Are JavaScript strings immutable? Do I need a “string builder” in JavaScript?

Question

Does javascript use immutable or mutable strings? Do I need a \"string builder\"?

Answer 1

They are immutable. You cannot change a character within a string with something like var myString = "abbdef"; myString[2] = 'c'. The string manipulation methods such as trim, slice return new strings.

In the same way, if you have two references to the same string, modifying one doesn't affect the other

let a = b = "hello";
a = a + " world";
// b is not affected

However, I've always heard what Ash mentioned in his answer (that using Array.join is faster for concatenation) so I wanted to test out the different methods of concatenating strings and abstracting the fastest way into a StringBuilder. I wrote some tests to see if this is true (it isn't!).

This was what I believed would be the fastest way, though I kept thinking that adding a method call may make it slower...

function StringBuilder() {
    this._array = [];
    this._index = 0;
}

StringBuilder.prototype.append = function (str) {
    this._array[this._index] = str;
    this._index++;
}

StringBuilder.prototype.toString = function () {
    return this._array.join('');
}

Here are performance speed tests. All three of them create a gigantic string made up of concatenating "Hello diggity dog" one hundred thousand times into an empty string.

I've created three types of tests

• Using Array.push and Array.join
• Using Array indexing to avoid Array.push, then using Array.join
• Straight string concatenation

Then I created the same three tests by abstracting them into StringBuilderConcat, StringBuilderArrayPush and StringBuilderArrayIndex http://jsperf.com/string-concat-without-sringbuilder/5 Please go there and run tests so we can get a nice sample. Note that I fixed a small bug, so the data for the tests got wiped, I will update the table once there's enough performance data. Go to http://jsperf.com/string-concat-without-sringbuilder/5 for the old data table.

Here are some numbers (Latest update in Ma5rch 2018), if you don't want to follow the link. The number on each test is in 1000 operations/second (higher is better)

| Browser          | Index | Push | Concat | SBIndex | SBPush | SBConcat |
---------------------------------------------------------------------------
| Chrome 71.0.3578 | 988   | 1006 | 2902   | 963     | 1008   | 2902     |
| Firefox 65       | 1979  | 1902 | 2197   | 1917    | 1873   | 1953     |
| Edge             | 593   | 373  | 952    | 361     | 415    | 444      |
| Exploder 11      | 655   | 532  | 761    | 537     | 567    | 387      |
| Opera 58.0.3135  | 1135  | 1200 | 4357   | 1137    | 1188   | 4294     | 

Findings

• Nowadays, all evergreen browsers handle string concatenation well. Array.join only helps IE 11

• Overall, Opera is fastest, 4 times as fast as Array.join

• Firefox is second and Array.join is only slightly slower in FF but considerably slower (3x) in Chrome.

• Chrome is third but string concat is 3 times faster than Array.join

• Creating a StringBuilder seems to not affect perfomance too much.

Hope somebody else finds this useful

Different Test Case

Since @RoyTinker thought that my test was flawed, I created a new case that doesn't create a big string by concatenating the same string, it uses a different character for each iteration. String concatenation still seemed faster or just as fast. Let's get those tests running.

I suggest everybody should keep thinking of other ways to test this, and feel free to add new links to different test cases below.

http://jsperf.com/string-concat-without-sringbuilder/7