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

The string type value is immutable, but the String object, which is created by using the String() constructor, is mutable, because it is an object and you can add new properties to it.

> var str = new String("test")
undefined
> str
[String: 'test']
> str.newProp = "some value"
'some value'
> str
{ [String: 'test'] newProp: 'some value' }

Meanwhile, although you can add new properties, you can't change the already existing properties

A screenshot of a test in Chrome console

In conclusion, 1. all string type value (primitive type) is immutable. 2. The String object is mutable, but the string type value (primitive type) it contains is immutable.