Lookup index in tuple

[OliverJAsh]

const x: [number, string] = [1, 'foo'];
const y = x[1]
y

TypeScript knows y is a string. However, if I ask for x[2], TypeScript tells me it is string | number. Why is this? I expected it not to compile.

[saschanaz]

It seems it needs more discussion. See also #5203.

[DanielRosenwasser]

This is because a tuple of length n is an anonymous type that has properties 0, ... n - 1, and which effectively extends an array of the union of all element types. Since it's a subtype of Array, it gets an index signature, whose output type is the union of all element types.