分配字符串后，process.env.VARIABLE变为未定义

Question

I running tests with Mocha and Node in Windows, and the env variables are showing an weird behavior. Look the code below

var stringMock = JSON.stringify(mock);
process.env.MOCKS = stringMock;
if(stringMock !== process.env.MOCKS) {
    console.log('typeof stringMock: ', typeof stringMock);
    console.log('typeof process.env.MOCKS: ', typeof process.env.MOCKS);
}

It's already very weird that the if statement evaluates to true , but the result of the log is:

    typeof stringMock: string
    typeof process.env.MOCKS: undefined

How can this be even possible? And if a ran this code in Codeship Linux, this doesn't happen, so probably can be a Node bug for Windows.

I suspect that Node has some kind of limitation about the size of the string for env variables, but I couldn't find anything useful about it.

Answer 1

The real questions are:

1. What is the value of mock ?
2. What is the value of stringMock ?
3. What is the value of process.env.MOCKS ?

Having only the types is just part of the data.

I can imagine only one value for mock that would make stringMock !== process.env.MOCKS evaluate to true, and that is undefined . That's because JSON.stringify() will return undefined instead of a string, and then thet undefined will get stringified when saved in the environment variables.

But strangely enough, the output for this program:

var mock = undefined;

var stringMock = JSON.stringify(mock);
process.env.MOCKS = stringMock;
if(stringMock !== process.env.MOCKS) {
    console.log('typeof stringMock: ', typeof stringMock);
    console.log('typeof process.env.MOCKS: ', typeof process.env.MOCKS);
}

is quite the opposite of what you're getting:

typeof stringMock:  undefined
typeof process.env.MOCKS:  string

That's because the first one in the undefined value and the second one is a string "undefined" . But here you have the opposite of that, which is quite strange and quite frankly I was unable to reproduce your result for any value that I have tried, like NaN , Infinity , 0 , '' , etc. Everything else serialized to JSON should either return a string (which would be the same in the env var) or throw an exception (for example for circular references).

The only hint that comes to my mind would be hitting some length limit of the operating system - like trying to save a very long string resulting in not saving the string at all to the environment.

And now when I thought about it I searched for it and it seems that there's a limit of 32K characters on Windows, see:

• https://blogs.msdn.microsoft.com/oldnewthing/20100203-00/?p=15083

The theoretical maximum length of an environment variable is around 32,760 characters. However, you are unlikely to attain that theoretical maximum in practice.

Try printing the stringMock.length and it may answer your question.

Answer 2

process.env values are implicitly converted to a string. So for instance:

> console.log('typeof process.env.MOCKS: ', typeof process.env.MOCKS);
typeof process.env.MOCKS:  undefined

> process.env.MOCKS = undefined
undefined
> process.env.MOCKS
'undefined'
> console.log('typeof process.env.MOCKS: ', typeof process.env.MOCKS);
typeof process.env.MOCKS:  string

So if you are trying to stringily something that returns undefined, you will end up assigning the string 'undefined' to process.env.MOCKS .

Answer 3

As noted by rsp , the issue is related to the length of the string. Sometimes stringMock has more than 40k characters.

process.env.MOCKS = stringMock.substr(0, 30000);
console.log(process.env.MOCKS); // prints the string

process.env.MOCKS = stringMock.substr(0, 40000);
console.log(process.env.MOCKS); // prints undefined