Нello！在javascript中，几乎所有值都可以成功检索其onda install snowflake-sqlalchemy：
(base) C:\Windows\system32>conda info
active environment : base
active env location : C:\ProgramData\Anaconda3
shell level : 1
user config file : C:\Users\localadmin\.condarc
populated config files : C:\Users\localadmin\.condarc
conda version : 4.5.2
conda-build version : 3.15.1
python version : 3.5.6.final.0
base environment : C:\ProgramData\Anaconda3 (writable)
channel URLs : https://repo.anaconda.com/pkgs/main/win-64
https://repo.anaconda.com/pkgs/main/noarch
https://repo.anaconda.com/pkgs/free/win-64
https://repo.anaconda.com/pkgs/free/noarch
https://repo.anaconda.com/pkgs/r/win-64
https://repo.anaconda.com/pkgs/r/noarch
https://repo.anaconda.com/pkgs/pro/win-64
https://repo.anaconda.com/pkgs/pro/noarch
https://repo.anaconda.com/pkgs/msys2/win-64
https://repo.anaconda.com/pkgs/msys2/noarch
package cache : C:\ProgramData\Anaconda3\pkgs
C:\Users\localadmin\AppData\Local\conda\conda\pkgs
envs directories : C:\ProgramData\Anaconda3\envs
C:\Users\localadmin\AppData\Local\conda\conda\envs
C:\Users\localadmin\.conda\envs
platform : win-64
user-agent : conda/4.5.2 requests/2.19.1 CPython/3.5.6 Windows/7 Wi
ndows/6.1.7601
administrator : True
netrc file : None
offline mode : False
(base) C:\Windows\system32>conda list
# packages in environment at C:\ProgramData\Anaconda3:
#
# Name Version Build Channel
_ipyw_jlab_nb_ext_conf 0.1.0 py35_0
alabaster 0.7.11 py35_0
anaconda-clean 1.1.0 py35_1
anaconda-client 1.7.2 py35_0
anaconda-navigator 1.9.2 py35_0
anaconda-project 0.8.2 py35_0
asn1crypto 0.24.0 py35_0
astroid 2.0.4 py35_0
astropy 3.0.4 py35hfa6e2cd_0
atomicwrites 1.3.0 py_0
attrs 19.1.0 py_0
automat 0.7.0 py35_0
babel 2.7.0 py_0
backcall 0.1.0 py35_0
backports 1.0 py_2
backports.shutil_get_terminal_size 1.0.0 py35_2
beautifulsoup4 4.6.3 py35_0
bitarray 0.8.3 py35hfa6e2cd_0
bkcharts 0.2 py35h4704c85_0
blas 1.0 mkl
blaze 0.11.3 py35_0
bleach 2.1.4 py35_0
blosc 1.14.3 he51fdeb_0
bokeh 0.13.0 py35_0
boto 2.49.0 py35_0
boto3 1.3.1 py35_0
botocore 1.4.26 py35_0
bottleneck 1.2.1 py35h452e1ab_1
bzip2 1.0.6 hfa6e2cd_5
ca-certificates 2019.5.15 0
certifi 2018.8.24 py35_1
cffi 1.6.0 py35_0
chardet 3.0.4 py35_1
click 6.7 py35h10df73f_0
cloudpickle 1.1.1 py_0
clyent 1.2.2 py35_1
colorama 0.3.9 py35h32a752f_0
comtypes 1.1.7 py35_0
conda 4.5.2 py35_0
conda-build 3.15.1 py35_0
