Je was op zoek naar: undefined (Perzisch - Engels)

undefined
undefined

returna + b;// returns undefined.
returna + b;// returns undefined.

شیوه‌ای بهتر این است که از (typeof x == undefined') استفاده کنید.
a more robust approach is to compare using (typeof x === 'undefined').

alert(null == undefined); // unenforced type during check, displays truealert(null === undefined); // enforce type during check, displays falsealert(typeof null == object'); // trueاعداد در جاوااسکریپت به صورت باینری در استانداد اعداد حقیقی ieee-754 با دقت ۱۶ رفم نمایش داده می‌شوند.
alert(null == undefined); // unenforced type during check, displays truealert(null === undefined); // enforce type during check, displays falsealert(typeof null === 'object'); // true===number===numbers are represented in binary as ieee-754 doubles, which provides an accuracy of nearly 16 significant digits.