节点 js 中的关闭未按预期工作

Question

I have this piece of code:

var responses = [];
for( var i=0; i < Number(process.argv[2]); i++) {
    responses.push(function () {
        var index = i;
        function bar() {
            console.log(index);
        }
        return bar;
    }());
}

responses.forEach(function(d){
    d();
});

where a closure is created by using an interim variable index . This outputs 0 1 as expected, printing the values I want to capture in a closure.

A similar code as http get callback doesn't work.

var http = require("http");

var urls = ["http://yahoo.com","http://google.com"];

for( var i=0; i < urls.length; i++) {
   http.get( urls[i] , function(res) {
        var j = i;
        res.setEncoding('utf-8');
        res.on("data", function(d) {
            console.log(j);
        });
    });
}

This code outputs: 2 2

What am I missing?

Answer 1

var j = i; will set the value of i which is length of the array on first response and that is the reason you get 2 every time you log it.

Invoke a anonymous function as a second argument of http.get which will return inner function to handle response and it will also remember the environment in which it is created. Value of passed argument i will be there in the memory to be used later.

Try this:

 var http = require("http"); var urls = ["http://yahoo.com", "http://google.com"]; for (var i = 0; i < urls.length; i++) { http.get(urls[i], (function(i) { return function(res) { res.setEncoding('utf-8'); res.on("data", function(d) { console.log(i); }); } })(i)); }

Answer 2

There is no block scope in javascript(es5). So the for loop that you used won't do the trick., Instead try:

  urls.forEach(function(url, i){
      http.get( url , function(res) {           
        res.setEncoding('utf-8');
        res.on("data", function(d) {
            console.log(i);
        });
    });