Structure means walls. There are tons of crazy things you can do 'outside' the walls with javascript. There are no checks if anything you reference exists or that your assignments are compatible. The browser's compiler will figure it all out and/or blow up at run-time.

Type and reference checking creates the walls your code must stay inside of otherwise there are compile errors. The compiler enforces a certain structure on your code that you must adhere to in order for the compilation to succeed.

Think of it like a coloring book where TypeScript keeps you inside the lines. Ever see a three year old's coloring book? :p