我有一个关于原生数组的问题。JavaScript的每一个实现都是异步的吗? 例如,如果我调用:
[many many elements].forEach(function () {lots of work to do})
这将是非阻塞的吗?
我有一个关于原生数组的问题。JavaScript的每一个实现都是异步的吗? 例如,如果我调用:
[many many elements].forEach(function () {lots of work to do})
这将是非阻塞的吗?
当前回答
这些代码片段将使您更好地理解forEach和forOf的比较。
/* eslint-disable no-console */ async function forEachTest() { console.log('########### Testing forEach ################ ') console.log('start of forEachTest func') let a = [1, 2, 3] await a.forEach(async (v) => { console.log('start of forEach: ', v) await new Promise(resolve => setTimeout(resolve, v * 1000)) console.log('end of forEach: ', v) }) console.log('end of forEachTest func') } forEachTest() async function forOfTest() { await new Promise(resolve => setTimeout(resolve, 10000)) //just see console in proper way console.log('\n\n########### Testing forOf ################ ') console.log('start of forOfTest func') let a = [1, 2, 3] for (const v of a) { console.log('start of forOf: ', v) await new Promise(resolve => setTimeout(resolve, v * 1000)) console.log('end of forOf: ', v) } console.log('end of forOfTest func') } forOfTest()
其他回答
如果您需要一个异步友好的Array版本。对于ach和类似的,它们在Node.js 'async'模块中可用:http://github.com/caolan/async…作为奖励,这个模块也可以在浏览器中工作。
async.each(openFiles, saveFile, function(err){
// if any of the saves produced an error, err would equal that error
});
下面是一个小例子,你可以运行来测试它:
[1,2,3,4,5,6,7,8,9].forEach(function(n){
var sum = 0;
console.log('Start for:' + n);
for (var i = 0; i < ( 10 - n) * 100000000; i++)
sum++;
console.log('Ended for:' + n, sum);
});
它将生成如下内容(如果花费的时间太少/太多,可以增加/减少迭代次数):
(index):48 Start for:1
(index):52 Ended for:1 900000000
(index):48 Start for:2
(index):52 Ended for:2 800000000
(index):48 Start for:3
(index):52 Ended for:3 700000000
(index):48 Start for:4
(index):52 Ended for:4 600000000
(index):48 Start for:5
(index):52 Ended for:5 500000000
(index):48 Start for:6
(index):52 Ended for:6 400000000
(index):48 Start for:7
(index):52 Ended for:7 300000000
(index):48 Start for:8
(index):52 Ended for:8 200000000
(index):48 Start for:9
(index):52 Ended for:9 100000000
(index):45 [Violation] 'load' handler took 7285ms
虽然数组。forEach不是异步的,你可以得到异步的“最终结果”。在下面的例子:
delayFunction(x) { 归还新承诺( (resolve) => setTimeout(() => resolve(x), 1000) ); } [1,2,3].forEach(async(x) => { console.log (x); console.log(等待delayFunction (x)); });
在npm中有一个包,可以为每个循环提供简单的异步。
var forEachAsync = require('futures').forEachAsync;
// waits for one request to finish before beginning the next
forEachAsync(['dogs', 'cats', 'octocats'], function (next, element, index, array) {
getPics(element, next);
// then after all of the elements have been handled
// the final callback fires to let you know it's all done
}).then(function () {
console.log('All requests have finished');
});
这也是allasync的另一个变体
这是一个不需要第三方库就可以使用的简短异步函数
Array.prototype.each = function (iterator, callback) {
var iterate = function () {
pointer++;
if (pointer >= this.length) {
callback();
return;
}
iterator.call(iterator, this[pointer], iterate, pointer);
}.bind(this),
pointer = -1;
iterate(this);
};