I haven't used Typescript either, but some of the differences are self evident to me. myFunction(x) can be called with any argument type; tooling can't detect an inappropriate type and output a warning/error(?), so you get to find certain mistakes the hard way; at runtime. "foo"+1 might make sense, but "foo"/1 is garbage, silently propagating 'undefined' up the stack in conventional Javascript, costing time and money for everyone. The user of myFunction(n: number | null) is provided with a meaningful signature during development, this saves time by providing precise information in a concise manner. Those of us that have spent time with statically typed languages and good tooling know what a blessing this is. The Typescript variant offers the possibility of a performance optimization; x+1 might compile down to a single instruction, as opposed to whatever runtime type check has to be invoked when the type of x isn't/can't be known at compile time. I don't know if that optimization is 'real' in practice given our contemporary javascript/typescript stack, but you can bet it will be eventually.