Question

One can use typeof to determine whether a value is primitive or boxed.

Consider:

typeof "foo"; // "string"
typeof new String("foo"); // "object"

In combination with Object.prototype.toString we could define the following two functions

var toString = Object.prototype.toString;

var is_primitive_string = function(s) {
  return toString.call(s) === "[object String]" && typeof s === "string";
};

var is_boxed_string = function(s) {
  return toString.call(s) === "[object String]" && typeof s === "object";
};

Are there any use cases for these two functions? (Or similar functions for Number, Boolean, etc).

The concept behind this question came from the following Comment by T.J.Crowder.

Should we ever care whether a value we have is primitive or boxed?

Answer 1

I’d say there’s virtually no point, you almost never care whether you’re dealing with a string primitive or a String object.

There are edge cases. For instance, a String object is an actual object, you can add properties to it. This lets you do things like this:

function test(arg) {
    arg.foo = "bar";
}

If calling code passes in a string primitive:

var s1 = "str";
test(s1);

…arg gets promoted to a String object and gets a property added to it, but that String object isn’t used by anything after test returns.

In contrast, if calling code passes in a String object:

var s2 = new String("str");
test(s2);

…then the property is added to that object and the calling code can see it. Consider (live copy):

var s1, s2;

s1 = "str";

display("[Before] typeof s1.foo = " + typeof s1.foo);
test(s1);
display("[After] typeof s1.foo = " + typeof s1.foo);

s2 = new String("str");

display("[Before] typeof s2.foo = " + typeof s2.foo);
test(s2);
display("[After] typeof s2.foo = " + typeof s2.foo);

function test(arg) {
  arg.foo = "bar";
}

Output:

[Before] typeof s1.foo = undefined
[After] typeof s1.foo = undefined
[Before] typeof s2.foo = undefined
[After] typeof s2.foo = string

Note that s2.foo is a string, but s1.foo isn’t (because s1 was a string primitive, the object created when we promoted it in test has nothing to do with the calling code).

Is there any use case for this? Dunno. I’d say it would be an extremely edgy edge case if so.