index.js

const walk = require('@bem/walk');
const config = require('bem-config')();
const istanbul = require('istanbul').utils;
const _ = require('lodash');

function getCoverage(level) {
    const files = {};

    return new Promise(function(resolve, reject) {
        walk([level], { levels: config.levelMapSync() })
            .on('data', filterFiles)
            .on('error', err => { reject(err); } )
            .on('end', () => { resolve(files); });

        function filterFiles(file) {
            if (file.cell.tech != 'test-result.json') return;

            let blockName = file.cell.entity.block;
            let obj = require(file.path);

            if (!obj.coverage) return;

            let coverage = istanbul.summarizeCoverage(obj.coverage);
            delete coverage.linesCovered;
            files[blockName] = coverage;
        }
    });
}

function getPct(coverage) {
    if (!coverage || !coverage.statements) return 0;

    return coverage.statements.pct || 0;
}

Promise.all([
    getCoverage('/Users/dima117a/projects/test.bundles'),
    getCoverage('/Users/dima117a/projects/test.bundles2')
]).then(function([obj1, obj2]) {

    var keys = _.uniq(Object.keys(obj1).concat(Object.keys(obj2)));

    var diffs = keys
        .map(function(blockName){
            let coverage1 = obj1[blockName];
            let coverage2 = obj2[blockName];

            let pct1 = getPct(coverage1);
            let pct2 = getPct(coverage2);

            let diff = Math.abs(pct2 - pct1);

            return {
                block: blockName,
                diff: diff,
                c1: coverage1,
                c2: coverage2
            };
        }).sort(function(el1, el2) { return el2.diff - el1.diff });

    console.log(JSON.stringify(diffs, null, 4));
});

package.json

{
  "name": "aa",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "@bem/walk": "^1.0.0-2",
    "bem-config": "^3.2.3",
    "bem-walk": "^1.0.0-1",
    "istanbul": "^0.4.5",
    "lodash": "^4.17.4"
  }
}