What's the simplest Typescript idiom for checking if an array exists and has nonzero length?

I would simply write it like this:

if (Array.isArray(thingies) && thingies.length > 0) {
  [...]
}

Not the shortest way but way more clear (and more correct) in my opinion.

Edit:

If it feels too long and you need it often, you can also simply define it as a function:

import {isNonEmptyArray} from "./utils";

if (isNonEmptyArray(thingies)) {
  [...]
}

// utils.ts
export const isNonEmptyArray = (array: unknown): array is unknown[] => (
  Array.isArray(array) && array.length > 0
)

Considering that both undefined and 0 are falsy values, you can just write:

if (array?.length)

Either array is undefined and this will return false, or it is defined and it will return false if length === 0.

Edit:
If you first wish to ensure that the object is an array, you can perform a check using Array.isArray beforehand:

if (Array.isArray(object) && object.length)

Answering my own question, a year later: I usually use thing?.length now, but I also frequently make use of the lodash size(thing) and isEmpty(thing) methods for this. That's particularly helpful when particularly when thing might be something else, like a set or an object, or when I need to check if the length is greater than n, where n > 0:

if ((thing?.length ?? 0) > 1) ...            // :(
if (isArray(thing) && thing.length > 1) ...  // :|
if (size(thing) > 1) ...                     // :)