conda-env 2.6.0 h36134e3_1
conda-verify 3.1.0 py35_0
console_shortcut 0.1.1 h6bb2dd7_3
constantly 15.1.0 py35h28b3542_0
contextlib2 0.5.5 py35h0a97e54_0
cryptography 1.4 py35_0
curl 7.60.0 h7602738_0
cycler 0.10.0 py35hcc71164_0
cython 0.28.5 py35h6538335_0
cytoolz 0.9.0.1 py35hfa6e2cd_1
dask 0.19.2 py35_0
dask-core 0.19.2 py35_0
datashape 0.5.4 py35_1
decorator 4.4.0 py_0
defusedxml 0.6.0 py_0
distributed 1.23.2 py35_0
docutils 0.14 py35h8ccb97f_0
entrypoints 0.2.3 py35_2
et_xmlfile 1.0.1 py35h2c13def_0
fastcache 1.0.2 py35hfa6e2cd_2
filelock 3.0.12 py_0
flask 1.0.2 py35_1
flask-cors 3.0.6 py35_0
freetype 2.8 h51f8f2c_1
future 0.16.0 py35_2
get_terminal_size 1.0.0 h38e98db_0
gevent 1.2.2 py35hb3d9581_0
glob2 0.6 py35_0
greenlet 0.4.15 py35hfa6e2cd_0
h5py 2.8.0 py35h3bdd7fb_2
hdf5 1.10.2 hac2f561_1
heapdict 1.0.0 py35_2
html5lib 1.0.1 py35_0
hyperlink 19.0.0 py_0
icc_rt 2017.0.4 h97af966_0
icu 58.2 ha66f8fd_1
idna 2.7 py35_0
imageio 2.4.1 py35_0
imagesize 1.1.0 py35_0
incremental 17.5.0 py35_0
intel-openmp 2018.0.0 8
ipykernel 4.10.0 py35_0
ipython 6.5.0 py35_0
ipython_genutils 0.2.0 py35ha709e79_0
ipywidgets 7.4.1 py35_0
isort 4.3.4 py35_0
itsdangerous 0.24 py35_1
jdcal 1.4.1 py_0
jedi 0.12.1 py35_0
jinja2 2.10 py35_0
jmespath 0.9.4 py_0
jpeg 9b hb83a4c4_2
jsonschema 2.6.0 py35h27d56d3_0
jupyter 1.0.0 py35_7
jupyter_client 5.2.3 py35_0
jupyter_console 5.2.0 py35_1
jupyter_core 4.4.0 py35_0
jupyterlab 0.34.9 py35_0
jupyterlab_launcher 0.13.1 py35_0
keyring 13.2.1 py35_0
kiwisolver 1.0.1 py35h6538335_0
lazy-object-proxy 1.3.1 py35hfa6e2cd_2
libcurl 7.60.0 hc4dcbb0_0
libiconv 1.15 h1df5818_7
libpng 1.6.34 h79bbb47_0
libsodium 1.0.16 h9d3ae62_0
libssh2 1.8.0 hd619d38_4
libtiff 4.0.9 hb8ad9f9_1
libxml2 2.9.8 hadb2253_1
libxslt 1.1.32 hf6f1972_0
llvmlite 0.24.0 py35h6538335_0
locket 0.2.0 py35h0dfcdd0_1
lxml 4.2.5 py35hef2cd61_0
lzo 2.10 h6df0209_2
m2w64-gcc-libgfortran 5.3.0 6
m2w64-gcc-libs 5.3.0 7
m2w64-gcc-libs-core 5.3.0 7
m2w64-gmp 6.1.0 2
m2w64-libwinpthread-git 5.0.0.4634.697f757 2
markupsafe 1.0 py35hfa6e2cd_1
matplotlib 2.2.2 py35h153e9ff_1
mccabe 0.6.1 py35_1
menuinst 1.4.14 py35hfa6e2cd_0
mistune 0.8.3 py35hfa6e2cd_1
mkl 2018.0.2 1
mkl-service 1.1.2 py35hb217b18_4
mkl_fft 1.0.1 py35h452e1ab_0
mkl_random 1.0.1 py35h9258bd6_0
more-itertools 4.3.0 py35_0
mpmath 1.0.0 py35_2
msgpack-python 0.5.6 py35he980bc4_1
