As long as you’re dealing with simple ASCII characters, and you’re happy to use built-in functions, this will work:
function reverse(s){
return s.split(“”).reverse().join(“”);
}
If you need a solution that supports UTF-16 or other multi-byte characters, be aware that this function will give invalid unicode strings, or valid strings that look funny. You might want to consider this answer instead.
[…s] is Unicode aware, a small edit gives:-
function reverse(s){
return […s].reverse().join(“”);
}
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’
As for the “in-place” part, see the other answers.