Code Coverage: Uncovered lines/statements in file that only defines interfaces.

• Issue

In a .ts file that contains only interface definitions/exports, code coverage reports uncovered lines and statements, even if the interface(s) are imported by unit tests. The mapping of uncovered code also appears to be nonsense.

Example file content:

/**
 * An interface.
 */
export interface MyInterface {
    name: string;
}

Resulting code coverage report:

Resulting code coverage JSON for this file:

{
    "path": "***redacted***/coverage-test.ts",
    "statementMap": {
        "0": {
            "start": {
                "line": 1,
                "column": 13
            },
            "end": {
                "line": 1,
                "column": 42
            }
        },
        "1": {
            "start": {
                "line": 1,
                "column": 42
            },
            "end": {
                "line": 1,
                "column": 55
            }
        },
        "2": {
            "start": {
                "line": 2,
                "column": 0
            },
            "end": {
                "line": 2,
                "column": 62
            }
        }
    },
    "fnMap": {},
    "branchMap": {},
    "s": {
        "0": 0,
        "1": 0,
        "2": 0
    },
    "f": {},
    "b": {}
}

• Expected behavior

The file has no executable lines/statements, so there should be nothing reported as “not covered”. It should report 0/0 (100%) coverage for both lines and statements.

• Link to a minimal repo that reproduces this issue

Should be simple enough to reproduce in ANY existing repo.

[marceloavf]

For me, it works when I set this option true in tsconfig.json

...
"sourceMap": true,
...

[JoshuaKGoldberg]

coveragePathIgnorePatterns works but isn’t enough.

Normally, we shouldn’t have to modify config files to write code outside of project setup. Someone adding code to a project could be very confused why their new interfaces-only file (or existing file that becomes interfaces-only). Also:

• It would be easy to forget to remove the listing in the config when adding real code to a file that used to be interfaces-only.
• Some larger projects have a whole bunch of interface-only files scattered throughout the project. It’s not maintainable to keep a complete list of them in JSON.