getModelCustomers(modelName) {
    const promise = new Promise((resolve, reject) => {
        this.getCustomers(modelName)
            .toPromise()
            .then(
                res => {
                    this.assetModel.addModelCustomers(res);
                    this.customers = {
                        data: this.getFormattedCustomers(this.assetModel.customers)
                    };

                    this.customersList.push(this.getFormattedCustomers(this.assetModel.customers));
                    resolve(this.assetModel.customers);
                },
                rej => {
                    reject(rej);
                }
            );
    });

    return promise;
}

I'm not sure how to write a test case for this code. Any help would be appreciated I'm stuck with the same