自作promiseそれなりには動くはず

promise.js

function promise(fn){
	var self = {
		status: "pending",
		value: undefined,
		nexts: [],
		__proto__: promise.prototype
	}
	
	function common(value, status){
		if(self.status !== "pending") return
		self.status = status
		self.value = value
		self.doNexts()
	}
	var resolve = value => common(value, "resolved")
	var reject = value => common(value, "rejected")
	
	try{
		fn(resolve, reject)
	}catch(e){
		reject(e)
	}
	
	return self
}
promise.prototype.then = function(fn){
	return this._common(fn, "resolved")
}
promise.prototype.catch = function(fn){
	return this._common(fn, "rejected")
}
promise.prototype._common = function(fn, status){
	return promise((aa,bb) => {
		var f = (value) => {
			if(this.status === status){
				try{
					var result = fn(value)
				}catch(e){
					return bb(e)
				}
				if(result && result instanceof promise){
					result.then(e => aa(e))
					result.catch(e => bb(e))
				}else{
					aa(result)
				}
			}else{
				this.status === "resolved" ? aa(value) : bb(value)
			}
		}
		
		if(this.status === "pending"){
			this.nexts.push(f)
		}else{
			f(this.value)
		}
	})
}
promise.prototype.doNexts = function(){
	while(this.nexts.length) this.nexts.shift()(this.value)
}
promise.resolve = () => ({
	status: "resolved",
	value: undefined,
	nexts: [],
	__proto__: promise.prototype
})
promise.reject = () => ({
	status: "rejected",
	value: undefined,
	nexts: [],
	__proto__: promise.prototype
})

usage.js

var p = promise((a,b) => setTimeout(a, 1000, 12))
p.then(e => console.log(e))
p.then(e => console.log(e+1))
p.then(e => console.log(e+2))

// 12
// 13
// 14


var p = promise((a,b) => setTimeout(a, 1000, 16))
p.then(e => e / 4).then(e => e + 2).then(e => console.log(e * e))
// 36

var p = promise((a,b) => setTimeout(b, 1000, 16))
p.then(e => console.log(e)).catch(e => console.log(-e))
// -16

promise((a,b) => setTimeout(a, 1000, 1))
	.then(e => promise((a,b) => setTimeout(a, 1000, e+1)))
	.then(e => console.log(e))

// 2

promise((a,b) => a())
	.then(e => promise.reject())
	.catch(e => console.log(111))
// 111


promise((a,b) => a(1)).then(e => no_defined_value).catch(e => console.log(e))
// ReferenceError: no_defined_value is not defined(…)


promise((a,b) => no_defined_value).then(e => 1).catch(e => console.log(e))
// ReferenceError: no_defined_value is not defined(…)