Source: client/script-loader.js

/**
Script loader
 */
export class ScriptLoader {
  static instance = null;
  constructor(contentBase) {
    // script url, loaded false/true
    this.scripts = {};
    this.contentBase = contentBase;
  }

  static getInstance(contentBase) {
    if ( !ScriptLoader.instance ) {
      ScriptLoader.instance = new ScriptLoader(contentBase);
    }
    return ScriptLoader.instance;
  }

  /** 
  Add a script to load path
  @param script url to load the script from
   */
  add(script) {
    if (typeof this.scripts[script] === 'undefined' ) {
      this.scripts[script] = false;
    }
    return this;
  }
  
  /**
  Load all scripts
  @param parallel default false - wait for each one to load before loading the next one
   */
  async load(parallel = false) {
    for ( var script in this.scripts ) {
      if ( ! this.scripts[script] ) {
        await this.loadScript(script, parallel);
        // debug to confirm scripts load only once
        //console.log("Loaded script "+script);
      }
    }
  }
  
  async loadScript(path, parallel) {
    return new Promise( (resolve, reject) => {
      const script = document.createElement('script');
      script.src = path;
      if (parallel) {
        document.body.appendChild(script);
        this.scripts[path] = true;
        resolve();
      } else {
        script.async = true;
        document.body.appendChild(script);
        script.onload = () => { 
          this.scripts[path] = true;
          resolve(); 
        }
      }
    });
  }
  
  /** 
  Utility method - load a script and append it to document head
  @param urls array containing URLs of scripts
  @param parallel optionally load in parallel
  */
  async loadScriptsToDocument(urls, parallel) {
    if ( Array.isArray(urls) ) {
      urls.forEach((url) => this.addScript(url));
    } else {
      this.addScript(urls);
    }
    return this.load(parallel);
  }
  
  addScript(url) {
    if ( url.startsWith('/') && this.contentBase ) {
      url = this.contentBase+url;
    }
    this.add(url);
  }
  
}