Without it you frequently run into objects passed to you after going through 10k lines of code and it becomes a wicked game to figure out which properties the object actually has at that point.
JavaScript, even ES6, is IMO crap for large codebases.
Does this object have the property I'm looking for? Did somebody overwrite it with the wrong type 5000 lines ago? Why is the prototype chain broken? Where's my damn inline documentation.
My company has abandoned JavaScript wholesale with great relief. If it was a physical object we would have tossed it off the balcony.