Infer type of a function argument as const in TypeScript
TypeScript infers literal types from function arguments of primitive types and creates more general types for inner members of complex data structures. I can illustrate that by creating a function that returns a value of the same type as the passed argument.
function inferReturnTypeFromArgument<T>(arg: T) {
return arg;
}
// primitive types
const exampleNumber = inferReturnTypeFromArgument(1);
const exampleString = inferReturnTypeFromArgument("hello");
// exampleNumber type: 1
// exampleString type: "hello"
// reference types
const exampleObject = inferReturnTypeFromArgument({ a: "hello" });
const exampleArray = inferReturnTypeFromArgument(["hello", "bye"]);
// exampleObject type: { a: string }
// exampleArray type: string[]
There may be a situation when you need a stricter type of inference for reference data types. In this case, you can add as const to function argument, and this is what I have been doing for ages. Today, I learned about a neat feature added to TypeScript 5.0 that solves this problem nicer - const Type Parameters. Have a look at the possible methods.
function inferReturnTypeFromArgument<T>(arg: T) {
return arg;
}
// reference types
const exampleObject = inferReturnTypeFromArgument({ a: "hello" } as const);
const exampleArray = inferReturnTypeFromArgument(["hello", "bye"] as const);
// exampleObject type: { readonly a: "hello" }
// exampleArray type: readonly ["hello", "bye"]
function inferReturnTypeFromArgument<const T>(arg: T) {
return arg;
}
// reference types
const exampleObject = inferReturnTypeFromArgument({ a: "hello" });
const exampleArray = inferReturnTypeFromArgument(["hello", "bye"]);
// exampleObject type: { readonly a: "hello" }
// exampleArray type: readonly ["hello", "bye"]
Helpful resources
- Announcing TypeScript 5.0
- Const type parameters bring ‘as const’ to functions by Matt Pocock
- 8 TypeScript Tips To Expand Your Mind (and improve your code by Matt Pocock)
Interesting read on TypeScript type inference! I've been looking up model pricing guides lately while comparing API costs for my projects. The const Type Parameters feature in TS 5.0 looks like a cleaner solution for type strictness. model pricing guides (https://ai-credits.ai/)