Convert `web3` asynchronous calls into Promises.

promisifyWeb3.js

module.exports = {
    promisify: function (web3) {
        // Pipes values from a Web3 callback.
        var callbackToResolve = function (resolve, reject) {
            return function (error, value) {
                    if (error) {
                        reject(error);
                    } else {
                        resolve(value);
                    }
                };
        };

        // List synchronous functions masquerading as values.
        var syncGetters = {
            db: [],
            eth: [ "accounts", "blockNumber", "coinbase", "gasPrice", "hashrate",
                "mining", "protocolVersion", "syncing" ],
            net: [ "listening", "peerCount" ],
            personal: [ "listAccounts" ],
            shh: [],
            version: [ "ethereum", "network", "node", "whisper" ]
        };

        Object.keys(syncGetters).forEach(function(group) {
            Object.keys(web3[group]).forEach(function (method) {
                if (syncGetters[group].indexOf(method) > -1) {
                    // Skip
                } else if (typeof web3[group][method] === "function") {
                    web3[group][method + "Promise"] = function () {
                        var args = arguments;
                        return new Promise(function (resolve, reject) {
                            args[args.length] = callbackToResolve(resolve, reject);
                            args.length++;
                            web3[group][method].apply(web3[group], args);
                        });
                    };
                }
            });
        });
    },
};

@renexdev You have to import PromisifyWeb3 like @xavierlepretre mentioned in the post above yours:

const PromisifyWeb3 = require("./promisifyWeb3.js");
PromisifyWeb3.promisify(web3);