I think this could be a possible solution:

const fs = require("fs");

let files = fs.readdirSync("library");

function mergeJson() {
  const output = files.reduce((acum, current) => {
    data = fs.readFileSync(`./library/${current}`);
    const newData = JSON.parse(data);

    return {
      ...acum,
      ...newData,
    };
  }, {});

  if (output) {
    fs.writeFileSync(`combine.json`, JSON.stringify(output), { flag: "a" });
  }
}

mergeJson();

Can do something like this using Promise:

const fs = require('fs');
const data = {
    "passed": 0,
    "fixtures": []
};
const dir = `${__dirname}/data/`;
fs.readdir(dir, (err, files) => {
    return new Promise((resolve, reject) => {
        if (err) reject(err);
        files.forEach(file => {
           console.log(file);
           let content = require(`${dir}${file}`);
           data['passed'] += content.passed;
           data['fixtures'] = data['fixtures'].concat(content['fixtures']);
        });
        resolve(data);
    }).then(data => {
        fs.writeFileSync('./final.json',JSON.stringify(data));
    });
})

Using Async/Await it can done like this:

const fs = require('fs');
const path = require('path');
const dir = path.join(__dirname, 'data');

let finalContent = { "fixtures": [], "passed": 0 };
const read_directory = async dir =>
    fs.readdirSync(dir).reduce((finalContent, file) => {
        filePath = path.join(dir, file);
        console.log(filePath);
        let content = require(filePath);
        finalContent.passed += content.passed;
        finalContent.fixtures = finalContent.fixtures.concat(content.fixtures);
        return finalContent;
    }, { "passed": 0, "fixtures": [] });

read_directory(dir).then(data => {
    fs.writeFileSync('./final.json', JSON.stringify(data));
});