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https://github.com/stereobooster/test-coverage-calculation
purely theoretical speculations about how code coverage may be calculated
https://github.com/stereobooster/test-coverage-calculation
c8 coverage istanbuljs javascript js monocart monocart-reporter vitest
Last synced: 13 days ago
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purely theoretical speculations about how code coverage may be calculated
- Host: GitHub
- URL: https://github.com/stereobooster/test-coverage-calculation
- Owner: stereobooster
- Created: 2024-08-10T11:09:39.000Z (3 months ago)
- Default Branch: main
- Last Pushed: 2024-08-16T20:29:15.000Z (3 months ago)
- Last Synced: 2024-08-16T21:39:46.360Z (3 months ago)
- Topics: c8, coverage, istanbuljs, javascript, js, monocart, monocart-reporter, vitest
- Language: JavaScript
- Homepage:
- Size: 78.1 KB
- Stars: 0
- Watchers: 1
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
Awesome Lists containing this project
README
# test-coverage-calculation
**This is purely theoretical speculations about how code coverage may be calculated.**
It's all started with this code:
```js
export function comp(a, b) {
if (a > b) return 1;
if (a < b) return -1;
return 0;
}
```
Most of code coverage tools would say this code has 4 branches. Which seemed strange to me. So I wondered why...
## Comparison of branch coverage
| | `@vitest/coverage-istanbul` | `vitest-monocart-coverage` | `@vitest/coverage-v8` | what I expect |
| ------------- | --------------------------- | -------------------------- | --------------------- | ------------- |
| example0.1.js | 0/0 | 0/0 | 1/1 | 1/1 |
| example0.2.js | 2/2 | 2/2 | 2/2 | 2/2 |
| example1.1.js | 4/4 | 4/4 | 4/4 | 3/3 or 4/4 |
| example1.2.js | 4/4 | 4/4 | [3/3][v6300] | 4/4 |
| example1.3.js | 4/4 | 4/4 | [3/3][v6300] | 4/4 |
| example1.4.js | 6/6 | 6/6 | [4/4][v6300] | 6/6 |
| example1.5.js | 4/4 | 4/4 | [5/5][v6300] | 3/3 or 4/4 |
| example2.1.js | 4/4 | 4/4 | 4/4 | 3/3 or 4/4 |
| example3.1.js | 2/2 | 2/2 | 2/2 | 2/2 or 1/1 |
| example3.2.js | 2/2 | 2/2 | 2/2 | 2/2 or 1/1 |
| example3.3.js | 2/2 | 2/2 | **3/3** | 2/2 or 1/1 |
| example3.4.js | 4/4 | 4/4 | **3/3** | 2/2 or 4/4 |
| example4.1.js | 2/2 | 2/2 | **3/3** | 2/2 |
| example4.2.js | [1/1][i795] | **1/1** | 2/2 | 2/2 |
| example4.3.js | 2/2 | 2/2 | 2/2 | 2/2 |
| example4.4.js | 3/3 | 3/3 | 3/3 | 3/3 |
| example5.1.js | 0/0 | 0/0 | 2/2 | 2/2 |
| example6.1.js | 0/0 | 0/0 | 2/2 | 2/2 or 1/1 |
| example6.2.js | 0/0 | 0/0 | 2/2 | 2/2 |
| example7.1.js | [0/0][i516] | 0/0 | 2/2 | 2/2 |
| example7.2.js | [0/0][i516] | 0/0 | 1/2 | 1/2 |
| example8.1.js | 0/0 | 0/0 | **1/2** | 1/3 |
**Note**: if you have 100% coverage you probably don't care if it is 3/3 or 5/5. This would make a difference it you have less than 100%, than numbers can be skewed.
[mcr68]: https://github.com/cenfun/monocart-coverage-reports/issues/68
[i795]: https://github.com/istanbuljs/istanbuljs/issues/795
[v6300]: https://github.com/vitest-dev/vitest/issues/6300
[i516]: https://github.com/istanbuljs/istanbuljs/issues/516
## Example 1
1 branch (or no branching):
```js
console.log(1);
```
```mermaid
flowchart LR
s --> e
```
2 branches:
```js
console.log(1);
if (a) {
console.log(2);
}
```
```mermaid
flowchart LR
s(s) --- if["if(a)"] -- true --> e
if -- false --> e(e)
```
**Important** even so for `a = true` it would visit all lines of code, we as well need to execute code with `a = false` to claim that all cases have been covered.
And this is basically explains why it counts 4 branches here:
```js
export function comp(a, b) {
if (a > b) return 1;
if (a < b) return -1;
return 0;
}
```
| | `a > b` | `a < b` |
| --- | ------- | ------- |
| 1 | true | true |
| 2 | true | false |
| 3 | false | true |
| 4 | false | false |
**But** there is no difference between cases 1 and 2. If the first condition is true we will never reach code in the second condition, because of early return.
