This function template is designed to work with a wide range of inputs, thanks to its generic nature. It allows a callback function to be executed with a set of parameters of any type

I've encountered an issue with the function I defined:

function someFunc<T extends any[]>(callback: (...args: T) => void, params: T) {}

Unexpected behavior occurs when calling it in TypeScript:

// this works
// hovering over a, b, and c reveals that they're numbers
someFunc((a, b, c) => {}, [1, 2, 3]);

// this doesn't work
// hovering over a, b, and c reveals that they're of type `number | string`
someFunc((a, b, c) => {}, [1, '2', 3]);

It seems like the issue lies in how TypeScript interprets the second argument of someFunc as an array rather than a tuple, resulting in types like number[] or (number | string)[].

How can I enforce each parameter's type to match appropriately within the parameters tuple?

typescriptgenerics

Answer №1

When it comes to TypeScript type inference, the compiler follows certain heuristic rules. For instance, when encountering an array literal like [1, '2', 3], it typically infers an unordered array type of arbitrary length such as (string | number)[]. However, if you specifically want a tuple type with a known order and length like [number, string, number], you can provide a hint to the compiler.

One way to indicate your preference for a tuple type is by encapsulating the type in a variadic tuple type using [...+]. According to information from microsoft/TypeScript#39094, this approach allows you to suggest the inference of tuple types.

To implement this change, modify the function as shown below:

function someFunc<T extends any[]>(
  callback: (...args: T) => void,
  params: [...T]) { }

Now, let's test it out with the following code snippets:

someFunc((a, b, c) => {
  console.log(a - 0, b - 1, c - 2)
}, [1, 2, 3]);

someFunc((a, b, c) => {
  console.log(a - 0, b.length, c - 2)
}, [1, '2', 3]);

The updated code now ensures that the compiler recognizes b in the second callback as a string, while a and c are treated as numbers.

Access the Playground link for the code snippet.

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