For-in Loop VS In-operator

I consider myself a JS veteran but just now for the first time I have realised that the for ... in loop does something very different from the in operator: 'length' in []; // true

Solution 1:

So this brings me to my question: why is the in operator at all present in the for ... in loop?

It isn't. The in token in the for … in loop construct is not an operator, just as much it is not one in the expression

x = {in: 5}.in

It's just one of the tokens that distinguishes a for ( … in … ) from a for (…; …; …) loop. Given that there is no for (…) statement, the role of the in token in a relational expression never collides with this.