Intl.RelativeTimeFormat and Intl.ListFormat can't be spied upon

[amannn]

Describe the bug

When using vi.spyOn(Intl, 'RelativeTimeFormat') or vi.spyOn(Intl, 'ListFormat'), the constructor can no longer be called due to:

Constructor Intl.RelativeTimeFormat requires 'new'

Note that Intl.DateTimeFormat works flawlessly.

Reproduction

StackBlitz

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (6) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 18.20.3 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.2.3 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    @vitest/ui: latest => 2.0.2 
    vite: latest => 5.3.3 
    vitest: latest => 2.0.2 

Used Package Manager

npm

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[sheremet-va]

Respecting new in a constructor would be a breaking change as it will change the value of mock.instances. However, you can already use tinyspy directly:

import { spyOn } from 'tinyspy'
const spy = spyOn(Intl, 'RelativeTimeFormat')

[amannn]

Many thanks for the super quick response! That seems to work, thanks for sharing a workaround.

Any idea why this works for Intl.DateTimeFormat and Intl.NumberFormat then?

[sheremet-va]

Any idea why this works for Intl.DateTimeFormat and Intl.NumberFormat then?

Because they are not implemented as classes internally.

[amannn]

Oh right, that's interesting: