JavaScript Patterns 3.7 Primitive Wrappers

Primitive value types: number, string, boolean, null, and undefined.

// a primitive number

var n = 100;

console.log(typeof n); // "number" 


// a Number object

var nobj = new Number(100);

console.log(typeof nobj); // "object" 

One reason to use the wrapper objects is when you want to augment the value and persist state.  Because primitives are not objects, they  cannot be augmented with properties. 

// primitive string

var greet = "Hello there";
 

// primitive is converted to an object

// in order to use the split() method

greet.split(' ')[0]; // "Hello"
 

// attemting to augment a primitive is not an error

greet.smile = true;
 

// but it doesn't actually work

typeof greet.smile; // "undefined" 

When used without new, wrapper constructors convert the argument passed to them to a primitive value:

typeof Number(1); // "number"

typeof Number("1"); // "number"

typeof Number(new Number()); // "number"

typeof String(1); // "string"

typeof Boolean(1); // "boolean"