今天看网上关于promise的实现源码，看到Promise.all()的实现的时候有些不理解了，这段代码是如何确保所有的promise都被执行完之后再去调用resolve(result)的，代码如下，请大神指点。

Promise.all = function(promises){
        if (!isArray(promises)) {
            throw new TypeError('You must pass an array to all.');
        }
        return Promise(function(resolve,reject){
            var i = 0,
                result = [],
                len = promises.length;

            function resolver(index) {
                return function(value) {
                    resolveAll(index, value);
                };
            }

            function rejecter(reason){
                reject(reason);
            }

            function resolveAll(index,value){
                result[index] = value;
                if(index == len - 1){
                    resolve(result);
                }
            }

            for (; i < len; i++) {
                promises[i].then(resolver(i),rejecter);
            }
        });
    }