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It is string comparison, although in terms of performance in JS string constant comparison is done by reference so its much faster. I believe this is closer to

by spion 7y ago

It is string comparison, although in terms of performance in JS string constant comparison is done by reference so its much faster. I believe this is closer to polymorphic variants than to actual sum types (global namespace like strings, comparison by name). You can tighten it a bit by switching to symbols instead of strings, which would guarantee unique tags, getting closer to classic variants. However TypeScript isn't big on nominal types, and for a good reason: the node module system for example can easily cause two versions of a module to be instantiated in the same program which in turn would cause two different and incompatible instances of the symbols and symbol types. Nevertheless I would still say that FP-like patterns are supported reasonably well.

crimsonalucard 7y ago
It may be string comparison on the JS level. However If it is string comparison on the TS level then that means it isn't type checked. Are you sure this is what you mean? That means I can add a new case for string "asdasfsdfds" and TS won't throw a type error. The variant type is not exclusively an FP paradigm.
- spion 7y ago
  If you hover over 'action.type' in the switch statement, you will see (property) type: "AddMovie" | "RemoveMovie" | "Reset" So you can't add a new case because while the value is a JS string, the type is a restricted subset of string i.e. a union of only 3 possible string constants. Therefore you are right - its not quite a string at the TS level - its a subset of string. p.s. you can make unions of any primitive values in TypeScript: let a:3 | "hello" | true = 3; a = "hello"; // valid a = true; // valid a = false; // not valid! or between any type and other values, really: let a: number | 'NotNumber' = 0; a = 1; // ok a = a - 1; // still ok, assignment narrowed the type to number a = 'NotNumber' // ok a = a - 1; // error! narrowed the type to the constant 'NotNumber', can't subtract anymore a = 'Hi' // ERROR TS is not always that smart about it (compared to Flow), but sometimes it works out quite well :)

by spion 7y ago

crimsonalucard 7y ago

It may be string comparison on the JS level. However If it is string comparison on the TS level then that means it isn't type checked. Are you sure this is what you mean? That means I can add a new case for string "asdasfsdfds" and TS won't throw a type error. The variant type is not exclusively an FP paradigm.

spion 7y ago
If you hover over 'action.type' in the switch statement, you will see (property) type: "AddMovie" | "RemoveMovie" | "Reset" So you can't add a new case because while the value is a JS string, the type is a restricted subset of string i.e. a union of only 3 possible string constants. Therefore you are right - its not quite a string at the TS level - its a subset of string. p.s. you can make unions of any primitive values in TypeScript: let a:3 | "hello" | true = 3; a = "hello"; // valid a = true; // valid a = false; // not valid! or between any type and other values, really: let a: number | 'NotNumber' = 0; a = 1; // ok a = a - 1; // still ok, assignment narrowed the type to number a = 'NotNumber' // ok a = a - 1; // error! narrowed the type to the constant 'NotNumber', can't subtract anymore a = 'Hi' // ERROR TS is not always that smart about it (compared to Flow), but sometimes it works out quite well :)