Suggestion: disallow for-in with Array types

[danvk]

Here's a function I just wrote:

/**
 * Read a property from a nested object, e.g.
 * getIn({a: {b: 2}}, ['a', 'b']) --> 2.
 */
export function getIn(o: Object, path: string[]): any {
  for (const property in path) {
    o = o[property];
    if (!o) return o;
  }
  return o;
}

It type checks fine, but it has a bug! The for .. in should really be for .. of.

If a variable is known to have an array type, then using for .. in to iterate over it is a very strange thing to do. It will iterate over the indices, converting them to strings, and skip over any holes in the array.

I'd propose that for .. in with an Array be an error. You probably want for .. of. Or array.forEach. If you really want for .. in for some reason, you could always use Object.keys(array).forEach.

This might be better implemented as a lint rule. See palantir/tslint#1380

[mhegazy]

I agree this is in the realm of linting. I would recommend writing a tslint rule for this.