devdocs/assets/javascripts/lib/ajax.js

const MIME_TYPES = {
  json: "application/json",
  html: "text/html",
};

function ajax(options) {
  applyDefaults(options);
  serializeData(options);

  const xhr = new XMLHttpRequest();
  xhr.open(options.type, options.url, options.async);

  applyCallbacks(xhr, options);
  applyHeaders(xhr, options);

  xhr.send(options.data);

  if (options.async) {
    return { abort: abort.bind(undefined, xhr) };
  } else {
    return parseResponse(xhr, options);
  }
}

ajax.defaults = {
  async: true,
  dataType: "json",
  timeout: 30,
  type: "GET",
};
// contentType
// context
// data
// error
// headers
// progress
// success
// url

var applyDefaults = function (options) {
  for (var key in ajax.defaults) {
    if (options[key] == null) {
      options[key] = ajax.defaults[key];
    }
  }
};

var serializeData = function (options) {
  if (!options.data) {
    return;
  }

  if (options.type === "GET") {
    options.url += "?" + serializeParams(options.data);
    options.data = null;
  } else {
    options.data = serializeParams(options.data);
  }
};

var serializeParams = (params) =>
  Object.entries(params)
    .map(
      ([key, value]) =>
        `${encodeURIComponent(key)}=${encodeURIComponent(value)}`,
    )
    .join("&");

var applyCallbacks = function (xhr, options) {
  if (!options.async) {
    return;
  }

  xhr.timer = setTimeout(
    onTimeout.bind(undefined, xhr, options),
    options.timeout * 1000,
  );
  if (options.progress) {
    xhr.onprogress = options.progress;
  }
  xhr.onreadystatechange = function () {
    if (xhr.readyState === 4) {
      clearTimeout(xhr.timer);
      onComplete(xhr, options);
    }
  };
};

var applyHeaders = function (xhr, options) {
  if (!options.headers) {
    options.headers = {};
  }

  if (options.contentType) {
    options.headers["Content-Type"] = options.contentType;
  }

  if (
    !options.headers["Content-Type"] &&
    options.data &&
    options.type !== "GET"
  ) {
    options.headers["Content-Type"] = "application/x-www-form-urlencoded";
  }

  if (options.dataType) {
    options.headers["Accept"] =
      MIME_TYPES[options.dataType] || options.dataType;
  }

  for (var key in options.headers) {
    var value = options.headers[key];
    xhr.setRequestHeader(key, value);
  }
};

var onComplete = function (xhr, options) {
  if (200 <= xhr.status && xhr.status < 300) {
    let response;
    if ((response = parseResponse(xhr, options)) != null) {
      onSuccess(response, xhr, options);
    } else {
      onError("invalid", xhr, options);
    }
  } else {
    onError("error", xhr, options);
  }
};

var onSuccess = function (response, xhr, options) {
  if (options.success != null) {
    options.success.call(options.context, response, xhr, options);
  }
};

var onError = function (type, xhr, options) {
  if (options.error != null) {
    options.error.call(options.context, type, xhr, options);
  }
};

var onTimeout = function (xhr, options) {
  xhr.abort();
  onError("timeout", xhr, options);
};

var abort = function (xhr) {
  clearTimeout(xhr.timer);
  xhr.onreadystatechange = null;
  xhr.abort();
};

var parseResponse = function (xhr, options) {
  if (options.dataType === "json") {
    return parseJSON(xhr.responseText);
  } else {
    return xhr.responseText;
  }
};

var parseJSON = function (json) {
  try {
    return JSON.parse(json);
  } catch (error) {}
};