javascript - 词法范围示例不起作用

Question

为什么这种关闭不起作用？

  var derivedKey;
  lightwallet.keystore.deriveKeyFromPassword(password, function (err, pwDerivedKey) {
    derivedKey = pwDerivedKey;
  });

  if (!derivedKey){
    console.error ("ERROR no derivedKey");
  }

错误。对此有什么可能的解释？