Cannot read property 'some' of undefined

Gostaria de uma ajuda para resolver esse erro.

Controller

class NegotiationController {

    constructor() {
        let $ = document.querySelector.bind(document);

        this._inputDate = $('#data');
        this._inputAmount = $('#quantidade');
        this._inputValue = $('#valor');

        let self = this;
        this._currentOrder = '';

        this._negotiationView = new NegotiationView($('#negotiation-view'));
        this._listNegotiation = new BindHelper(new ListNegotiation(), 
                this._negotiationView, 'add', 'delete', 'order', 'inverseOrder');

        this._messageView = new MessageView($('#message-view'));
        this._message = new BindHelper(new Message(), this._messageView, 'msg');

        this._util = new NegotiationUtil();

        this._init();
    }

    _init() {
        this._util
            .list()
            .then(negotiations => {
                negotiations.forEach(negotiation => this._listNegotiation.add(negotiation))
            }).catch(err => this._message.msg = err);

        setInterval(() => {
            this.import();
        }, 3000);
    }

    import() {
        this._util
            .import(this._listNegotiation.list)
            .then(negotiations => {
                negotiations.forEach(negotiation => this._listNegotiation.add(negotiation));
            }).catch(err => this._message.msg = err);
    }

Util

class NegotiationUtil {

    constructor() {
        this._http = new HttpUtil();
    }

    negotiationsPromise() {
        return Promise.all([
            this._weekNegotiations(),
            this._weeksAgoNegotiations(),
            this._twoWeeksAgoNegotiations()
        ]).then(periods => periods.reduce((data, period) => data.concat(period), [])
        ).catch(err => {
            throw new Error(err);
        });
    }

    import(currentList) {
        return this.negotiationsPromise()
        .then(negotiations => {
            negotiations.filter(negotiation => {
                !currentList.some(negotiationExisting => {
                    JSON.stringify(negotiation) == JSON.stringify(negotiationExisting);
                });
            });
        }).catch(err => {
            console.log(err);
            throw new Error('Access daneid');
        });
    }