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

问题

This question already has answers here:

What's the point of new String(“x”) in JavaScript? (8 answers)

Closed 4 years ago.

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()?

回答1:

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"
  

来源：https://stackoverflow.com/questions/30984463/difference-between-string-primitive-and-string-wrapper-object-in-javascript