Property ‘allSettled’ does not exist on type ‘PromiseConstructor’.ts(2339)

The types for Promise.allSettled() were only merged in January, and will apparently be released in TypeScript 3.8.

As an interim solution, you can declare a mock-ish type for the function yourself:

declare interface PromiseConstructor {
    allSettled(promises: Array<Promise<any>>): Promise<Array<{status: 'fulfilled' | 'rejected', value?: any, reason?: any}>>;
}