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> Typescript is designed to accept any valid Javascript program That's not strictly true though. `a<b,c>(d)` is valid Javascript but Typescript treats it as a

• qayxc 4y ago
  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 4y ago
    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.
• thoughtspile 4y ago
  Lol, this is brilliant!
• oblosys 4y ago
  With these declarations your fragment is valid in both JavaScript and TypeScript: class b {} class c {} const d = JSON.parse const a = Promise.prototype.then.bind(Promise.resolve(1)) console.log(a<b,c>(d)) // TS output: Promise { <state>: "pending" } // JS output: false false TS playground: https://tsplay.dev/mAdeZN https://tsplay.dev/mAdeZN
• sirmarksalot 4y ago
  I guess this is a distinction between intent and results. Typescript is intended to be a strict superset of Javascript. In practice, there are edge cases like these.