Today I Learned

TIL, 2018-05-16, Thinking about How Components can be Refactored


  • Warning: React.createElement: type is invalid -- expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in, or you might have mixed up default and named imports. => Got this from importing Tabs and Tab from react, not react-bootstrap.
  • JS can’t sort if there’s a null. You need to make them ‘’ if ever.
  • Destructuring a two level hash
class Listing extends Component {
  render() {
    const {
      listing: {
        location: {
    } = this.props;

return (
      <p>Location: {city}, {state}, {country}</p>
  • Stateless functional components: should not have functions inside lol. Bind them from outside. Reference
  • Prefetching, preloading, prebrowsing
    • Pre-fetching: A way to hint to the browser about resources that are going to or might be used in the future.
    • DNS pre-fetching: Notifies the client that there are assets we’ll need later from a specific URL so the browser can resolve the DNS as quickly as possible. <link rel="dns-prefetch" href="//">


  • map then tap is ugly but it works lol.


  • ASIC: Application-specific integrated circuit, an IC customized for a particular use (voice recording, Bitcoin mining).

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