Question

这是我最简单的代码：

var this_version =null;

this.timervar = setTimeout(function () {
    try {
        // Firefox 4 and later; Mozilla 2 and later
        Components.utils.import("resource://gre/modules/AddonManager.jsm");
        AddonManager.getAddonByID("parasites@maafire.com", function(addon) {
            this_version = addon.version;
            alert("r "+this_version);
        });
    } catch (ex) {
        // Firefox 3.6 and before; Mozilla 1.9.2 and before
        var em = Components.classes["@mozilla.org/extensions/manager;1"]
            .getService(Components.interfaces.nsIExtensionManager);
        var addon = em.getItemForID("parasites@maafire.com");
        this_version = addon.version;
        alert("rr " + this_version);
    }
    alert("rrr " + this_version);
}, 2000);

它真是太奇怪了，因为这会引发两次alert("r "+this_version);，但这个alert("rrr "+this_version);总是让我无效;（

还在学习JS，所以这很令人费解......请帮忙！

谢谢！

Answer 1

设置this_version变量的代码位于回调中，因此对变量的更改不会反映在第三个alert执行的范围内。

Answer 2

这是在JavaScript世界中实现它的正确方法：

function done() {
    alert("rrr "+this_version);
}

var this_version =null;
try {
    // Firefox 4 and later; Mozilla 2 and later
    Components.utils.import("resource://gre/modules/AddonManager.jsm");
    AddonManager.getAddonByID("parasites@maafire.com", function(addon) {
        this_version = addon.version;
        alert("r "+this_version);
        done();
    });
} catch (ex) {
    // Firefox 3.6 and before; Mozilla 1.9.2 and before
    var em = Components.classes["@mozilla.org/extensions/manager;1"]
        .getService(Components.interfaces.nsIExtensionManager);
    var addon = em.getItemForID("parasites@maafire.com");
    this_version = addon.version;
    alert("rr " + this_version);
    done();
}

甚至更好：

function done(version) {
    alert("rrr "+version);
}

然后这样称呼：

done(this_version);

Answer 3

好的，只需移动计时器以外的所有内容即可解决它：

var this_version =null;
try {
    // Firefox 4 and later; Mozilla 2 and later
    Components.utils.import("resource://gre/modules/AddonManager.jsm");
    AddonManager.getAddonByID("parasites@maafire.com", function(addon) {
        this_version = addon.version;
        alert("r "+this_version);
    });
} catch (ex) {
    // Firefox 3.6 and before; Mozilla 1.9.2 and before
    var em = Components.classes["@mozilla.org/extensions/manager;1"]
        .getService(Components.interfaces.nsIExtensionManager);
    var addon = em.getItemForID("parasites@maafire.com");
    this_version = addon.version;
    alert("rr " + this_version);
}

this.timervar = setTimeout(function ()
                            { 



                                    alert("rrr "+this_version);


                            }, 2000);

Javascript，变量没有被分配...为什么？

3 个答案: