Singleton

singleton.js

((doc, name)=>{
  var store = {};

  function memoize(f, cache){
    return function(k){
      if (k === null) return undefined;
      if (cache[k]) return cache[k];
      return cache[k] = f(k);
    }
  }

  function cnt(){
    return doc.getElementById(name) ||
      (()=>{
        var div = doc.createElement("div");
        div.id = name;
        doc.body.insertBefore(div, doc.body.childNodes[0]);
        return div;
      })();
  }

  var get = memoize(function(name){
    if (name === "htm") return htm();
    if (name === "cnt") return cnt();
    if (name === "log") return log();
    return -1;
  }, store);

  function htm(){
    return `
<div class="-cnt">
  <div class="-name">${name}</div>
  <pre class="-log">${get("log")}</pre>
</div>
<style>
  #${name} .-cnt {
    background-color: #ededed;
  }
  #${name} .-log {
    text-align: left;
  }
</style>
`;
  }

  function render(){
    if (get("cnt").innerHTML !== get("htm"))
      get("cnt").innerHTML = get("htm");
    return true;
  }   

  const Singleton = (()=>{
    var instance;
    return {
      get: ()=> instance || (()=>{
        return instance = new Date()
      })()
    };
  })();   

  function log(){
    function assert(val, expect){
      if (!expect) return val;
      return val === expect;
    }

    function it(describe, test) {
      try {
        var result = test();
      } catch (e) {
        return e;
      }
      
      if (result) {
        return `✓ ${describe}`;
      } else {
        return `:( ${describe}`;
      }
    }
  
var readyStatus = () => 1;
var test = ()=>{
  var a, b;
  a = Singleton.get();
  b = Singleton.get();
  return [a, b];    
}

    return `
${it("is ready", function() {
  return assert(readyStatus() , 1);
})}
${test.toString()}
${it("has 'a' equal to 'b'", function() {
  var result = test();
  return assert(result[0], result[0]);
})}
`;
  }  
  
  return render();
})(document, "Singleton");