Node.js 同步与异步

Can you please detail the reason why would the second example be better? I'll be happy to get an ever wider explanation that would help me understand the concept..

谢谢！！

第一个例子:

var fs = require('fs');

function calculateByteSize() {
    var totalBytes = 0,
        i,
        filenames,
        stats;
    filenames = fs.readdirSync(".");
    for (i = 0; i < filenames.length; i ++) {
        stats = fs.statSync("./" + filenames[i]);
        totalBytes += stats.size;
    }
    console.log(totalBytes);
}

calculateByteSize();

第二个例子:

var fs = require('fs');

var count = 0,
    totalBytes = 0;

function calculateByteSize() {
    fs.readdir(".", function (err, filenames) {
        var i;
        count = filenames.length;

        for (i = 0; i < filenames.length; i++) {
            fs.stat("./" + filenames[i], function (err, stats) {
                totalBytes += stats.size;
                count--;
                if (count === 0) {
                    console.log(totalBytes);
                }
            });
        }
    });
}

calculateByteSize();

推荐答案

你的第一个例子是所有阻塞 I/O.换句话说，您需要等到 readdir 操作完成后才能遍历每个文件.然后，您需要阻止(等待)每个单独的同步统计操作运行，然后再转到下一个文件.calculateByteSize() 调用后，在所有操作完成之前，没有代码可以运行.