allow undefined to be used alongside null for args

[BrianHung]

Typically, fractional indices are stored in an array or an object / map (?); and out-of-bounds or not-found keys into those return undefined. It'd be nice to have to avoid null check or null casting for just undefined when pass in values for a and b.

E.g.

  insertCols(insertAtIndex: number, number: number = 1) {
    const orders = generateNKeysBetween(
      this.colArray[insertAtIndex - 1]?.order ?? null, // undefined ?? null not necessary
      this.colArray[insertAtIndex]?.order ?? null, // undefined ?? null not necessary
      number
    );
    ...
  }

[arv]

I think this is fine.

I'm generally a bit averse to code that cop out and allows both null and undefined. I also feel like it is too easy to get an unintentional undefined.

[arv]

LGTM

Thanks