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Why is the 'in' operator throwing an error with a string literal instead of logging false?

As per MDN the in operator returns true if the property exists and accordingly the first example logs true. But when using a string literal, why is it throwing an error instead of logging false?
let let1 = new String('test');
console.log(let1.length);
console.log('length' in let1)
var let1 = 'test';
console.log(let1.length);
console.log('length' in let1);

In a sense it is a matter of timing. String literals do not have any properties. The reason that you can call methods and lookup properties on primitive strings is because JavaScript automatically wraps the string primitive in a String object when a method call or property lookup is attempted. JavaScript does not interpret the in operator as a method call or property lookup so it does not wrap the primitive in an object and you get an error (because a string primitive is not an object).
See Distinction between string primitives and String objects
Also, the same docs referenced in your question specifically note that using in on a string primitive will throw an error.
You must specify an object on the right side of the in operator. For
example, you can specify a string created with the String constructor,
but you cannot specify a string literal.

It throws an error because in is an operator for objects:
prop in object
but when you declare a string as `` (` string(template) literals) or "" '' (",' string literals) you don't create an object.
Check
typeof new String("x") ("object")
and
typeof `x` ("string").
Those are two different things in JavaScript.

JavaScript operator in only applicable to an Objects instances.
When you using constructor new String('abc') this will causing creating of a String object instance.
In other side, when you using only string literals or call function String('abc') without new it creates an string primitive. (like Number and Boolen)
Some behaviour of primitives and objects is differrent, look at this simple example's output:
console.log(typeof (new String('ddd'))) // "object"
console.log(typeof ('ddd')) // "string"
console.log(eval('1 + 2')) // 3
console.log(eval(new String('1 + 2'))) // {"0":"1","1":" ","2":"+","3":" ","4":"2"}
In code where you use methods on string primitives javascript engine automatically wraps primitives with corresponding objects to perform methods call.
But in it is not an method call, its language operator an in this case wrapping is not applied.
PS: Sorry for my english.

typeof('test') == string (string literal)
typof(new String('test')) == object (string object)
you can't use in with a string literal.
The in operator returns true if the specified property is in the specified object or its prototype chain.

The in operator can only be used to check if a property is in an
object. You can't search in strings, or in numbers, or other primitive
types.
The first example works and prints 'true' because length is a property of a string object.
The second example doesn't work and gives you an error because you are trying to look for a property length in something (a string) that is not an object.

What is the purpose of using function `String()` with function argument?

I'm currently reading 'Singe Page Web Applications' book. I encountered the following example:
// This is single pass encoder for html entities and handles
// an arbitrary number of characters
encodeHtml = function ( input_arg_str, exclude_amp) {
var input_str = String( input_arg_str), regex, lookup_map;
...
return input_str.replace(regex, function ( match, name ){
return lookup_map[ match ] || '';
});
};
I wonder, what is the purpose of using function String() with argument input_arg_str. I know that by using String() function I can convert different object to string, but I never met with such a feature using String().
I'm curious what you think about this and top thank you for your help.

#Amit Joki's answer is correct, of course, but there are several ways you could convert an object to a string, why use String(...)?
I'd guess the main reason here is that it safely handle's null and undefined whereas .toString would obviously fail.
String(undefined) // "undefined"
String(null) // "null"
In short, it's a more defensive way to convert an object to a string than .toString. Here's a note about it on MDN:
It's possible to use String as a "safer" toString alternative, as
although it still normally calls the underlying toString, it also
works for null and undefined.
I believe you get the same results with string concatenation:
var input_str = '' + input_arg_str; // also handles `null` and `undefined`

Can't say I've ever found a reason to use it, but MDN does suggest there are some subtle differences between string literals and string objects (emphasis mine):
Note that JavaScript distinguishes between String objects and primitive string values. (The same is true of Boolean and Numbers.)
String literals (denoted by double or single quotes) and strings returned from String calls in a non-constructor context (i.e., without using the new keyword) are primitive strings. JavaScript automatically converts primitives to String objects, so that it's possible to use String object methods for primitive strings. In contexts where a method is to be invoked on a primitive string or a property lookup occurs, JavaScript will automatically wrap the string primitive and call the method or perform the property lookup.
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 these reasons, code may break when it encounters String objects when it expects a primitive string instead, although generally authors need not worry about the distinction.
source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String
As others have said, in the context of the code you posted, it does ensure that whatever was passed as input_arg_str is converted to an actual string.

