Difference between string primitive and string wrapper object in Javascript [duplicate]

Question

I'm very confused about what the wrapper objects for primitives. For example, a string primitive and a string created with the string wrapper object.

var a = "aaaa";
var b = new String("bbbb");
console.log(a.toUpperCase()); // AAAA
console.log(b.toUpperCase()); // BBBB
console.log(typeof a);        // string
console.log(typeof b);        // object

Both give access to String.prototype methods, and seem to act just like a string literal. But one is not a string, it's an object. What is the practical difference between a and b? Why would I create a string using new String()?

There's no real use case actually. The practical difference is that the string wrapper acts as an object (and for example can have own properties). — Bergi
– Bergi, Commented Jun 22, 2015 at 15:55

Oriol · Accepted Answer · 2015-06-22 16:03:13Z

2

A primitive string is not an object. An object string is an object.

Basically, that means:

Object strings are compared by reference, not by the string they contain

"aaa" === "aaa";                         // true
new String("aaa") === new String("aaa"); // false

Object strings can store properties.

function addProperty(o) {
  o.foo = 'bar'; // Set a property
  return o.foo;  // Retrieve the value
}
addProperty("aaa");             // undefined
addProperty(new String("aaa")); // "bar"

edited Jun 22, 2015 at 16:03

answered Jun 22, 2015 at 15:56

Oriol

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