How to use the fusion-core.assetUrl function in fusion-core

// @flow
import App from 'fusion-core';
import {assetUrl} from 'fusion-core';
import * as jsonData from './static/test.json';

import serverAsset from './server-asset.js';

const hoistedUrl = assetUrl('./static/test.css');
if (typeof window !== 'undefined') {
  window.__hoistedUrl__ = hoistedUrl;
}

export default (async function() {
  const app = new App('element', el => el);
  __NODE__ &&
    app.middleware((ctx, next) => {
      if (ctx.url.startsWith('/_static')) {
        ctx.set('x-test', 'test');
      } else if (ctx.url === '/test') {
        ctx.body = assetUrl('./static/test.css');
      } else if (ctx.url === '/dirname') {
        ctx.body = __dirname;
      } else if (ctx.url === '/filename') {
        ctx.body = __filename;

// @noflow
import {assetUrl} from 'fusion-core';

export default assetUrl('./static/test-server-asset.txt');

// @noflow
import App from 'fusion-core';
import {assetUrl} from 'fusion-core';
import {key as jsonField} from './static/test.json';

import serverAsset from './server-asset.js';

const hoistedUrl = assetUrl('./static/test.css');
if (typeof window !== 'undefined') {
  window.__hoistedUrl__ = hoistedUrl;
}

export default (async function() {
  const app = new App('element', el => el);
  __NODE__ &&
    app.middleware((ctx, next) => {
      if (ctx.url.startsWith('/_static')) {
        ctx.set('x-test', 'test');
      } else if (ctx.url === '/test') {
        ctx.body = assetUrl('./static/test.css');
      } else if (ctx.url === '/dirname') {
        ctx.body = __dirname;
      } else if (ctx.url === '/filename') {
        ctx.body = __filename;

app.middleware((ctx, next) => {
      if (ctx.url.startsWith('/_static')) {
        ctx.set('x-test', 'test');
      } else if (ctx.url === '/test') {
        ctx.body = assetUrl('./static/test.css');
      } else if (ctx.url === '/dirname') {
        ctx.body = __dirname;
      } else if (ctx.url === '/filename') {
        ctx.body = __filename;
      } else if (ctx.url === '/hoisted') {
        ctx.body = hoistedUrl;
      } else if (ctx.url === '/json') {
        ctx.body = assetUrl('./static/test.json');
      } else if (ctx.url === '/json-import') {
        ctx.body = JSON.stringify(jsonData);
      } else if (ctx.url === '/server-asset') {
        ctx.body = serverAsset;
      }
      return next();
    });

componentDidMount() {
    fetch(assetUrl('../static/test.json'))
      .then(result => result.json())
      .then(result => {
        this.setState({
          content: result.content,
        });
      });
  }

export default async function() {
  const app = new App('element', el => el);
  assetUrl('./assets/SVG_logo.svg');
  return app;
}

export const serverAsset = () => assetUrl('./assets/server-asset.txt');

app.middleware((ctx, next) => {
      if (ctx.url.startsWith('/_static')) {
        ctx.set('x-test', 'test');
      } else if (ctx.url === '/test') {
        ctx.body = assetUrl('./static/test.css');
      } else if (ctx.url === '/dirname') {
        ctx.body = __dirname;
      } else if (ctx.url === '/filename') {
        ctx.body = __filename;
      } else if (ctx.url === '/hoisted') {
        ctx.body = hoistedUrl;
      } else if (ctx.url === '/json') {
        ctx.body = assetUrl('./static/test.json');
      } else if (ctx.url === '/json-import') {
        ctx.body = JSON.stringify(jsonData);
      } else if (ctx.url === '/server-asset') {
        ctx.body = serverAsset;
      }
      return next();
    });

componentDidMount() {
    fetch(assetUrl('../static/test.json'))
      .then(result => result.json())
      .then(result => {
        this.setState({
          content: result.content,
        });
      });
  }

middleware: () => {
    const todoCssUrl = assetUrl("../../node_modules/todomvc-app-css/index.css");

    return (ctx, next) => {
      applyTodoMvcStyling(ctx, todoCssUrl);
      return next();
    };
  }
});