study-promise.js

function test() {
  return new Promise(function(resolve, reject) {
    var r = Math.random();
    r > 0.5 ? resolve(r) : reject(r);
  });
}

function throwError() {
  return new Promise(function(resolve, reject) {
    throw new Error("Boom!");
  });
}

for(var i = 0; i < 20; i++) {
 test().then(function(v) {
    console.log("resolved", v);
  }).catch(function(v) {
    console.log("rejected", v);
  });
}

throwError().then(function(v) {
  console.log("resoleved", v);
}).catch(function(v) {
  console.log("rejected", v);
});