Empty test

empty.js

/**
 * empty array
 * empty string
 * function of zero arity
 * empty object
 * null
 * undefined
 */
var empty = x => Array.isArray(x) || typeof x === 'string' || typeof x === 'function' ? 
  !x.length : null == x || Object.keys(x).length === 0 && x.constructor === Object

/**
 * also test for 0
 */
var empty = x => Array.isArray(x) || typeof x === 'string' || typeof x === 'function' ? !x.length :
    typeof x === 'number' ? x === 0 :
      null == x || Object.keys(x).length === 0 && x.constructor === Object