post-form.js

const formData = new FormData()
formData.append("hoge","hogehoge")
formData.append("foo","bar")

fetch("url", {
  headers:{
    'Accept': 'application/json, */*',
    //'Content-type':'application/json'    
  },
  method:"post",
  body:formData
})
.then(response => {
  return response.json().then(json => ({
    status: response.status,
  json }))
})
.then(({
    status,
    json
  }) => {
  if (callback) callback(status, json)
})

post-json.js

fetch("url", {
  headers:{
    'Accept': 'application/json, */*',
    'Content-type':'application/json'    
  },
  method:"post",
  body:JSON.stringify({hoge:"hogehoge",foo:"bar"})
})
.then(response => {
  return response.json().then(json => ({
    status: response.status,
  json }))
})
.then(({
    status,
    json
  }) => {
  if (callback) callback(status, json)
})