Angular 5 synchronous HTTP call

Try using await/async

async getResult(): Promise<MyCustomObject> {
    if (typeof this.result === 'undefined') 
    {
        // save result
        this.result = await this.service.call()
        .toPromise()
        .then(resp =>resp as MyCustomObject);//Do you own cast here

    }
    return this.result;
}