Reversing a string in JavaScript

Question

I'm trying to reverse an input string

var oneway = document.getElementById('input_field').value(); var backway = oneway.reverse(); 

but firebug is telling me that oneway.reverse() is not a function. Any ideas?

Thank you

Answer 1

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.

Answer 2

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'