How does eval() treat a string object differently from a primitive string value?

Question

I was reading in my Javascript book and it was talking about the difference between these two statements.

var s = "hello world"; // A primitive string value
var S = new String("hello world"); // A String object

I understand the difference but the book also mentioned (as like a little side note) that eval() will handle these differently but didn't mention how.

I tried looking around a google and couldn't find anything so I want to example.com and started messing around with it. I couldn't see a difference in the way that they are handled.

My question is: How does the eval() method treat these differently?

Answer 1

From the MDN:

String primitives and String objects also give different results when using eval. Primitives passed to eval are treated as source code; String objects are treated as all other objects are, by returning the object. For example:

s1 = "2 + 2";               // creates a string primitive
s2 = new String("2 + 2");   // creates a String object
console.log(eval(s1));      // returns the number 4
console.log(eval(s2));      // returns the string "2 + 2"

Answer 2

Consider this:

str = "alert('foo')";
obj = new String("alert('foo')");

eval(str); // produces an alert popup with 'foo' in it
eval(obj); // returns "alert('foo')" as a string


str = "arglebargle";
obj = new String('arglebargle');

eval(str); // reference error: arglebargle is not defined
eval(obj); // string: "arglebargle"