Vue.js/test/helpers/to-have-been-warned.js

function noop () {}

if (typeof console === 'undefined') {
  window.console = {
    warn: noop,
    error: noop
  }
}

// avoid info messages during test
console.info = noop

let asserted

function createCompareFn (spy) {
  const hasWarned = msg => {
    var count = spy.calls.count()
    var args
    while (count--) {
      args = spy.calls.argsFor(count)
      if (args.some(containsMsg)) {
        return true
      }
    }

    function containsMsg (arg) {
      return arg.toString().indexOf(msg) > -1
    }
  }

  return {
    compare: msg => {
      asserted = asserted.concat(msg)
      var warned = Array.isArray(msg)
        ? msg.some(hasWarned)
        : hasWarned(msg)
      return {
        pass: warned,
        message: warned
          ? 'Expected message "' + msg + '" not to have been warned'
          : 'Expected message "' + msg + '" to have been warned'
      }
    }
  }
}

// define custom matcher for warnings
beforeEach(() => {
  asserted = []
  spyOn(console, 'warn')
  spyOn(console, 'error')
  jasmine.addMatchers({
    toHaveBeenWarned: () => createCompareFn(console.error),
    toHaveBeenTipped: () => createCompareFn(console.warn)
  })
})

afterEach(done => {
  const warned = msg => asserted.some(assertedMsg => msg.toString().indexOf(assertedMsg) > -1)
  let count = console.error.calls.count()
  let args
  while (count--) {
    args = console.error.calls.argsFor(count)
    if (!warned(args[0])) {
      done.fail(`Unexpected console.error message: ${args[0]}`)
      return
    }
  }
  done()
})