Import module function (assuming <script type="module"> is implemented)

0-usage.js

// Dynamic module loading using runtime-composed strings, decisions, etc.

for (const m of ["cool", "awesome", "fun", "whee"]) {
  if (Math.random() > 0.5) {
    importModule(`/js/${m}.js`).then(
      module => console.log("Module instance object for " + m, module),
      e => console.error(e)
    );
  }
}

import-module-current.js

function importModule(url) {
  return new Promise((resolve, reject) => {
    const script = document.createElement("script");
    const tempGlobal = "__tempModuleLoadingVariable" + Math.random().toString(32).substring(2);
    script.type = "module";
    script.textContent = `import * as m from "${url}"; window.${tempGlobal} = m;`;

    script.onload = () => {
      resolve(window[tempGlobal]);
      delete window[tempGlobal];
      script.remove();
    };
    
    script.onerror = () => {
      reject(new Error("Failed to load module script with URL " + url));
      delete window[tempGlobal];
      script.remove();
    };
    
    document.documentElement.appendChild(script);
  });
}

import-module-maybe.js

// Let's say we specced HTMLScriptElement.prototype.module to return the module instance object

function importModule(url) {
  return new Promise((resolve, reject) => {
    const script = document.createElement("script");
    script.type = "module";
    script.src = url;

    script.onload = () => {
      resolve(script.module);
      script.remove();
    };
    
    script.onerror = () => {
      reject(new Error("Failed to load module script with URL " + url));
      script.remove();
    };
    
    document.documentElement.appendChild(script);
  });
}

// Much less hacky... still have to insert into the document though.