What scope does unit testing cover in a weakly typed language?

user_17804016 0
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The best way I can express my question is with an example.

class foo{
  constructor(size) {
    this.bar = new Array(size);
  }
  baz(index) {
    this.body[index] = true;
  }
}

Ignoring the constructor, assume we're unit testing baz(). baz is logically expected to take an input index that's between 0 and bar.length - 1, but the following can also compile:

  1. baz(['hey'])
  2. baz(5) // with an array size of 2
  3. baz(true)
  4. baz('0')

Now, I've been doing a lot of research on what we're actually testing with unit tests, and the best answer I can gather from what I read all around, is that, we're testing the function's behavior within the bounds of its logical contract with what/who's calling it. AKA, test implementation while implicitly assuming that it' called correctly.

I'm fully satisfied with this, but what I want to know is, where do we draw that line? None of the examples above will cause a typeerror, but I can disregard 1,3 and probably 4 if I'm to follow what I learned above.

But should I care about 2? Or do I also assume that the function's 'contract' will be upheld not only in terms of the inputs' implicit typing, but also their logical bounds.

Should I test and account for example 2 in my code?

testing unit-testing javascript
1 Answers

It depends whether the contract says "…and for all other inputs, the function throws an exception" - in which case you should test that it is thrown as expected - or whether the contract simply says (or implies) "…and don't call it with anything else, it may not work or do unpredictable things" (i.e. undefined behaviour) - in which case you don't need to test it.

A number outside of the expected range (your example case 2), and a value of an unexpected type (cases 1, 3, 4) may independently fall on either side of that distinction. You can draw the line wherever you want. Just make sure to gain bonus points by documenting where you drew it.

user_10485720
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