Serializes a form's elements into a ready-to-send POST body string

serialize-multipart-form.js

const serializeMultipart = async form => {
  const data = {}

  const formdata = Array.from(form.querySelectorAll('input, select, textarea, button')).map(input => {
    if (input.disabled) return false

    switch (input.nodeName) {
      case 'INPUT':
        switch (input.type.toLowerCase()) {
          case 'checkbox':
          case 'radio':
            if (input.selected) {
              data[input.name] = input.value
            }
            break

          case 'file':
            data[input.name] = []
            const promises = Array.prototype.slice.call(input.files).map(file => {
              return new Promise((resolve, reject) => {
                const fr = new FileReader()
                fr.onload = () => {
                  data[input.name].push({
                    file: file,
                    binary: fr.result
                  })
                  resolve()
                }
                fr.onerror = e => {
                  console.error(e)
                  reject(e)
                }
                fr.readAsBinaryString(file)
              })
            })

            return Promise.all(promises)

          default:
            data[input.name] = input.value
        }
        break

      case 'TEXTAREA':
        data[input.name] = input.value
        break

      case 'SELECT':
        if (input.type === 'select-one') {
          data[input.name] = input.value
        } else {
          data[input.name] = []
          for (const option of input.childNodes) {
            if (option.selected) {
              data[input.name].push(option.value)
            }
          }
        }
        break

      case 'BUTTON':
        data[input.name] = input.value
        break
    }

    return true
  })

  await Promise.all(formdata)

  delete data['']

  const output = []
  const boundary = '--Boundary-' + Math.round(Math.random() * 100000000)
  for (const name in data) {
    if (Array.isArray(data[name])) {
      for (const item of data[name]) {
        if (typeof item === 'object') {
          output.push('Content-Disposition: form-data name="' + name + '" filename="' + item.file.name + '"\r\nContent-Type: ' + item.file.type + '\r\n\r\n' + item.binary.toString())
        } else {
          output.push('Content-Disposition: form-data name="' + name + '"\r\n\r\n' + data[name])
        }
      }
    } else {
      output.push('Content-Disposition: form-data name="' + name + '"\r\n\r\n' + data[name])
    }
  }

  return {
    boundary: boundary,
    object: data,
    string () { return boundary + '\r\n' + output.join('\r\n' + boundary + '\r\n') + '\r\n' + boundary }
  }
}