typeof更安全,因为它允许从未声明过标识符:
if(typeof neverDeclared === "undefined") // no errorsif(neverDeclared === null) // throws ReferenceError: neverDeclared is not defined

typeof更安全,因为它允许从未声明过标识符:
if(typeof neverDeclared === "undefined") // no errorsif(neverDeclared === null) // throws ReferenceError: neverDeclared is not defined