Skill Maps

How do strings get methods?

You noticed that a primitive string has methods (toUpperCase), even though it's not an object. This is JavaScript " magic " called primitive wrappers.

1. The Wrapper Mechanism

When you access a primitive's property, the JS engine:

  1. Creates a temporary wrapper object (e.g., new String).
  2. Calls the method on this object.
  3. Immediately destroys the wrapper object.

2. Trying to add a property

let str = "Hello";
str.test = 5; // Temporary object created, property added...
alert(str.test); // undefined (object is already deleted, this is a NEW temporary object)
Caution

Never use constructors like new Number(1) or new String("a"). This creates a fully-fledged object, which breaks type logic: typeof (new Number(1)) will return "object", not "number".

Important

The null and undefined primitives don't have wrappers. Trying to call a method on them will lead to the most common JS error: Cannot read property ... of null.