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import { differenceWith } from './differenceWith.js'
test('happy', () => {  const foo = [ { a : 1 }, { a : 2 }, { a : 3 } ]  const bar = [ { a : 3 }, { a : 4 } ]  const fn = function (r, s){    return r.a === s.a  }  const result = differenceWith(    fn, foo, bar  )  expect(result).toEqual([ { a : 1 }, { a : 2 } ])})