Check index array length

Search Terms

Indexing, Array, Strict check

Suggestion

Strict check to indexing a array

Use Cases

Safety with arrays usages

Examples

const arr: House[] = .... // same as [] | House[]
arr[0] // throw error
if(arr.length > 0) {
  arr[0] // OK
  arr[200] // throw error
}
const arr2: [House, House] = ...
arr[0] // OK
arr[1] // OK

Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript/JavaScript code (with a flag)
• 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.

[Jamesernator]

I would definitely find even just basic cases working better than nothing. At current I use a custom helper that takes an array and returns if it’s a tuple of a given length e.g.:

function isLength<T>(arr: T[], length: 0): arr is [];
function isLength<T>(arr: T[], length: 1): arr is [T];
function isLength<T>(arr: T[], length: 2): arr is [T, T];
// etc
function isLength<T>(arr: T[], length: number): arr is T[] {
    return arr.length === length;
}

I use it quite a lot as I’m also using --noUncheckedIndexedAccess, but it’d be super nice to be builtin so I can just use arr.length === 0 rather than using a custom function that only exists for the type system (or more accurately forgetting to use it, and then having an error that value could be undefined). Having <, <=, >, >= would be even better for things like const [first, ...rest] and so on.

[Kingwl]

@Jamesernator

Well… Just for fun:

playground