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❗undeclared identifier

JS ⟩ identifier ⟩ undeclared

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

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()❗

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Last updated 3 years ago