Is String an instance of the object String()?

Question

I am currently learning javascript and there is something that I don't understand.

//This means that I am using a method from the String.prototype
"ThisIsMyString".length

So, if I use ("ThisIsMyString" instanceof String) was suppose to return true, wasn't it? But turns out that returns false.. and I belive that is because the primitive type.

Here is my question: If "ThisIsMyString" is not an instance of String, how it can access property from that Object? What's the part of the puzzle that I don't know?

Check this http://stackoverflow.com/questions/17256182/javascript-string-literal-vs-string-object — PSL, Nov 12 '13 at 22:22
Be careful with terms: Javascript has no "primitive types", but it has "literals" (as most languages). They are not considered separate types however but language constructs that don't really fit the OO model. — Johannes H., Nov 12 '13 at 22:29
@JohannesH. Not quite... [§4.3.2 primitive value](http://www.ecma-international.org/ecma-262/5.1/#sec-4.3.2) — Jonathan Lonowski, Nov 12 '13 at 22:30
@JonathanLonowski: oh. ok, well then ;) I have to admit, I never read the spec, but most references only use the term literals, that'S why I was under the impression that it is the only term used here. Doesn't mka too much difference though ^^ — Johannes H., Nov 12 '13 at 22:31
@JohannesH. Yeah. "*Literals*" just refers to the syntax involved ([§7.8](http://www.ecma-international.org/ecma-262/5.1/#sec-7.8)), which most do create primitive values at runtime. — Jonathan Lonowski, Nov 12 '13 at 22:33

maček · Answer 1 · 2013-11-12T22:32:38.280

1

String.length is not

a method from the String.prototype

length is a property of String

See the MDN docs for String.length

To answer your question though, the reason "hello" instanceof String returns false lies within how instanceof actualy works

Object.getPrototypeOf("hello")
// TypeError: Object.getPrototypeOf called on non-object

However, this is how your string literal has access to these methods/properties

"my string".constructor === String
// true

"my string".__proto__ === String.prototype
// true

If you want an actual instance of String

var str = new String("hello");
str instanceof String
// true

edited Nov 12 '13 at 22:32

answered Nov 12 '13 at 22:20

maček

76,434
37
167
198

The facts that `"my string".constructor === String` and `"my string".__proto__ === String.prototype` are not the answer, but part of the same behaviour. The reason is the dot operator, which converts a string literal temporarily into a String object to perform the desired operation. – basilikum Nov 12 '13 at 23:34

Farmer Joe · Answer 2 · 2013-11-12T22:24:10.457

0

You are able to use such methods because the language recognizes and converts the primitive type to a temporary instance of the String object and returns the length property of that object.

edited Nov 12 '13 at 22:24

answered Nov 12 '13 at 22:18

Farmer Joe

6,020
1
30
40

Just in case you noticed: Downvoted your initialy response, as it just wasn't true. Better now ;) – Johannes H. Nov 12 '13 at 22:27
@JohannesH. haha no worries! You were more than right to down vote me before I fixed my silliness. – Farmer Joe Nov 12 '13 at 22:29

score 0 · Answer 3 · answered Nov 12 '13 at 22:24

Taken from http://oreilly.com/javascript/excerpts/learning-javascript/javascript-datatypes-variables.html: "JavaScript implicitly wraps the string primitive in a String object, processes the String object property or method call, and then discards the object"

Read further for details.

Is String an instance of the object String()?

3 Answers3