How to generate full coverage data in out.json for all source files?

Question

I am using babel-plugin-istanbul to generate code coverage report, however, I found that the out.json file only includes the coverage data for tested source files as below:

"C:\\path\\to\\file\\a.ts": {
    "path": "C:\\path\\to\\file\\a.ts",
    "statementMap": {
        "0": {
            "start": {
                "line": 4,
                "column": 2
            },
            "end": {
                "line": 4,
                "column": 116
            }
        },
    }
    skip other data...
}

the coverage data for un-tested source files is missing as below:

"C:/path/to/file/b.ts": {
    "path": "C:/path/to/file/b.ts",
    "statementMap": {},
    "fnMap": {},
    "branchMap": {},
    "s": {},
    "f": {},
    "b": {}
}

Based on the out.json file, the generated coverage report only displays coverage for the tested source files and ignores un-tested source files, so the total coverage is incorrect.

May I know is there any method to generate all coverage data in out.json for all source files including tested and un-tested source files? Thanks in advance.