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Incrementally concatenating strings means a lot of intermediate strings get allocated only to be thrown away. The result is O(n**2) in both space and time. See this post about “Shlemiel the painter’s algorithm” for a good description about the quadratic behavior involved:

Storing each fragment in an array and joining them all together in the end at least avoids allocating ever-longer intermediate strings. I have not run any benchmarks on very long strings,
though. That might still be interesting to do.

Closes #12.

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Esrever Build status Code coverage status Dependency status

Esrever is a Unicode-aware string reverser written in JavaScript. It allows you to easily reverse any string of Unicode symbols, while handling combining marks and astral symbols just fine. Here’s an online demo.

Why not just use string.split('').reverse().join('')?

The following code snippet is commonly used to reverse a string in JavaScript:

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

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.

But there’s more:

naiveReverse('mañana mañana');
// → 'anãnam anañam'
// Wait, so now the tilde is applied to the `a` instead of the `n`? WAT.

In order to correctly reverse any given string, Esrever implements an algorithm that was originally developed by Missy ‘Misdemeanor’ Elliot in September 2002:

I put my thang down, flip it, and reverse it. I put my thang down, flip it, and reverse it.

And indeed: by swapping the position of any combining marks with the symbol they belong to, as well as reversing any surrogate pairs before further processing the string, the above issues are avoided successfully. Thanks, Missy!


Via npm:

npm install esrever

Via Bower:

bower install esrever

Via Component:

component install mathiasbynens/esrever

In a browser:

<script src="esrever.js"></script>

In Narwhal, Node.js, and RingoJS:

var esrever = require('esrever');

In Rhino:


Using an AMD loader like RequireJS:

    'paths': {
      'esrever': 'path/to/esrever'
  function(esrever) {



A string representing the semantic version number.


This function takes a string and returns the reversed version of that string, correctly accounting for Unicode combining marks and astral symbols.

Usage example

var input = 'Lorem ipsum 𝌆 dolor sit ameͨ͆t.';
var reversed = esrever.reverse(input);

// → '.teͨ͆ma tis rolod 𝌆 muspi meroL'

esrever.reverse(reversed) == input;
// → true

Using the esrever binary

To use the esrever binary in your shell, simply install Esrever globally using npm:

npm install -g esrever

After that you will be able to reverse strings from the command line:

$ esrever 'I put my thang down, flip it, and reverse it.'
.ti esrever dna ,ti pilf ,nwod gnaht ym tup I

$ esrever 'H̹̙̦̮͉̩̗̗ͧ̇̏̊̾Eͨ͆͒̆ͮ̃͏̷̮̣̫̤̣ ̵̞̹̻̀̉̓ͬ͑͡ͅCͯ̂͐͏̨̛͔̦̟͈̻O̜͎͍͙͚̬̝̣̽ͮ͐͗̀ͤ̍̀͢M̴̡̲̭͍͇̼̟̯̦̉̒͠Ḛ̛̙̞̪̗ͥͤͩ̾͑̔͐ͅṮ̴̷̷̗̼͍̿̿̓̽͐H̙̙̔̄͜'
H̙̙̔̄͜Ṯ̴̷̷̗̼͍̿̿̓̽͐Ḛ̛̙̞̪̗ͥͤͩ̾͑̔͐ͅM̴̡̲̭͍͇̼̟̯̦̉̒͠O̜͎͍͙͚̬̝̣̽ͮ͐͗̀ͤ̍̀͢Cͯ̂͐͏̨̛͔̦̟͈̻ ̵̞̹̻̀̉̓ͬ͑͡ͅEͨ͆͒̆ͮ̃͏̷̮̣̫̤̣H̹̙̦̮͉̩̗̗ͧ̇̏̊̾

$ cat foo.txt
These are the contents of `foo.txt`.
This is line two.

$ esrever -f foo.txt
.owt enil si sihT
.`txt.oof` fo stnetnoc eht era esehT

$ esrever -l foo.txt
.`txt.oof` fo stnetnoc eht era esehT
.owt enil si sihT

Why not just use the good old rev command instead? Glad you asked. rev doesn’t account for Unicode combining marks:

$ rev <<< 'mañana mañana'
anãnam anañam

On the other hand, the esrever binary returns the expected result:

$ esrever 'mañana mañana'
anañam anañam

See esrever --help for the full list of options.


Esrever has been tested in at least Chrome 27-29, Firefox 3-22, Safari 4-6, Opera 10-12, IE 6-10, Node.js v0.10.0, io.js v1.0.0, Narwhal 0.3.2, RingoJS 0.8-0.11, PhantomJS 1.9.0, and Rhino 1.7RC4.

Unit tests & code coverage

After cloning this repository, run npm install to install the dependencies needed for Esrever development and testing. You may want to install Istanbul globally using npm install istanbul -g.

Once that’s done, you can run the unit tests in Node using npm test or node tests/tests.js. To run the tests in Rhino, Ringo, Narwhal, and web browsers as well, use grunt test.

To generate the code coverage report, use grunt cover.


Mathias Bynens


Esrever is available under the MIT license.


A Unicode-aware string reverser written in JavaScript.







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