Immutable

An immutable value is one whose content cannot be changed without creating an entirely new value, in comparison with mutable values.

In JavaScript, primitive values are immutable — once a primitive value is created, it cannot be changed, although the variable that holds it may be reassigned another value. By contrast, objects and arrays are mutable by default — their properties and elements can be changed without reassigning a new value.

It can be beneficial to use immutable objects for several reasons:

To improve performance (no planning for the object’s future changes) To reduce memory use (make object references instead of cloning the whole object) Thread-safety (multiple threads can reference the same object without interfering with one other) Lower developer mental burden (the object’s state won’t change and its behavior is always consistent)