Clean Code in JavaScript

Primitive wrappers

You'll remember that we mentioned that primitive values don't have methods as they are not objects. So, how exactly are we able to call toUpperCase on the preceding string? Is that not a method? Yes, it is. And to allow us to access this method, JavaScript wraps primitive values in their respective wrapper objects at the time of property access. This occurs for all primitive values, apart from null and undefined.

Primitive values, in these moments of being wrapped, remain immutable but, via their wrapped instance, provide access to properties and methods. A string value would be wrapped in a String instance, while a number value would be wrapped in a Number instance. The same would occur for all other non-null and non-undefined ...