Typescript (an extension to JS that adds type-safety) essentially does option 3 as well. As the sibling noted, there are instances during runtime where the typ

Typescript (an extension to JS that adds type-safety) essentially does option 3 as well. As the sibling noted, there are instances during runtime where the type checking fails, especially because Typescript is compiling to Javascript. I have had moments where a numeric observable in KnockoutJS had to be forced to a number when receiving a new value; the form that the observable is bound to is simply returning a string, and while the TS interface for KnockoutJS says the observable is a number, the actual JS doesn't care and just returns what the form gives it. In some ways this is a failing of Typescript; it shouldn't allow strongly-type interfaces to be wrapped around JS that can return varying types.