在 javascript 函数中使用 done 作为参数是什么意思？

Question

我正在学习 Javascript，似乎将 done 作为函数中的参数是一个难以理解的概念。我想知道它为什么会这样（作为参数完成（我猜是完成的过程信号），如果有一些好书或在线资源可以进一步研究这个概念。 例如，我正在跟随教程和它使用完成作为参数，问题是当我通过 gulp (gulpfile.js) 在节点上运行代码时，使用完成时进程永远不会停止，如果我选择跳过它运行顺利的代码中的完成。我试图跟踪解决问题，我知道问题是作为参数完成的，（我已经多次检查过）。

gulp.task('clean-styles', function(done) {
    var files = config.temp + '**/*.css';
    clean(files, done);
});


function clean(path, done) {
    log('Cleaning: ' + $.util.colors.blue(path));
    del(path, done).then(function(path) {
        console.log("path=",util.inspect(path,false,null))
        console.log('Deleted Files\/Folders:\n', path.join('\n'));
        console.log('Finishing clean')
    });
}

• 节点版本：0.12.4
• npm 版本：2.10.1
• 吞咽版本：3.9.0

非常感谢您的帮助，将不胜感激。问候。

Answer 1

can only explain the concept. what you are trying to achieve is not clear enough.

done is just a non-official standard name for a function (a.k.a callback) that informs the calling function (parent in stacktrace) that a task is completed.

recall that javascript is asynchronous and functions can be passed around as variables.

now, imagine a function startPrinting that has to call printText1, printText2 and printText3 and then output message that process is completed. We have:

function startPrinting() {
    printText1();
    printText2();
    printText3();
    console.log("completed");
}
function printText1() {
    $.get('http://ps-web.cloudapp.net/proxy.php?url=http://pastebin.com/raw.php?i=3uPCavLN', function(response){
        console.log(response)
    });
}
function printText2() {
    $.get('http://ps-web.cloudapp.net/proxy.php?url=http://pastebin.com/raw.php?i=jZjqKgNN', function(response){
        console.log(response)
    });
}
function printText3() {
    $.get('http://ps-web.cloudapp.net/proxy.php?url=http://pastebin.com/raw.php?i=SreCbunb', function(response){
        console.log(response)
    });
}

here, there is no assurance that completed will ALWAYS be printed after all three functions have been executed. since they execute asynchronously.

in order to sort this, javascript ninjas will introduce a done function so that startPrinting will only print completed when all three functions have been executed. Notice how a function is passed to printText1 ... 2 below:

function startPrinting() {
    /* START OF DONE ROUTINE */
    var count = 0;
    var printCompleted = function() {
        count+=1;

        if(count == 3)
            console.log("completed");
    }
    /* END */
    printText1(printCompleted);
    printText2(printCompleted);
    printText3(printCompleted);
}
function printText1(done) {
    $.get('http://ps-web.cloudapp.net/proxy.php?url=http://pastebin.com/raw.php?i=3uPCavLN', function(response){
        console.log(response)
        done();
    });
}
function printText2(done) {
    $.get('http://ps-web.cloudapp.net/proxy.php?url=http://pastebin.com/raw.php?i=jZjqKgNN', function(response){
        console.log(response)
        done();
    });
}
function printText3(done) {
    $.get('http://ps-web.cloudapp.net/proxy.php?url=http://pastebin.com/raw.php?i=SreCbunb', function(response){
        console.log(response)
        done();
    });
}

I hope you are able to apply this principle to better understanding your context.