```mermaid
flowchart LR
s(s) --- if1["if(a > b)"] -- true --> e
if1 -- false --- if2[" if(a < b)"] -- true --> e
if2 -- false --> e(e)
```
On the other hand, code like this:
```js
export function comp(a, b) {
let result = 0;
if (a > b) result = 1;
if (a < b) result = -1;
return result;
}
```
Indeed has 4 branches:
```mermaid
flowchart LR
s(s) --- if1["if(a > b)"]
if1 -- true --> if2
if1 -- false --> if2
if2["if(a < b)"] -- true --> e
if2 -- false --> e(e)
```
### Branches != paths
In example above we have 4 branches and coincidentally 4 paths. All branches can be reached, but not all paths, because there are no such values that `a > b` and `a < b`.
Let's take a different example:
```js
export function experiment(a, b) {
let result = 0;
if (a) result += 1;
if (b) result += 2;
return result;
}
```
All branches can be covered with two tests:
```js
expect(experiment(false, false)).toBe(0);
expect(experiment(true, true)).toBe(3);
```
But to cover all paths you need 2 more tests:
```js
expect(experiment(true, false)).toBe(1);
expect(experiment(false, true)).toBe(2);
```
One more example:
```js
export function experiment(a, b, c) {
let result = 0;
if (a) result += 1;
if (b) result += 2;
if (c) result += 4;
return result;
}
```
It has 6 branches, but 8 paths.
### 100% branch or path coverage may be not enough
Let's take the same example, we started with:
```js
export function comp(a, b) {
if (a > b) return 1;
if (a < b) return -1;
return 0;
}
```
And write 100% test coverage:
```js
expect(comp(1, 1)).toBe(0);
expect(comp(1, 0)).toBe(1);
expect(comp(0, 1)).toBe(-1);
```
We still miss edge cases for `NaN`:
```js
(comp(1, 1) === comp(1, NaN)) === comp(NaN, 1);
```
Which may be not a desired behaviour.
## Example 2
```js
export function comp(a, b) {
let result;
if (a === b) result = 0;
else if (a > b) result = 1;
else result = -1;
return result;
}
```
This code has 3 or 4 branches (depending on how you define "branches"):
```mermaid
flowchart LR
s(s) --- if1["if(a === b)"]
if1 -- true --> e
if1 -- false --- if2
if2["if(a > b)"] -- true --> e
if2 -- false --> e(e)
```
## Example 3
So far we talked only about `if/else`. Let's talk about other "branching" constructs
```js
a && b();
// is the same as
if (a) b();
a || b();
// is the same as
if (!a) b();
a ? b() : c():
// is the same as
if (!a) b(); else c();
```
Which makes sense. But what about this example:
```js
if (a || b) {
console.log(1);
}
```
Using logic above this code can be estimated to have 4 branches. But it seems more natural to count it as 2 branches (4 paths?). WDYT?
With exceptions if second operand (`b`) is a function call (`b()`) or property accessor (`b.something`), which may be [a getter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get).
Shall we count code like this:
```js
let x = a ? 1 : 2;
```
as 2 branches or as 1 branch (but 2 paths)?
## Example 4
This should count as 2 branches:
```js
switch (a) {
case 1:
//...
break;
default:
//...
}
```
This is 2 branches as well:
```js
switch (a) {
case 1:
//...
break;
}
```
This is 2 branches as well:
```js
switch (a) {
case 1:
//...
default:
//...
}
```
This is 3 branches:
```js
switch (a) {
case 1:
//...
case 3:
//...
default:
//...
}
```
Is this 2 or 3 branches:
```js
switch (a) {
case 1:
case 3:
//...
default:
//...
}
```
## Example 5
This should count as 2 branches (?):
```js
try {
a(x);
b(y);
//...
} catch (e) {
//...
}
```
But what if each function (`a`, `b`) can throw an exception. Shall we count it as 3 (or 4) branches? On the other hand there is no way to know this from statical analysis unless we have type system with effects, like in [koka](https://koka-lang.github.io/koka/doc/book.html#why-effects).
## Example 6
This should count as 2 branches
```js
for (let i = 0; i < j; i++) {
//
}
```
Because depending on the value of `j` we may or may not "get inside" `for` statement. On the other hand - this is 1 branch:
```js
for (let i = 0; i < 10; i++) {
//
}
```
Same argument applies to `while`:
```js
while (j < 3) {
//...
}
```
`do` always counts as 1 branch
```js
do {
//...
} while (j < 3);
```
## Example 7
Do we count [optional chaining](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining) as branching?
```js
let x = a?.something;
```
It should be counted the same way as:
```js
let x = a == null ? undefined : a.something;
```
Do we count whole chain as 2 branches or do we add branch for each link:
```js
let x = a?.something?.else;
```
Same goes to [nullish coalescing](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing) and [nullish coalescing assignment](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_assignment)
## Example 8
Do we count each `yield` in [generator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator) as branch?
```js
const x = function* () {
yield "a";
yield "b";
yield "c";
};
```