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

from the rhino book:

In JavaScript, strings are immutable objects, which means that the characters within them may not be changed and that any operations on strings actually create new strings. Strings are assigned by reference, not by value. In general, when an object is assigned by reference, a change made to the object through one reference will be visible through all other references to the object. Because strings cannot be changed, however, you can have multiple references to a string object and not worry that the string value will change without your knowing it

Answer 2

Regarding your question (in your comment to Ash's response) about the StringBuilder in ASP.NET Ajax the experts seem to disagree on this one.

Christian Wenz says in his book Programming ASP.NET AJAX (O'Reilly) that "this approach does not have any measurable effect on memory (in fact, the implementation seems to be a tick slower than the standard approach)."

On the other hand Gallo et al say in their book ASP.NET AJAX in Action (Manning) that "When the number of strings to concatenate is larger, the string builder becomes an essential object to avoid huge performance drops."

I guess you'd need to do your own benchmarking and results might differ between browsers, too. However, even if it doesn't improve performance it might still be considered "useful" for programmers who are used to coding with StringBuilders in languages like C# or Java.

Answer 3

Strings are immutable – they cannot change, we can only ever make new strings.

Example:

var str= "Immutable value"; // it is immutable

var other= statement.slice(2, 10); // new string

Answer 4

JavaScript strings are indeed immutable.

Answer 5

It's a late post, but I didn't find a good book quote among the answers.

Here's a definite except from a reliable book:

Strings are immutable in ECMAScript, meaning that once they are created, their values cannot change. To change the string held by a variable, the original string must be destroyed and the variable filled with another string containing a new value... —Professional JavaScript for Web Developers, 3rd Ed., p.43

Now, the answer which quotes Rhino book's excerpt is right about string immutability but wrong saying "Strings are assigned by reference, not by value." (probably they originally meant to put the words an opposite way).

The "reference/value" misconception is clarified in the "Professional JavaScript", chapter named "Primitive and Reference values":

The five primitive types...[are]: Undefined, Null, Boolean, Number, and String. These variables are said to be accessed by value, because you are manipulating the actual value stored in the variable. —Professional JavaScript for Web Developers, 3rd Ed., p.85

that's opposed to objects:

When you manipulate an object, you’re really working on a reference to that object rather than the actual object itself. For this reason, such values are said to be accessed by reference.—Professional JavaScript for Web Developers, 3rd Ed., p.85

Answer 6

Strings in Javascript are immutable