javascript - 我们可以强制函数调用完成并返回，然后再继续执行 node.js 中的下一条语句吗？

Question

这里我有一个简单的 HTTP 服务器。当 foo() 被调用时，它会根据键获取一个值。但事实证明，当 foo(key, redisClient) 被调用时，它打印了

I am inside foo

然后马上去汇报

x is null

此时异步redis.get调用结束，现在我明白了

About to return from foo with result: 1

这是我期望的值。但现在我的错误检查已经结束，它已经在 HTTP 响应中写入了错误。在主服务器线程中继续执行任何其他操作之前，我如何确保从 foo() 中实际获得正确的返回值以存储到 x 中？

var http = require('http');
var redis = require("redis");
http.createServer(function (req, res) {

    var x = null;
    var key = "key";
    var redisClient = redis.createClient();

    x = foo(key, redisClient);

    if(x == null)
    {
        // report error and quit
                console.log('x is null');
                // write error message and status in HTTP response
    }
    // proceed
        console.log('Proceeding...');
        // do some stuff using the value returned by foo to var x
        // .........
        // .........
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello World\n');
}).listen(1400, '127.0.0.1');
console.log('Server running at http://127.0.0.1:1400/');


function foo(key, redisClient)
{
    console.log('I am inside foo');
    redisClient.get(key, function(error, result) {
        if(error) console.log('error:' + error);
        else
            {
                    console.log('About to return from foo with result:' + result);
                    return result;
            }
    }
}

Answer 1

redisClient.get() 调用中的返回不会传递给 foo() 的返回。您需要在回调中将值传回。这是修改代码:

var http = require('http');
var redis = require("redis");
var me = this;
http.createServer(function (req, res) {

    var x = null;
    var key = "key";
    var redisClient = redis.createClient();

    me.foo(key, redisClient, function(err, result) {
       x = result;
       if(x == null)
       {
       // report error and quit
               console.log('x is null');
               // write error message and status in HTTP response
       }
       // proceed
        console.log('Proceeding...');
        // do some stuff using the value returned by foo to var x
        // .........
       // .........
       res.writeHead(200, {'Content-Type': 'text/plain'});
       res.end('Hello World\n');
    });

}).listen(1400, '127.0.0.1');
console.log('Server running at http://127.0.0.1:1400/');


function foo(key, redisClient, callback)
{
  console.log('I am inside foo');
  redisClient.get(key, function(error, result) {
    if(error)  {
        console.log('error:' + error);
        callback (error);
    } else {
        console.log('About to return from foo with result:' + result);
        callback(null, result);
    }
  }
}