tester.js

class Tester {
  constructor(app) {
    this.app = app
  }
  
  test(cases) {
    cases.forEach(tc => {
      if (app(tc.input) !== tc.output) {
        throw new Error()
      }      
    })
  }
}


function app(input) {
  if (input == '123') {
    return '456'
  }  
  return '123'
}

const cases = [{
  input: '123',
  output: '456'
}, {
  input: '234',
  output: '123'
}]

const tester = new Tester(app)
tester.test(cases)