I would like to write unit tests (JavaScript, Jest) for my class, but I don’t understand something. When writing a test, I need to import a class file into the test. But import calls the entire file. So, if I had a class call with the original data in the imported file, the class with the original data would be called first, and next with the test data.

1. Does this mean my class is not well isolated for test?
2. Shouldn’t I create a class instance in the same file as the class body? I haven’t found any contraindications anywhere, but maybe it’s bad from an architectural point of view?
3. Is there a way to get around this, but I just haven’t found it yet?

Sample class:

class ABC {
    constructor(settings) {
        this.a = settings.a
        this.b = settings.b
        this.logText = settings.logText

        this.sum(this.a, this.b, this.logText)
    }

    sum() {
        console.log("log text:", this.logText)
        return this.a + this.b
    }
}

//Simplified settings. Originally it contains, for example: an 
//asynchronous method for calling endpoint.
let settingsArray = [{
    a: 1,
    b: 1,
    logText: "original"
}]

function createInstance () {
    for(let setting from settingsArray) {
      new ABC(setting);
    }
}
createInstance(settingsArray)

Specifically about the problem to understand:

1. in the test I import a class ABC
2. the class is called, the createInstance method creates instances of the class with the original data (settingsArray). I can see log “log text: original”. I think, in correctly done test I shouldn’t see it, because in my test settingsArray is overwritten and logText != “original”.
3. I’m just taking the test now
4. in the test, I have overwritten settingsArray where for example: logText: “from test”
5. only now I am seeing an instance of the class with test data
6. even if the test passes, if the original settings contain e.g. external methods, errors are returned along the way (or later, if there were e.g. asynchronous methods). And I guess it shouldn’t be like that.

Can someone help me understand what should be good and what is wrong with my way of thinking? Maybe this is no problem, and I don’t understand correctly what is isolation for unit test?