JavaScript and String as primitive value

In JavaScript a String is a primitive value.
But is also a String object...
A primitive value is a value put directly into a variable.
So my question is:
var d = "foo";
does d contain directly foo or a reference to a string object like other languages?
Thanks.

If I understand it correctly, d will contain the string literal "foo", and not a reference to an object. However, the JavaScript engine will effectively cast the literal to an instance of String when necessary, which is why you can call methods of String.prototype on string literals:
"some string".toUpperCase(); //Method of String.prototype
The following snippet from MDN may help to explain it further (emphasis added):
String literals (denoted by double or single quotes) and strings
returned from String calls in a non-constructor context (i.e., without
using the new keyword) are primitive strings. JavaScript automatically
converts primitives and String objects, so that it's possible to use
String object methods for primitive strings. In contexts where a
method is to be invoked on a primitive string or a property lookup
occurs, JavaScript will automatically wrap the string primitive and
call the method or perform the property lookup.
This is all explained in detail in the specification, but it's not exactly easy reading. I asked a related question recently (about why it is possible to do the above), so it might be worth reading the (very) detailed answer.

if you define
var d = "foo";
than d contains directly foo
but, if you define
var S = new String("foo");
then S is an Object
Example:
var s1 = "1";
var s2 = "1";
s1 == s2 -> true
var S1 = new String("2");
var S2 = new String("2");
S1 == S2 -> false

I think that every variable in Javascript actually represents an Object. Even a function is an Object.

I found two useful articles detailing this, located here and here. Seems like primitive types in JavaScript are passed by VALUE (i.e. when you pass if to a function it gets "sandboxed" within the function and the original variable's value won't change), while reference types are passed, you guessed it, by REFERENCE and passing it through to a function will change the original variable.
Primitive types in JavaScript are text (string), numeric (float / int), boolean and NULL (and the dreaded "undefined" type). Any custom objects, functions or standard arrays are considered reference types. I haven't researched the Date type though, but I'm sure it will fall into the primitive types.

Found this page about javascript variables, seems that:
Primitive type for javascript are booleans, numbers and text.

I believe there are no primitives in Javascript, in the Java sense at least - everything is an object of some kind.
So yes it is a reference to an object - if you extend the String object, d would have that extension.
If you mean primitives as in those types provided by the language, you've got a few, boolean, numbers, strings and dates are all defined by the language.

Are strings objects? [duplicate]

This question already has answers here:
How is a Javascript string not an object?
(2 answers)
Closed 7 years ago.
Here are two reasons to think strings are objects. First, you can create a string in the following way:
var mystring = new String("asdf");
I'm under the impression that the constructor function following the new operator has to return an object. Second, strings seem to have properties and methods. For example:
mystring.toUpperCase();
BUT, if strings were objects, then we'd expect something like the following to work:
function string_constructor() {
return "asdf";
}
var mystring = new string_constructor();
But it doesn't, and I've been told it doesn't because strings aren't objects. So are strings objects or not? And, either way, how can I make sense of everything I've listed?

Speaking about language types, Strings are values of the String type.
The language has five primitive types, which are String, Number, Boolean, Null and Undefined.
There are String objects (also for Number or Boolean), they are called primitive wrappers, they are created when you use the constructor function associated with them, for example:
typeof new String('foo'); // "object"
typeof 'foo'; // "string"
But don't get confused with them, you will rarely need to use primitive wrappers, because even if primitive values are not objects, you can still access their inherited properties, for example, on a string, you can access all members of String.prototype, e.g.:
'foo'.indexOf('o'); // 2
That's because the property accessor (the dot in this case) temporarily converts the primitive value to an object, for being able to resolve the indexOf property up in the prototype chain.
About the constructor function you have in your question, as you know, it won't return the string.
Functions called with the new operator return an implicit value, which is a new object that inherits from that function's prototype, for example:
function Test () {
// don't return anything (equivalent to returning undefined)
}
new Test() instanceof Test; // true, an object
If an object is returned from the constructor, that newly created object (this within the constructor) will be lost, so the explicit returned object will come out the function:
function Test2() {
return {foo: 'bar'};
}
new Test2().foo; // 'bar'
But in the case of primitive values, they are just ignored, and the new object from the constructor is implicitly returned (for more details check the [[Construct]] internal operation, (see step 9 and 10)).

In JavaScript, strings come in two flavors:
There is a String language type which contains values like "foo" and 'bar'. Those values are primitive values. Read about the String type here
Then there is a String constructor. (A constructor is a function object which is used to create new instances of a certain "class" (or pseudo-class)). So this: new String("foo")
will create a new object (a value of the type Object), which contains the primitive value "foo". Read about the String constructor here
In practice you don't use the new String('foo') notation, but the string literal notation 'foo'.
So to answer your question:
In JavaScript, strings are not objects. They are primitive values. However, there exist String objects which can be used to store string values, but those String objects are not used in practice.

Primitive strings behaves like objects in JavaScript because they are automatically converted to objects when you call an object method:
https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/String
String objects may be created by
calling the constructor new String().
The String object wraps JavaScript's
string primitive data type with the
methods described below. The global
function String() can also be called
without new in front to create a
primitive string. String literals in
JavaScript are primitive strings.
Because JavaScript automatically
converts between string primitives and
String objects, you can call any of
the methods of the String object on a
string primitive. JavaScript
automatically converts the string
primitive to a temporary String
object, calls the method, then
discards the temporary String object.
For example, you can use the
String.length property on a string
primitive created from a string
literal

String are both primitive and object type. Think about int, float, char ... which have real Object classes like Integer, Float and Char in Java.

String is a wrapper around the primitive string datatype. When you do something like var s = "My String" then a String wrapper object is created behind the scenes when needed..
It is interesting however that typeof s = 'string' instead of 'object'. Anybody know why that is?

Strings are objects, but what you are doing in your example is not creating a new String. By using 'new' you are creating an instance of a Object, not a 'string'
var F = function() {
return 'c';
};
var a = new String('a');
var b = 'b';
var c = new F();
alert("a is: " + typeof a + ", b is: " + typeof b + ", c is: " + typeof c);
// alerts: a is: object, b is: string. c is: object
You shouldn't use 'new' for strings regardless (or arrays, or 'simple' objects.)

How to use JSON to re-build the Javascript Object?

I have an object like this:
var someObj = Class.create ({
initialize: function(objName){
this.objName = objName;
}
});
I can use
o = new someObj("objName");
to make an obj. I can use Object.toJSON(o) to change the o to become a JSON String,
but I want the JSON String convert back to someObj, so, I use eval() to pass the
JSON String to become an object, but the question is, it can become a JS Obj,
but the constructor of "o" is not someObj. How can I eval the JSON String by using
"someObj" as the constructor?

JSON strings cannot represent objects with member functions, so the only thing you will get out of a JSON string is raw data. Assuming the toJSON method results in a JSON string representing an object with all the non-function members of your class instance, you should be able to take the resulting object and attach the prototype to get all the functions back. For example, using jQuery's handy extend function:
var o = new someObj("objName");
var json = Object.toJSON(o);
var json_obj = eval(json);
$.extend(json_obj, someObj.prototype);
json_obj.someMethodDefinedOnsomeObj()
Depending on how the framework you are using to represent classes in JavaScript makes use of the prototypal object model, your milage may very with the above example. Also, using eval() creates a security hole, so if you do not trust where the JSON string is coming from, you should use a different de-serialization method. Just for full coverage, here is how I did it with raw prototypes:
function Animal(name){
this.name = name;
}
Animal.prototype.talk = function(){
console.log("My name is "+this.name);
}
var a = new Animal("Brendan Eich");
a.talk();
var json = '{name: "Tim Berners-Lee"}'
var b = eval(b);
$.extend(b, Animal.prototype);
b.talk();
In a firebug console this produces the output:
My name is Brendan Eich
My name is Tim Berners-Lee

See JSON revivers at http://json.org/js.html
var myObject = JSON.parse(myJSONtext, reviver);
The optional reviver parameter is a
function that will be called for every
key and value at every level of the
final result. Each value will be
replaced by the result of the reviver
function. This can be used to reform
generic objects into instances of
pseudoclasses, or to transform date
strings into Date objects.

you're using prototypejs right? i've always found that tricky and ended up just making my own initializer that read in an object that had been evaled or the json string itself.