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15. Undefined > JavaScript ECMAScript 3 Edition (and Later) Declares the undefi...
JavaScript ECMAScript 3 Edition (and Later) Declares the undefined Variable in the Global Scope
Unlike previous versions, JavaScript ECMAScript 3 Edition (and
later) has a global variable called undefined declared in the global scope. Because
the variable is declared, and not assigned a value, the undefined variable
is set to undefined.
<!DOCTYPE html><html lang="en"><body><script> // confirm that undefined is a property of the global scope console.log(undefined in this); // logs true </script></body></html>
