TypeScript lets you annotate that a parameter will only ever be a number, but the JIT can find that out for itself by seeing that it’s only ever a number when it runs the program and a number is all you pass to it, using stamps and class profiling. The JIT can also see types beyond that, like that it’s only ever a number and it’s always below 0xff, using stamps and value profiling. The JIT has to guard that the type is as expected, yes, but so would the static compiler in order to do the more specific types.

Lets say you've built a house in lego. But the computer do not know it's actually a house. Something called a compiler has to take apart the lego house, then package the parts in groups of 8,16,32 or 64 (dissembles the house into stacks with 8 lego bit's in each). Then the compILer have to tell the compUter what to do with the stacks, and what each stack is. For example take this stack with red pieces and this stack with green pieces and build a wall, then take these bits, and combine with these bits to build a roof, etc. The computer is dumb, but it's very fast.