Tuple allows access at any index

[OliverJAsh]

Version: 2 rc

Given:

    const cell: [ number, number ] = [ 0, 1 ];
    const y = cell[100]
    y

I expect cell[100] to error, but it doesn't. y has a type of number. This means it's very easy to me to have bugs whereby I'm asking for an index that doesn't exist (like asking for a property that doesn't exist on an object).

[aluanhaddad]

This works because indexing is not verified. You can always index. What is weird is that cell[100] has type number, I would expect it have the same type as cell['100'], that is type any.

[yortus]

The current behaviour is to give 'unknown' elements the union of the tuple element types. It's clearer to see if the tuple has different element types:

const cell: [ string, number ] = [ '0', 1 ];
const u = cell[0];
u // u is string
const v = cell[1];
u // u is number
const y = cell[100]
y // y is string | number

But personally I don't think that's a logical deduction for the type system to make. All bets are off for indexes outside the tuple range. At runtime they are likely to return undefined.

[aluanhaddad]

@yortus what I find odd is this

const cell: [ string, number ] = [ '0', 1 ];
const u = cell['0'];
u // u is string
const v = cell['1'];
u // u is number
const y = cell[100]
y // y is string | number
const y = cell['100']
y // y is any

[Igorbek]

See a proposal of fixing this #6229

[RyanCavanaugh]

Duplicate #5203

[OliverJAsh]

I noticed this as well, sort of related. Can anyone explain why?

    const xs = [1,2,3];
    const x5 = xs[5]; // type is number, expected number | undefined