mouad.js

// suppose this is the array we want to send 

let arr = [
  {
    key: 'key1',
    value: 'value1'
  },
  {
    key: 'key2',
    value: 'value3'
  }
]

// client code

let str = {arr}

function sendWithAjax(url, method, data) {
  return new Promise((res, rej) => {
    let xhr = new XMLHttpRequest()
    if (xhr.readyState === 4) {
      if (xhr.status === 200) {
        res(xhr.response)
      } else {
        rej(xhr)
      }
    }
    xhr.open(url, method, true)
    xhr.send(data)
  })
}

sendWithAjax('/create', 'POST', JSON.stringify(str))
    .then(res => {
      // success here 
    })

// server code

app.post('/create', (req, res) => {
  let obj = req.body
  console.log('object is ', obj)
})