2 replies

Inferred Type Parameter from a Fixed Union is Different from a Parameter that is a Fixed Union

So this took me embarrassingly long to figure out how to articulate, hoping it saves someone a lot of time

export const FixedInferredType = <T extends QueryRoutes>(name: T) =>

export const FixedInferredType = <T extends QueryRoutes>(name: T) =>

export const ParameterAsUnion = (name: QueryRoutes) =>

export const ParameterAsUnion = (name: QueryRoutes) =>

These are very meaningfully different in that if you pass in a name to the second one it doesn't narrow the return type at all, you'll get a type error if you don't explicitly type guard the possibilities
(even if all the inputs you are pulling from are strongly typed and would not error)

Even though with both you'll get an error if you pass in a random value

Is this obvious advice that exists somewhere? trying to figure out how to explain this to my team or if there is just some resource i'm missing.

I guess if you're building types for DX like I am, then you should be inferring or else you're not going to get the narrowing you really want.

---

The Copilot output that made it click
It makes sense to me and explains why I couldn't get the specificity in places where I really wanted it.

The fundamental difference between these two lies in how TypeScript infers types:

export const FixedInferredType = <T extends QueryRoutes>(name: T) =>: This is a generic function. The type T is inferred based on the argument passed to the function. If you call FixedInferredType("getBusinesses"), TypeScript infers that T is "getBusinesses", not just any member of QueryRoutes. This allows for more precise type checking and intellisense within the function body.

export const ParameterAsUnion = (name: QueryRoutes) =>: This is a non-generic function. The type of name is QueryRoutes, which is a union of several string literals. TypeScript only knows that name is some member of QueryRoutes, but it doesn't know which specific member it is. This results in less precise type checking and intellisense within the function body.

The fundamental difference between these two lies in how TypeScript infers types:

export const FixedInferredType = <T extends QueryRoutes>(name: T) =>: This is a generic function. The type T is inferred based on the argument passed to the function. If you call FixedInferredType("getBusinesses"), TypeScript infers that T is "getBusinesses", not just any member of QueryRoutes. This allows for more precise type checking and intellisense within the function body.

export const ParameterAsUnion = (name: QueryRoutes) =>: This is a non-generic function. The type of name is QueryRoutes, which is a union of several string literals. TypeScript only knows that name is some member of QueryRoutes, but it doesn't know which specific member it is. This results in less precise type checking and intellisense within the function body.

TTC

Theo's Typesafe Cult•3y ago•

2 replies

tendies123

Inferred Type Parameter from a Fixed Union is Different from a Parameter that is a Fixed Union

So this took me embarrassingly long to figure out how to articulate, hoping it saves someone a lot of time

export const FixedInferredType = <T extends QueryRoutes>(name: T) =>

export const FixedInferredType = <T extends QueryRoutes>(name: T) =>

export const ParameterAsUnion = (name: QueryRoutes) =>

export const ParameterAsUnion = (name: QueryRoutes) =>

The fundamental difference between these two lies in how TypeScript infers types:

export const FixedInferredType = <T extends QueryRoutes>(name: T) =>: This is a generic function. The type T is inferred based on the argument passed to the function. If you call FixedInferredType("getBusinesses"), TypeScript infers that T is "getBusinesses", not just any member of QueryRoutes. This allows for more precise type checking and intellisense within the function body.

export const ParameterAsUnion = (name: QueryRoutes) =>: This is a non-generic function. The type of name is QueryRoutes, which is a union of several string literals. TypeScript only knows that name is some member of QueryRoutes, but it doesn't know which specific member it is. This results in less precise type checking and intellisense within the function body.

The fundamental difference between these two lies in how TypeScript infers types:

export const FixedInferredType = <T extends QueryRoutes>(name: T) =>: This is a generic function. The type T is inferred based on the argument passed to the function. If you call FixedInferredType("getBusinesses"), TypeScript infers that T is "getBusinesses", not just any member of QueryRoutes. This allows for more precise type checking and intellisense within the function body.

export const ParameterAsUnion = (name: QueryRoutes) =>: This is a non-generic function. The type of name is QueryRoutes, which is a union of several string literals. TypeScript only knows that name is some member of QueryRoutes, but it doesn't know which specific member it is. This results in less precise type checking and intellisense within the function body.

Inferred Type Parameter from a Fixed Union is Different from a Parameter that is a Fixed Union

Inferred Type Parameter from a Fixed Union is Different from a Parameter that is a Fixed Union

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