typescript - Infer tuple type instead of union type

Question

Does anybody know how to make compiler to infer tuple type automatically ?

// Now: (string | number)[]
// Wanted: [string, number][]
const x = [ ["a", 2], ["b", 2] ];

Answer 1

This can be done if we use an extra function to help type inference a bit:

function tupleArray<T1, T2, T3>(arr:[T1, T2, T3][]) : typeof arr 
function tupleArray<T1, T2>(arr:[T1, T2][]) : typeof arr 
function tupleArray<T1>(arr:[T1][]) : typeof arr 
function tupleArray(arr:any[]) : any[]{
    return arr;
}

var t = tupleArray([ ["a", 2], ["b", 2] ]) // [string, number][]

Edit

Better version with fewer overrides:

const tupleArray = <T extends ([any] | any[])[]>(args: T): T => args
tupleArray([["A", 1], ["B", 2]]) // [string, number][]

You can add more overloads if you need more then 3 items in the tuple.