Infer type of a function argument as const in TypeScript

Published: 2024.02.07 · 2 minutes read

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"]

Infer type of a function argument as const in TypeScript

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