const deferred = function() {
this.allFunc = [];
this.lastResult;
}
deferred.prototype.then = function (newFunc) {
this.allFunc.push(newFunc);
}
deferred.prototype.resolve = function (arg) {
this.lastResult = arg;
for(let i = 0; i < this.allFunc.length; i++) {
const item = this.allFunc[i];
const res = item(this.lastResult);
if (res instanceof deferred) {
const lastFuncs = this.allFunc.slice(i+1);
res.allFunc = lastFuncs;
break;
} else {
this.lastResult = res;
}
}
}

const d = new deferred();

d.then(function(res){
console.log('1', res);
var d1 = new deferred();
setTimeout(function(){ d1.resolve("a"); }, 3000);
return d1;
});

d.then(function(res){ console.log("2 ", res); return "b"; });
d.then(function(res){ console.log("3 ", res); return "c"; });
d.resolve("hello");