异步控制流程的目的：

• 对异步操作提供类似同步遍历的操作
• 本文的Promise兼容均为在原本只支持node回调的方式下修改

基础方法

onlyOnce 只执行一次，第二次报错

• 只执行一次，因为第二次func变成了抛出错误

function onlyOnce(func) {return function(err, res) {var fn = func;func = throwError;fn(err, res);};
}

once 只执行一次，第二次无效

• 只执行一次，因为第二次func变成了空函数

 function once(func) {return function(err, res) {var fn = func;func = noop;fn(err, res);};}

iteratorSymbol 判断是否具有迭代器并返回迭代器

var iteratorSymbol = typeof Symbol === func && Symbol.iterator;

arrayEach 普通数组遍历

- iterator:每次遍历的回调- callback:内部有个计数器，执行遍历完成后的回调function arrayEach(array, iterator, callback) {var index = -1;var size = array.length;// 三个参数情况if (iterator.length === 3) {while (++index < size) {// 传入value、key、手动计数最后遍历完成回调（只有第一次执行有效）iterator(array[index], index, onlyOnce(callback));}} else { // 两个参数情况while (++index < size) {iterator(array[index], onlyOnce(callback));}}
}

baseEach 对象类型遍历

function baseEach(object, iterator, callback, keys) {var key;var index = -1;var size = keys.length;// 三个参数情况if (iterator.length === 3) {while (++index < size) {key = keys[index];iterator(object[key], key, onlyOnce(callback));}} else { // 两个参数情况while (++index < size) {iterator(object[keys[index]], onlyOnce(callback));}}
}

symbolEach 具有迭代器类型遍历

  function symbolEach(collection, iterator, callback) {var iter = collection[iteratorSymbol]();var index = 0;var item;// 三个参数情况if (iterator.length === 3) {while ((item = iter.next()).done === false) {iterator(item.value, index++, onlyOnce(callback));}} else {// 两个参数情况while ((item = iter.next()).done === false) {index++;iterator(item.value, onlyOnce(callback));}}return index;}

异步遍历

each

• 实现 forEach 效果
• 原理：每次回调中执行计数器，判断计数完毕时，执行结束回调

exports.each=createEach(arrayEach, baseEach, symbolEach)function createEach(arrayEach, baseEach, symbolEach) {return function each(collection, iterator, callback) {let promise=new Promise((resolve,reject)=>{callback = once(callback || noop);var size, keys;var completed = 0;if (isArray(collection)) {size = collection.length;arrayEach(collection, iterator, done);} else if (iteratorSymbol && collection[iteratorSymbol]) {size = symbolEach(collection, iterator, done);size && size === completed && callback(null);} else if (typeof collection === obj) {// Object.keyskeys = nativeKeys(collection);size = keys.length;baseEach(collection, iterator, done, keys);}if (!size) {callback(null);reject('value should be an object');}function done(err, bool) {if (err) {callback = once(callback);callback(err);reject(err);} else if (++completed === size) {callback(null);resolve('null')} else if (bool === false) {callback = once(callback);callback(null);reject('cancel');}}})return promise.then((res)=>res).catch(err=> Promise.reject(err))};}

基本使用：

const array = { a: 1, b: 3, c: 2 };;
const iterator = function(key, value,done) {setTimeout(function() {done()// done() done只有第一次执行有效// done(err)、done(null,false) 结束遍历并执行最终回调}, key * 1000);
};// 回调方式
nac.each(array, iterator, function(err) {console.log('done')
});// Promise方式
async function neo(){try {let res=await nac.each(array, iterator);console.log('success',res);    } catch (error) {console.log('error',error)}}