Add undefined to tuple index signature, or reject keys that are not "keyof tuple"

Search Terms

Tuple index signature

Suggestion

I am opening up this issue as Seperate from https://github.com/microsoft/TypeScript/issues/13778 since I am ok with the current interpretation of the Array.

declare var strArray: string[] // infinitely long list of type string
var a : strArray[10 as number]  // ok; a is string

but this fails in conjunction with tuples

declare var strTuple: [number, number] // length = 2
var a : strTuple[10 as number]  // absolutely not ok; a is string; 

Better Options:

var a : strTuple[10 as number] should either be string | undefined or it should give a type error similar to:

var a = { 0:  10, 1: 20} as const;
a[10 as number]; // type 'number' can't be used to index type

Tuples are already aware of boundaries and properly return undefined for more specific number types:

const t0: [string][0] = 'hello' // ok
const t1: [string][1] = 'hello' // Type '"hello"' is not assignable to type 'undefined'.

// therefore this is already true!
const t2: [string][0|1] // string | undefined 

why would type number act any differently?

Use Cases

// Easy fix for for loops

for (let i = 0 as 0 | 1; i < 2; i++) { 
  var definitly_string = strTuple[i] // type string
  var j = i + someNumber;
  var not_definitly_string  = strTuple[j] //lets you know you need to be careful
}

You could use keyof typeof strTuple instead of 0 | 1 when you fix this https://github.com/microsoft/TypeScript/issues/27995

for (let i = 0 as keyof typeof strTuple; i < strTuple.length; i++) { 
  var definitly_string = strTuple[i] // type string
  var j = i + someNumber;
  var not_definitly_string = strTuple[j] //lets you know you need to be careful
} 

Given that strictNullChecks still allow typed arrays to do this:

var notString1 = strArray[Infinity]; // no error, not undefined
var notString2 = strArray[NaN]; // no error, not undefined

It would be nice to have a mechanism that could allow you to declare arrays with better type safety.

Examples

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start by reproducing the tuple-indexing examples in the issue, including numeric, out-of-range, and keyof cases, and compare them with array indexing. Evaluate both proposed outcomes: adding undefined to number-indexed tuple access or rejecting keys outside keyof the tuple. Done means the chosen behavior is specified consistently and covered for the examples shown.