Type guard, so that void/undefined can be excluded from an array's type, after filtering them out, using JSDoc type annotations.

typescript-type-guard-function-using-jsdoc.js

/**
 * Type guard, so that void/undefined can be excluded from an array's type, after filtering them out.
 * @template T
 * @param {T | void} val
 * @returns {val is T}
 */
const isDefined = (val) => val && typeof val === 'object'

/** @type {number[]} */
const numbers = [-1, 1, 2]

/** @type {number[]} */
const numbersAboveZero = numbers
  .map((n) => n > 0 ? n : undefined) // => 😓 type: (number | undefined)[]
  .filter(isDefined)                 // => 😌 type: number[]