/binary, so you can easily reverse strings from your terminal if you want.

var input = 'foo 𝌆 bar mañana mañana';
esrever.reverse(input);
// → 'anañam anañam rab 𝌆 oof'