node_modules/axios/lib/helpers/resolveConfig.js

import platform from "../platform/index.js";
import utils from "../utils.js";
import isURLSameOrigin from "./isURLSameOrigin.js";
import cookies from "./cookies.js";
import buildFullPath from "../core/buildFullPath.js";
import mergeConfig from "../core/mergeConfig.js";
import AxiosHeaders from "../core/AxiosHeaders.js";
import buildURL from "./buildURL.js";

export default (config) => {
  const newConfig = mergeConfig({}, config);

  let { data, withXSRFToken, xsrfHeaderName, xsrfCookieName, headers, auth } = newConfig;

  newConfig.headers = headers = AxiosHeaders.from(headers);

  newConfig.url = buildURL(buildFullPath(newConfig.baseURL, newConfig.url, newConfig.allowAbsoluteUrls), config.params, config.paramsSerializer);

  // HTTP basic authentication
  if (auth) {
    headers.set('Authorization', 'Basic ' +
      btoa((auth.username || '') + ':' + (auth.password ? unescape(encodeURIComponent(auth.password)) : ''))
    );
  }

  if (utils.isFormData(data)) {
    if (platform.hasStandardBrowserEnv || platform.hasStandardBrowserWebWorkerEnv) {
      headers.setContentType(undefined); // browser handles it
    } else if (utils.isFunction(data.getHeaders)) {
      // Node.js FormData (like form-data package)
      const formHeaders = data.getHeaders();
      // Only set safe headers to avoid overwriting security headers
      const allowedHeaders = ['content-type', 'content-length'];
      Object.entries(formHeaders).forEach(([key, val]) => {
        if (allowedHeaders.includes(key.toLowerCase())) {
          headers.set(key, val);
        }
      });
    }
  }  

  // Add xsrf header
  // This is only done if running in a standard browser environment.
  // Specifically not if we're in a web worker, or react-native.

  if (platform.hasStandardBrowserEnv) {
    withXSRFToken && utils.isFunction(withXSRFToken) && (withXSRFToken = withXSRFToken(newConfig));

    if (withXSRFToken || (withXSRFToken !== false && isURLSameOrigin(newConfig.url))) {
      // Add xsrf header
      const xsrfValue = xsrfHeaderName && xsrfCookieName && cookies.read(xsrfCookieName);

      if (xsrfValue) {
        headers.set(xsrfHeaderName, xsrfValue);
      }
    }
  }

  return newConfig;
}