index.js

(function (global, factory) {
  if (typeof require === "function" && typeof exports === "object" && typeof module === "object") {
    // CommonJS support
    module.exports = factory();
  } else if (typeof define === "function" && define.amd) {
    // Do AMD support
    define(["LoadJS"], factory);
  } else {
    // Do browser support
    global.LoadJS = factory();
  }
})(this, function () {
  var cache = {};
  var head = document.getElementsByTagName("head")[0] || document.documentElement;

  function exec(options) {
    if (typeof options === "undefined") {
      return Promise.resolve();
    }

    if (typeof options === "string") {
      options = {
        url: options
      };
    }

    var cacheId = options.id || options.url;
    var cacheEntry = cache[cacheId];

    if (cacheEntry) {
      // console.log("load-js: cache hit", cacheId);
      return cacheEntry;
    }
    else if (options.allowExternal !== false) {
      var el = getScriptById(options.id) || getScriptByUrl(options.url);

      if (el) {
        var promise = Promise.resolve(el);

        if (cacheId) {
          cache[cacheId] = promise;
        }

        return promise;
      }
    }

    if (!options.url && !options.text) {
      throw new Error("load-js: must provide a url or text to load");
    }

    var pending = (options.url ? loadScript : runScript)(head, createScript(options));

    if (cacheId && options.cache !== false) {
      cache[cacheId] = pending;
    }

    return pending;
  }

  function runScript(head, script) {
    head.appendChild(script);
    return Promise.resolve(script);
  }

  function loadScript(head, script) {
    return new Promise(function (resolve, reject) {
      // Handle Script loading
      var done = false;

      // Attach handlers for all browsers.
      //
      // References:
      // http://stackoverflow.com/questions/4845762/onload-handler-for-script-tag-in-internet-explorer
      // http://stevesouders.com/efws/script-onload.php
      // https://www.html5rocks.com/en/tutorials/speed/script-loading/
      //
      script.onload = script.onreadystatechange = function () {
        if (!done && (!script.readyState || script.readyState === "loaded" || script.readyState === "complete")) {
          done = true;

          // Handle memory leak in IE
          script.onload = script.onreadystatechange = null;
          resolve(script);
        }
      };

      script.onerror = reject;

      head.appendChild(script);
    });
  }

  function createScript(options) {
    options.type = options.type || detectScriptType(options.url);
    if (options.type == "text/css") {
      var script = document.createElement("link");
      script.rel = "stylesheet";
      script.href = options.url;
    } else {
      var script = document.createElement("script");
      script.charset = options.charset || "utf-8";
      script.async = !!options.async;

      if (options.url) {
        script.src = options.url;
      }

      if (options.text) {
        script.text = options.text;
      }
    }

    script.type = options.type;
    script.id = options.id || options.url;
    script.loadJS = "watermark";

    return script;
  }

  function detectScriptType(url) {
    return url.includes('.css') ? "text/css" : "text/javascript";
  }

  function getScriptById(id) {
    var script = id && document.getElementById(id);

    if (script && script.loadJS !== "watermark") {
      console.warn("load-js: duplicate script with id:", id);
      return script;
    }
  }

  function getScriptByUrl(url) {
    var script = url &&
      (document.querySelector("script[src='" + url + "']") ||
        document.querySelector("link[href='" + url + "']"));

    if (script && script.loadJS !== "watermark") {
      console.warn("load-js: duplicate script with url:", url);
      return script;
    }
  }

  return function load(items) {
    return items instanceof Array ?
      Promise.all(items.map(exec)) :
      exec(items);
  }
});