This might get tricky because arrayOfNumbers[index] has type number even if arrayOfNumbers is number[] (!), and this is intentional (microsoft/TypeScript#9235) although it's at least quite controversial if you ask me.

const array: number[] = [];

array[5000] = 5;

const a = array[0]; // a is a `number`, not `number | undefined` !!!

const b = List().toArray();

// **EDIT**: this is not actually accurate for JavaScript:
// b.forEach(v => { /* v has `number` type but can have `undefined` value! */ } );
// b.map(v => { /* v has `number` type but can have `undefined` value! */ } );
// ...

So one has to decide should Immutable.js mimic this behavior or should it have more type safety?
If we choose type safety then which way? Make all methods like forEach, find, map, sortBy, ... to go with T | undefined for values because Immutable.js iterates over undefined after List resize? mapper: (v: number | undefined, k: number | undefined) => M for Set<number>? Does it make any sense? What about toArray() and toObject()?

Or should we instead change all mutation methods which may potentially introduce undefined values to return List<T | undefined>? Like set, setSize, or List constructor:

function of<T>(...values: T[]): List<T>;
function List<T>(): List<T>;
function List<T>(array: Array<T>): List<T | undefined>;

interface List<T> extends Collection.Indexed<T> {
    // ...
    push(...values: T[]): List<T>; // push can't add undefined values, so we keep List<T> type
    set(index: number, value: T): List<T | undefined>; // can't guarantee that new list won't have undefined values
    setSize(size: number): List<T | undefined>; // can't guarantee that new list won't have undefined values
}

I don't know...

EDIT: I wasn't quite accurate: JavaScript actually does not iterate over undefined values if one does a[50000] = 1, and Immutable.js behaves in a different way. Still there's a question about type of value parameters in iteration methods.

Uh oh!

Non-nullable types support for TypeScript 2.0 #914

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