🔸undefined

🚧 under construction

• JS ⟩ value ⟩ primitive ⟩ undefined

• JS ⟩ scope ⟩ global ⟩ global object ⟩ property ⟩ undefined

( read-only global object property )❗ automatically assigned to

• variable(s) that have just been declared.

• parameter(s) that have no corresponding arguments.

⭐️ 重點

undefined

• is not a keyword❗

• is not a literal❗

• is an immutable & read-only property of the global object❗

• JS doesn't throw any errors if you declare it in the global scope.

• has no primitive wrapper, can't call methods, has no properties.

null and undefined are the only two values that do not have property.❗

with a function in block (FiB)

• sloppy mode: ✅ hoisted / initialized to undefined (in outer scope)

• strict mode: ⛔ ReferenceError (invisible in outer scope)❗

👥 相關

• 👉 compare： null

• ℹ️ undefined is nullish.

• ❗uninitialized variable has no initial value, not undefined even❗

📗 參考

• TeckTutorialHub ⟩ JavaScript Keywords & Reserved Words ⭐️

📘 手冊

• undefined

• difference between null and undefined

🚧

• null vs. undefined