--- title: "Meaning & Key Takeaway of: undefined and NaN are not constants. They…" description: "\"“undefined and NaN are not constants. They are global variables, and you can change their values. That should not be possible, and yet it is. Don’t do ..." image: "https://sms4smile.com/quotes/i/4f8/douglas-crockford-19095_technology_undefined-and-nan-are-not-constants-they_1258135.jpg" --- # undefined and NaN are not constants. They are global… ![“undefined and NaN are not constants. They are global variables, and you can change their values. That should not be possible, and yet it is. Don’t do it.” quote by Douglas Crockford](https://sms4smile.com/quotes/i/4f8/douglas-crockford-19095_technology_undefined-and-nan-are-not-constants-they_1258135.jpg) [Download](https://sms4smile.com/quotes/i/4f8/douglas-crockford-19095_technology_undefined-and-nan-are-not-constants-they_1258135.jpg) [Open image](https://sms4smile.com/quotes/i/4f8/douglas-crockford-19095_technology_undefined-and-nan-are-not-constants-they_1258135.jpg) > ““undefined and NaN are not constants. They are global variables, and you can change their values. That should not be possible, and yet it is. Don’t do it.”” [Douglas Crockford](https://sms4smile.com/quotes/author/douglas-crockford/) ## About This Quote This interpretation was drafted with AI assistance. It is one reading of the quote, not the author's own explanation. NaN and undefined are mutable global variables, contrary to expectations of constant behavior. **In simple terms:** These values can be altered, which is unsafe. Key Takeaway Never reassign NaN or undefined. Themes Programming JavaScript variables Mood cautionary technical Type best‑practice warning When to use this quote - Code reviews - static analysis - team coding standards Key Concepts Mutable globals language design runtime safety Questions to Reflect On - Why were NaN and undefined designed as mutable? - What safeguards can prevent accidental changes? A Different Perspective Changing them can break libraries and cause bugs. ★ ★ ★ ★ ★ No ratings yet [Technology](https://sms4smile.com/quotes/about/technology/) ## Related quotes > [“When you link to multiple JavaScript files from your page, all the global variables are defined in the same global space.”](https://sms4smile.com/quotes/eric-freeman/when-you-link-to-multiple-javascript-files-from-your-page-all-the/) [Eric Freeman](https://sms4smile.com/quotes/author/eric-freeman/) > [“the road to programming hell is paved with global variables,”](https://sms4smile.com/quotes/steve-mcconnell/the-road-to-programming-hell-is-paved-with-global-variables/) [Steve McConnell](https://sms4smile.com/quotes/author/steve-mcconnell/) > [Don't ask where the rest of this book is!" It is a shrill cry that comes from an undefined spot among the shelves. 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