JS: Declarative FormData

index.js

/**
 * Declarative FormData
 *
 * @usage formdata({ username: '' });
 * @param {object} obj
 * @return FormData
 */
export default function formdata(obj) {
  const form = new FormData();
  const flattened = flattenKeys(obj)
  
  Object.keys(flattened).forEach((key) => {
    form.append(key, flattened[key]);
  })

  return form;
}

function flattenKeys(obj, parent = true) {
  let output = {}
  
  function flatten(obj, parentKey) {
    Object.keys(obj).forEach(key => {
      const value = obj[key]

      const newKey = parentKey
        ? `${parentKey}[${key}]`
        : key
  
      if ((typeof value === 'object' || Array.isArray(value)) && !(value instanceof File)) {
        flatten(value, newKey)
      } else {
        output[newKey] = value
      }
    })
  }

  flatten(obj)
  
  return output
}