There are two answers to this: at runtime, there's nothing wrong with Javascript. That's why TypeScript compiles to Javascript. At design time, however Javascript doesn't have features of modern languages. For example, class-based programming. Some of these features are planned for in future Javascript releases, but as a language it's really behind other languages and will likely remain so. This makes large-scale Javascript programs more difficult to maintain.
Not all programming languages need classes and static types, and these features are not requirements for large-scale projects.
And there is plenty wrong with JavaScript at runtime. Lack of proper arrays is just one of the many things that come to mind. It's just a convenient runtime because it's good enough and ubiquitous.
> You can thank Microsoft for that. They refused ES4 spec.
Do not forget to thank Yahoo and Google as well.
"Yahoo, Microsoft, Google, and other 4th edition dissenters formed their own subcommittee to design a less ambitious update of ECMAScript 3, tentatively named ECMAScript 3.1"
There are two answers to this: at runtime, there's nothing wrong with Javascript. That's why TypeScript compiles to Javascript. At design time, however Javascript doesn't have features of modern languages. For example, class-based programming. Some of these features are planned for in future Javascript releases, but as a language it's really behind other languages and will likely remain so. This makes large-scale Javascript programs more difficult to maintain.