Are JavaScript Strings Immutable? Do I Need a "String Builder" in JavaScript

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

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)

var myObject = new String('my value');
var myPrimitive = 'my value';

function myFunc(x) {
  x.mutation = 'my other value';
}

myFunc(myObject);
myFunc(myPrimitive);

console.log('myObject.mutation:', myObject.mutation);
console.log('myPrimitive.mutation:', myPrimitive.mutation);

myObject.mutation: my other value
myPrimitive.mutation: undefined

String.prototype.replaceAt=function(index, replacement) {
    return this.substr(0, index) + replacement+ this.substr(index + replacement.length);
}

var hello="Hello World"
hello = hello.replaceAt(2, "!!")) //should display He!!o World

let x = 2;
x += 2;
// The above line is equivalent to the following.
x = x + 2;

let str = "hello";
str += "world"; // is the same as 
str = str + "world";