vue/test/unit/features/instance/init.spec.ts

import Vue from 'vue'

describe('Initialization', () => {
  it('without new', () => {
    try {
      Vue()
    } catch (e) {}
    expect(
      'Vue is a constructor and should be called with the `new` keyword'
    ).toHaveBeenWarned()
  })

  it('with new', () => {
    expect(new Vue() instanceof Vue).toBe(true)
  })
})