Otdfbt

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.