Key derivation in NodeJS

derive.js

const { pbkdf2: deriveKey } = require("pbkdf2");

const HMAC_KEY_SIZE = 32;
const PASSWORD_KEY_SIZE = 32;

function pbkdf2(password, salt, rounds, bits) {
    return new Promise((resolve, reject) => {
        deriveKey(password, salt, rounds, bits / 8, "sha256", (err, key) => {
            if (err) {
                return reject(err);
            }
            return resolve(key);
        });
    });
}

function deriveFromPassword(password, salt, rounds) {
    if (!password) {
        return Promise.reject(new Error("Failed deriving key: Password must be provided"));
    }
    if (!salt) {
        return Promise.reject(new Error("Failed deriving key: Salt must be provided"));
    }
    if (!rounds || rounds <= 0 || typeof rounds !== "number") {
        return Promise.reject(new Error("Failed deriving key: Rounds must be greater than 0"));
    }
    const bits = (PASSWORD_KEY_SIZE + HMAC_KEY_SIZE) * 8;
    return pbkdf2(password, salt, rounds, bits)
        .then(derivedKeyData => derivedKeyData.toString("hex"))
        .then(function(derivedKeyHex) {
            const dkhLength = derivedKeyHex.length;
            const keyBuffer = new Buffer(derivedKeyHex.substr(0, dkhLength / 2), "hex");
            const output = {
                salt: salt,
                key: keyBuffer,
                rounds: rounds,
                hmac: new Buffer(derivedKeyHex.substr(dkhLength / 2, dkhLength / 2), "hex")
            };
            return output;
        });
}