String immutability in Javascript & Strings in Salesforce Commerce Cloud

Bhuman Soni
Bhuman Soni January 6, 2021
Updated 2021/01/27 at 8:30 AM

Like the previous post on this blog, this post focuses on Salesforce Commerce Cloud. If you would like to get a background on Salesforce Commerce Cloud I suggest you read my post on How to sort a list – Salesforce Commerce Cloud. More specifically this post talks about String immutability in Javascript, focuses on how to build a large string in Salesforce Commerce Cloud using a class similar to a StringBuilder in Java.

String immutability

What constitutes to being mutable? If you look at the literal meaning of being mutable, it means something that can change, hence by that logic immutable would mean it cannot change. So when we say that a string is immutable we can think of the literal meaning and determine that it cannot change i.e. we cannot re-assign values to a string without re-initialising it. Makes sense doesn’t it?

String immutability in Javascript

An immutable String in Javascript would mean once a String variable is initialised, it cannot be re-initialised. I mean if you try to concatenate another string to it, the concatenated String will be a new String i.e. stored in a different block of memory than the original string. Ok, may be let’s have a look at some code to understand it better

let name = "Bhuman"; //first name only
name += "Soni"; //append last name to it
//now let's append an address to it
name += "\n"; //append a new line
name += "42 Wallaby way Sydney"; //add an address to it
/*should print 'Bhuman Soni
                42 Wallaby way Sydney' 
to console*/

The above can be explained as follows

  • Initialise a variable name with value “Bhuman”
  • Concatenate the last name ‘Soni’ to the variable name which now points to a newly created space in memory, while marking the old memory space for garbage collection
  • Concatenate a new line character to name hence re-initialising it and pointing to a new memory space
  • Lastly, concatenate the address string to the name variable, again re-initialising it and pointing to a new memory space

You can also read an article on MDN to understand this concept.

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The problem

You see the problem above? Re-initialising the string every time we append a value can result in bad performance when we doing it for a several strings. Example: say we are building a large string as we read/process a large csv file, we have to re-initialise the string every time. Therefore finding a new empty memory block every time and pointing to it. Over time this would end up being a very expensive process, hence not very efficient. So how do we solve it?

Salesforce Commerce Cloud StringWriter

In Salesforce Commerce Cloud, we can solve it using the StringWriter class. You can know more about it in the official documentation

It’s a JSP so you cannot bookmark the class, you would just have to enter StringWriter in the search box. Have a look at the screenshot below to see how

Salesforce B2C Documentation

Sample code

Here’s the code on how to use the StringWriter

var StringWriter = require("dw/io/StringWriter"); 
var nameAndAdd = new StringWriter(); // create a variable
nameAndAdd.write("Soni"); // append the last name
nameAndAdd.write("\n"); // append a new line
nameAndAdd.write("42 Wallaby way Sydney"); // append address
//now let's get the final string
var finalString = nameAndAdd.toString();
/*should have 'Bhuman Soni
                42 Wallaby way Sydney' 
to console*/

You see? This is very similar to Java’s StringBuilder or StringBuffer class. Of course they both do the same thing in Java except StringBuffer is thread-safe and StringBuilder isn’t.


In this post we learned a little bit what immutability means with respect to Strings in Javascript. Then we had a look at the code samples of how we can build a large string in Javascript without re-initialising it every time. This post shows examples of how to build a large string in Salesforce Commerce Cloud using a StringWriter. There is also a reference to Java documentation for StringBuilder and StringBuffer classes in this post.

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