❗undeclared identifier

JS ⟩ identifier ⟩ undeclared

👥 相關

• assignment (=) to an undeclared identifier in sloppy mode will result in an accidental global variable in sloppy mode❗️

• temporal dead zone

💈範例

• 👉 related：class body has its own "this" context

typeof undeclared === "undefined"     // true

🧨 雷區

assignment (=) to undeclared identifier will result in

• accidental global variable in sloppy mode❗️ (in sloppy mode)

• ⛔️ ReferenceError：'...' is not defined❗ (in strict mode)

there is NO way to write a function to check if an identifer is declared.

// assume f is such a function
f(undeclared)          // ⛔ ReferenceError

// JS will throw a ReferenceError on accessing `undeclared`
// BEFORE you even have a chance to call f()❗

📗 參考

• 3 Ways to Check if a Variable is Defined in JavaScript ⭐️

📘 手冊

• typeof ⟩ Interaction with undeclared and uninitialized variables

• ReferenceError: "x" is not defined

🗣 討論

• JavaScript check if variable exists (is defined/initialized)