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
甚至可以像这样编码解决方案,例如:
var loop = function(i, data, callback) {
if (i < data.length) {
//TODO("SELECT * FROM stackoverflowUsers;", function(res) {
//data[i].meta = res;
console.log(i, data[i].title);
return loop(i+1, data, errors, callback);
//});
} else {
return callback(data);
}
};
loop(0, [{"title": "hello"}, {"title": "world"}], function(data) {
console.log("DONE\n"+data);
});
另一方面,它比“for”慢得多。
否则,优秀的Async库可以这样做:https://caolan.github.io/async/docs.html#each
其他回答
甚至可以像这样编码解决方案,例如:
var loop = function(i, data, callback) {
if (i < data.length) {
//TODO("SELECT * FROM stackoverflowUsers;", function(res) {
//data[i].meta = res;
console.log(i, data[i].title);
return loop(i+1, data, errors, callback);
//});
} else {
return callback(data);
}
};
loop(0, [{"title": "hello"}, {"title": "world"}], function(data) {
console.log("DONE\n"+data);
});
另一方面,它比“for”慢得多。
否则,优秀的Async库可以这样做:https://caolan.github.io/async/docs.html#each
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
});
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()
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
不,它堵塞了。看一下算法的说明。
然而,MDN上给出了一个可能更容易理解的实现:
if (!Array.prototype.forEach)
{
Array.prototype.forEach = function(fun /*, thisp */)
{
"use strict";
if (this === void 0 || this === null)
throw new TypeError();
var t = Object(this);
var len = t.length >>> 0;
if (typeof fun !== "function")
throw new TypeError();
var thisp = arguments[1];
for (var i = 0; i < len; i++)
{
if (i in t)
fun.call(thisp, t[i], i, t);
}
};
}
如果你必须为每个元素执行大量代码,你应该考虑使用不同的方法:
function processArray(items, process) {
var todo = items.concat();
setTimeout(function() {
process(todo.shift());
if(todo.length > 0) {
setTimeout(arguments.callee, 25);
}
}, 25);
}
然后用:
processArray([many many elements], function () {lots of work to do});
这将是非阻塞的。这个例子来自于高性能JavaScript。
另一个选择可能是网络工作者。