DDGarfield 2022-06-23 19:02:41 阅读数:779
javascript in ,
promise yes ES6 Chinese language standard , Save an event that will end in the future （ This is usually an asynchronous operation ） Result .const promise = new Promise(function(resolve, reject) {
// ... some code
if (/* Asynchronous operation succeeded */){
resolve(value);
} else {
reject(error);
}
});
In the work , There is a business function ： Periodic scan task , Each cycle will scan out data , Then save it to... According to the number of cycles elasticsearch, Now it is necessary to check whether the data between each cycle has overlapping and related data to support Periodic snapshot function , adopt Query DSL +kibana Visual query , Or it can be compared with the naked eye , It's hard work .kibana through http post request , Then return the data json, Then we can simulate kibana request , get data , Then compare the adjacent period data through the code , The output file , The idea of a small crawler and a small tool for data analysis came to mind .
Because the real code is in the company , The sample code has been modified ,url Replaced by Baidu ,post Replace with get
const http = require('https');
function spider() {
for (i = 0; i < 9; i++) {
//i For a period parameter
http.get('https://www.baidu.com/', function (res) {
console.log(` At present i:${i}`);
console.log(` Status code ${res.statusCode}`);
})
}
}
spider()
node .\src\server\index.js
At present i:9
Status code 200
At present i:9
Status code 200
At present i:9
Status code 200
At present i:9
Status code 200
At present i:9
Status code 200
At present i:9
Status code 200
At present i:9
Status code 200
At present i:9
Status code 200
At present i:9
Status code 200
Why is that ？ because http.get It's an asynchronous method , Not waiting , The loop will continue ,i The value will also change , In this case, the asynchronous method is used to i The reference of becomes 9. We definitely want to output something different i value , To view the data of the corresponding period . What do I do ？ Anonymous functions
const http = require('https');
function spider() {
for (i = 0; i < 9; i++) {
// Closure
// Anonymous functions + Execute now
(function (i) {
http.get('https://www.baidu.com/', function (res) {
console.log(` At present i:${i}`);
console.log(` Status code ${res.statusCode}`);
})
})(i)
}
}
spider()
node .\src\server\index.js
At present i:1
Status code 200
At present i:6
Status code 200
At present i:5
Status code 200
At present i:2
Status code 200
At present i:4
Status code 200
At present i:3
Status code 200
At present i:0
Status code 200
At present i:8
Status code 200
At present i:7
Status code 200
Closure = function + Citation environment , Functions are anonymous functions , The reference environment is the parameter i value
If the requirement is to view the cycle ： Cycle data , such key:value The needs of , So the above requirements have been basically met , If you think anonymous functions + Immediate execution is not easy to understand , The transformation is as follows , Also understand .
const http = require('https');
//let map=new Map();
function getData(i) {
http.get('https://www.baidu.com/', function (res) {
console.log(` At present i:${i}`);
console.log(res.statusCode);
//map.set ...
})
}
function spider() {
for (i = 0; i < 9; i++) {
getData(i)
}
}
spider()
But now , We need to save every cycle and cycle data , Then do data analysis . In other words , We need to create multiple loops http Asynchronous requests , All done , And return data , Coexist , To do analysis . Don't forget that this is asynchronous , Define global variables let map=new Map(); And in asynchronous callbacks map.set This will not work . This is the time. Promise On the stage .
const http = require('https');
// Define an array
let array = Array();
// Define an array
let promiseArray = []
// Define a map
let map=new Map();
function getData(i) {
return new Promise((resolve,reject) => {
http.get('https://www.baidu.com/', function (res) {
console.log(` At present i:${i}`);
console.log(res.statusCode);
array.push(i);
map.set(i,res.statusCode);
resolve();
})
})
}
function spider() {
console.log(' Start cycle creation promiseArray');
for (i = 0; i < 9; i++) {
promiseArray.push(getData(i))
}
console.log(' End cycle creation promiseArray');
}
spider();
//Promise.all(iterable) Method returns a Promise example , This example is in iterable All in parameters promise all “ complete （resolved）” Or parameter does not contain promise When the callback completes （resolve）; If the parameter promise There is a failure （rejected）, This instance callback failed （reject）, The reason for failure is the first failure promise Result .
Promise.all(promiseArray).then(function(){
console.log(array.length);
console.log(array);
console.log(map);
})
Each asynchronous request creates a Promise object , And put it into a storage Promise An array of objects , And then call Promise.all, Or return one Promise object , His callback completion is Promise Each of the objects in the array is resolve, That is, all asynchronous requests have been completed .
Start cycle creation promiseArray
End cycle creation promiseArray
At present i:7
200
At present i:4
200
At present i:1
200
At present i:2
200
At present i:6
200
At present i:5
200
At present i:8
200
At present i:3
200
At present i:0
200
9
[
7, 4, 1, 2, 6,
5, 8, 3, 0
]
Map {
7 => 200,
4 => 200,
1 => 200,
2 => 200,
6 => 200,
5 => 200,
8 => 200,
3 => 200,
0 => 200
}
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