msys2-conda-epoch 20160418 1
multipledispatch 0.6.0 py35_0
navigator-updater 0.2.1 py35_0
nbconvert 5.5.0 py_0
nbformat 4.4.0 py35h908c9d9_0
networkx 2.3 py_0
nltk 3.3.0 py35_0
nose 1.3.7 py35_2
notebook 5.6.0 py35_0
numba 0.39.0 py35h830ac7b_0
numexpr 2.6.5 py35hcd2f87e_0
numpy 1.14.3 py35h9fa60d3_1
numpy-base 1.14.3 py35h555522e_1
numpydoc 0.9.1 py_0
odo 0.5.1 py35hc850252_0
olefile 0.46 py35_0
openpyxl 2.6.2 py_0
openssl 1.0.2s he774522_0
packaging 17.1 py35_0
pandas 0.23.4 py35h830ac7b_0
pandoc 1.19.2.1 hb2460c7_1
pandocfilters 1.4.2 py35_1
parso 0.4.0 py_0
partd 0.3.10 py_0
path.py 11.1.0 py35_0
pathlib2 2.3.2 py35_0
patsy 0.5.0 py35_0
pep8 1.7.1 py35_0
pickleshare 0.7.4 py35h2f9f535_0
pillow 5.1.0 py35h0738816_0
pip 10.0.1 py35_0
pkginfo 1.4.2 py35_1
pluggy 0.11.0 py_0
ply 3.11 py35_0
prometheus_client 0.3.1 py35h28b3542_0
prompt_toolkit 1.0.15 py35h89c7cb4_0
psutil 5.4.7 py35hfa6e2cd_0
py 1.6.0 py35_0
pyasn1 0.4.5 py_0
pyasn1-modules 0.2.2 py35_0
pycodestyle 2.4.0 py35_0
pycosat 0.6.3 py35hfa6e2cd_0
pycparser 2.19 py35_0
pycrypto 2.6.1 py35hfa6e2cd_9
pycurl 7.43.0.2 py35h74b6da3_0
pyflakes 2.0.0 py35_0
pygments 2.4.2 py_0
pylint 2.1.1 py35_0
pyodbc 4.0.24 py35h6538335_0
pyopenssl 0.16.0 py35_0
pyparsing 2.4.0 py_0
pyqt 5.9.2 py35h1aa27d4_0
pysocks 1.6.8 py35_0
pytables 3.4.4 py35he6f6034_0
pytest 3.8.1 py35_0
pytest-arraydiff 0.2 py35h39e3cac_0
pytest-astropy 0.4.0 py35_0
pytest-doctestplus 0.1.3 py35_0
pytest-openfiles 0.3.0 py35_0
pytest-remotedata 0.3.0 py35_0
python 3.5.6 he025d50_0
python-dateutil 2.7.3 py35_0
pytz 2019.1 py_0
pywavelets 1.0.0 py35h452e1ab_0
pywin32 223 py35hfa6e2cd_1
pywinpty 0.5.4 py35_0
pyyaml 3.13 py35hfa6e2cd_0
pyzmq 17.1.2 py35hfa6e2cd_0
qt 5.9.5 vc14he4a7d60_0 [vc14]
qtawesome 0.5.7 py_0
qtconsole 4.5.1 py_0
qtpy 1.7.1 py_0
requests 2.19.1 py35_0
rope 0.14.0 py_0
ruamel_yaml 0.15.46 py35hfa6e2cd_0
scikit-image 0.14.0 py35h6538335_1
scikit-learn 0.19.1 py35h2037775_0
scipy 1.1.0 py35h672f292_0
seaborn 0.9.0 py35_0
send2trash 1.5.0 py35_0
setuptools 40.2.0 py35_0
simplegeneric 0.8.1 py35_2
singledispatch 3.4.0.3 py35h33f66b4_0
sip 4.19.8 py35h6538335_0
six 1.11.0 py35_1
snappy 1.1.7 h777316e_3
snowballstemmer 1.2.1 py35h4c55bfa_0
snowflake_connector_python 1.2.1.1 py35_0 snowflakedb
sortedcollections 1.0.1 py35_0
sortedcontainers 2.0.5 py35_0
sphinx 2.1.0 py_0
sphinxcontrib 1.0 py35_1
