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

Question

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);

Answer 1

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.

Answer 2

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.