const v8toIstanbul = require('v8-to-istanbul')
// create a script object from a source-file. the source file
// is loaded from disk and this is used to determine the original
// line count.
const script = v8toIstanbul('./path-to-instrumented-file.js')
// provide an array of coverage information in v8 format.
script.applyCoverage([
  {
    "functionName": "",
    "ranges": [
      {
        "startOffset": 0,
        "endOffset": 520,
        "count": 1
      }
    ],
    "isBlockCoverage": true
  },
  // ...
])
// output coverage information in a form that can
// be consumed by Istanbul.
console.info(JSON.stringify(script.toIstanbul()))