[qayxc]

It is strictly true. a<b,c>(d) is only valid JavaScript iff a, b, c, and d are values. The TypeScript expression a<b,c>(d) where b and c are types is NOT JavaScript, since JavaScript doesn't have type expressions. The statement that every JavaScript program is a valid TypeScript program does not imply the opposite - hence strict superset.

edit: there is indeed an edge case with the parentheses that throws the TypeScript parser off even if only values are involved.

[lhorie]

See the sibling comment from oblosys for an example where the exact same code can give different results in vanilla JS vs after a tsc pass. As you can see there, the problem is that an identifier can simultaneously represent a value and a type.

This is different than JSX or hashbang, where the set of non-JS syntax cannot legally overlap with existing syntax/semantics.