incomplete promise impl

promise.js

function MyPromise(fn) {
  let status = 0; // 0 = unfulfilled, 1 = resolved, 2 = rejected
  
  function makeQueue(neededStatus) {
  	return {
      neededStatus,
      fns: [],
      do() {
        if (status === neededStatus) {
          if (neededStatus === 2 && this.fns.length === 0) {
            console.error('uncaught error');
          }          
          while(this.fns.length) {
            this.fns.shift()(this.value);
          }
        }
      },
      handle(v) {
        if (status) throw Error('can not set, already handled');
        status = neededStatus;
        this.value = v;
        this.do();
      },
    };
  }
  
  const thenQueue = makeQueue(1);
  const catchQueue = makeQueue(2);
  const resolve = thenQueue.handle.bind(thenQueue);
  const reject = catchQueue.handle.bind(catchQueue);
    
  function makeChain(queue) {
    return function(fn) {
      let fns;
      const promise = new MyPromise(function(resolve, reject) {
        fns = [undefined, resolve, reject];
      });
      queue.fns.push(function(r) {
        fns[queue.neededStatus](fn(r));
      });
      queue.do();
      return promise;
    };
  }
  
  this.then = makeChain(thenQueue);
  this.catch = makeChain(catchQueue);
  
  try {
    fn(resolve, reject);
  } catch (e) {
    reject(e);
  }
}