sphinxcontrib-applehelp 1.0.1 py_0
sphinxcontrib-devhelp 1.0.1 py_0
sphinxcontrib-htmlhelp 1.0.2 py_0
sphinxcontrib-jsmath 1.0.1 py_0
sphinxcontrib-qthelp 1.0.2 py_0
sphinxcontrib-serializinghtml 1.1.3 py_0
sphinxcontrib-websupport 1.1.2 py_0
spyder 3.3.1 py35_1
spyder-kernels 0.2.6 py35_0
sqlalchemy 1.2.11 py35hfa6e2cd_0 anaconda
sqlite 3.23.1 h35aae40_0
statsmodels 0.9.0 py35h452e1ab_0
sympy 1.2 py35_0
tblib 1.4.0 py_0
terminado 0.8.1 py35_1
testpath 0.3.1 py35h06cf69e_0
tk 8.6.7 hcb92d03_3
toolz 0.9.0 py35_0
tornado 5.1.1 py35hfa6e2cd_0
tqdm 4.32.1 py_0
traitlets 4.3.2 py35h09b975b_0
twisted 17.5.0 py35_0
typed-ast 1.1.0 py35hfa6e2cd_0
typing 3.6.6 py35_0
unicodecsv 0.14.1 py35h0d88516_0
urllib3 1.23 py35_0
vc 14.1 h21ff451_3 anaconda
vs2015_runtime 15.5.2 3 anaconda
wcwidth 0.1.7 py35h6e80d8a_0
webencodings 0.5.1 py35_1
werkzeug 0.15.4 py_0
wheel 0.31.1 py35_0
widgetsnbextension 3.4.1 py35_0
win_inet_pton 1.0.1 py35_1
win_unicode_console 0.5 py35h56988b5_0
wincertstore 0.2 py35hfebbdb8_0
winpty 0.4.3 4
wrapt 1.10.11 py35hfa6e2cd_2
xlrd 1.1.0 py35_1
xlsxwriter 1.1.8 py_0
xlwings 0.11.8 py35_0
xlwt 1.3.0 py35hd04410a_0
yaml 0.1.7 hc54c509_2
zeromq 4.2.5 hc6251cf_0
zict 0.1.3 py35_0
zlib 1.2.11 h8395fce_2
zope 1.0 py35_1
zope.interface 4.5.0 py35hfa6e2cd_0
我也知道您可以创建一个其实例不具有constructor属性的类：
console.log((33).constructor); // Number
console.log((false).constructor); // Boolean
console.log(('hello').constructor); // String
console.log(({}).constructor); // Object
console.log(([]).constructor); // Array
console.log((v => v).constructor); // Function
我感兴趣的是在尝试检索其构造函数时会导致错误的值。我只知道两个这样的值：
constructor我的问题是： let O = function(){};
O.prototype = Object.create(null);
console.log((new O()).constructor)); // undefined
和console.log((null).constructor); // TypeError!
console.log((undefined).constructor); // TypeError!
是仅有两个可能具有这种行为的值吗？
答案 0 :(得分：4)
我的问题是：具有这种行为的唯一两个可能值是null和undefined吗？
默认情况下是，因为如果对象或其任何原型上不存在属性，则将获得值undefined。抛出null和undefined的原因是您无法尝试从它们获取属性。当您尝试从中获取属性时，它们是唯一表现出这种行为的东西。
但是，当您直接获得constructor属性时，可以构造任何对象引发错误：
const o = {
get constructor() {
throw new Error("Don't do that!");
}
};
console.log(o.constructor);
或在陷阱阻止的情况下使用代理：
const o = new Proxy({}, {
get(target, propName, receiver) {
if (propName === "constructor") {
throw new Error("Don't do that!");
}
return Reflect.get(target, propName, receiver);
}
});
console.log(o.constructor);