A solution for https://github.com/github/fetch/issues/263

zepto-post-param.js

// codes from https://github.com/madrobby/zepto/blob/601372ac4e3f98d502c707bf841589fbc48a3a7d/src/ajax.js#L344-L37
// serialize the form data for `Content-Type: application/x-www-form-urlencoded`
//
// Usage Example:
// ```javascript
// const YOUR_DATA = {foo: "bar", some: "baz"}
// fetch('/api/result', {
//   method: 'POST',
//   headers: {
//     'Content-Type': 'application/x-www-form-urlencoded; charset=utf-8'
//   },
//   body: param(YOUR_DATA)
// })
//  ```


function serialize(params, obj, traditional, scope) {
  let type;
  let array = Array.isArray(obj);
  let hash = isPlainObject(obj);

  Object.keys(obj).forEach(function(key) {
    let value = obj[key];
    type = typeof value;
    if (scope)
      key = traditional
        ? scope
        : scope +
          '[' +
          (hash || type == 'object' || type == 'array' ? key : '') +
          ']';
    // handle data in serializeArray() format
    if (!scope && array) params.add(value.name, value.value);
    else if (type == 'array' || (!traditional && type == 'object'))
      // recurse into nested objects
      serialize(params, value, traditional, key);
    else params.add(key, value);
  });
}

function param(obj, traditional) {
  let params = [];
  params.add = function(key, value) {
    if (typeof value === 'function') value = value();
    if (value == null) value = '';
    this.push(encodeURIComponent(key) + '=' + encodeURIComponent(value));
  };
  serialize(params, obj, traditional);
  return params.join('&').replace(/%20/g, '+');
}