[NaN](<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN>)
stands for “Not a Number.” When a mathematical function or operation in JavaScript cannot return a specific number, it returns the value NaN
instead.
It is a property of the global object, and a reference to [Number.NaN](<https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Number/NaN>)
window.hasOwnProperty('NaN'); // true
NaN; // NaN
Perhaps confusingly, NaN
is still considered a number.
typeof NaN; // 'number'
Don’t check for NaN
using the equality operator. See [isNaN](<https://stackoverflow.com/documentation/javascript/700/built-in-constants/1760/testing-for-nan-using-isnan#t=201607272211248187198>)
instead.
NaN == NaN // false
NaN === NaN // false