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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

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 instead.

NaN == NaN // false
NaN === NaN // false