javascript - 停止执行进一步的代码
问题描述
我有个请求:
Character.count({'character.ownerid': msg.author.id}, function (err, count) {
if (err) {
throw err;
}
if (count > 3) {
err.message = 'Too many characters';
//create error explanation and throw it
throw err;
}
})
如果确实发生任何错误,我需要退出整个父函数。我不能在这个请求中返回,因为它只退出这个方法。我认为有可能进行如下回调:
Character.count({'character.ownerid': msg.author.id}, function (err, count, stop) {
但是如何使用它呢?它在一个匿名函数中,我不知道在哪里放置它的内容。我也尝试使用try
/catch
但由于 ,我无法向外部处理程序抛出错误Error: Unhandled "error" event. ([object Object])
,请参见下面的代码:
Character.count({'character.ownerid': msg.author.id}, function (err, count) {
if (err) {
throw err;
}
if (count > 3) {
var err = {};
err.message = 'Too many characters';
throw err;
}
}).then((count) => {
console.log('all good we may continue');
console.log(count);
}).catch((err) => {
if (err != undefined && err.length > 0) {
msg.reply(err.message);
return 0; //exit parent function?
}
});
但即使这有效,我也不确定这段代码是否能满足我的需要。请求是异步的,那么如果其余代码之前被触发then
怎么办?这甚至可能吗?
所以我基本上需要以return 0;
某种方式访问父函数,如果有任何错误,我需要为它们提供处理程序。对此有什么想法吗?
解决方案
你似乎错过了这个概念。首先,如前所述,所有 mongoose 操作都返回一个 Promise 或至少一个“类似 Promise”的对象,该对象可以立即通过 a 解决,then()
而不是传入回调函数。这可以以两种方式呈现。
使用async/await
语法和try..catch
块:
const { Schema } = mongoose = require('mongoose');
const uri = 'mongodb://localhost/test';
mongoose.set('debug', true);
mongoose.Promise = global.Promise;
const characterSchema = new Schema({
name: String
});
const Character = mongoose.model('Character', characterSchema);
const log = data => console.log(JSON.stringify(data,undefined,2));
const doCount = async () => {
let count = await Character.count();
if (count > 3)
throw new Error("too many charaters");
return count;
};
(async function() {
try {
const conn = await mongoose.connect(uri);
await Promise.all(Object.entries(conn.models).map(([k,m]) => m.remove()))
await Character.insertMany(
["Huey","Duey","Louie"].map(name => ({ name }))
);
let count = await doCount();
log({ count });
await Character.create({ name: 'Donald' });
let newCount = await doCount();
console.log("never get here");
} catch(e) {
console.error(e)
} finally {
mongoose.disconnect();
}
})()
或者使用标准then()
和catch()
语法:
const { Schema } = mongoose = require('mongoose');
const uri = 'mongodb://localhost/test';
mongoose.set('debug', true);
mongoose.Promise = global.Promise;
const characterSchema = new Schema({
name: String
});
const Character = mongoose.model('Character', characterSchema);
const log = data => console.log(JSON.stringify(data,undefined,2));
function doCount() {
return Character.count()
.then(count => {
if (count > 3)
throw new Error("too many charaters");
return count;
});
};
(function() {
mongoose.connect(uri)
.then(conn => Promise.all(
Object.entries(conn.models).map(([k,m]) => m.remove())
))
.then(() => Character.insertMany(
["Huey","Duey","Louie"].map(name => ({ name }))
))
.then(() => doCount())
.then(count => log({ count }))
.then(() => Character.create({ name: 'Donald' }))
.then(() => doCount())
.then(() => console.log("never get here"))
.catch(e => console.error(e))
.then(() => mongoose.disconnect() );
})()
两个清单的输出是一样的:
Mongoose: characters.remove({}, {})
Mongoose: characters.insertMany([ { _id: 5b0f66ec5580010efc5d0602, name: 'Huey', __v: 0 }, { _id: 5b0f66ec5580010efc5d0603, name: 'Duey', __v: 0 }, { _id: 5b0f66ec5580010efc5d0604, name: 'Louie', __v: 0 } ], {})
Mongoose: characters.count({}, {})
{
"count": 3
}
Mongoose: characters.insertOne({ _id: ObjectId("5b0f66ec5580010efc5d0605"), name: 'Donald', __v: 0 })
Mongoose: characters.count({}, {})
Error: too many charaters
at doCount (/home/projects/characters/index.js:20:11)
at <anonymous>
at process._tickCallback (internal/process/next_tick.js:188:7)
As you can see the function happily returns it's value where the count
remains 3 or under, but then throws an exception which stops further execution when the count
would be greater than 3 since the "never get here"
message never gets logged.
So there is no need for "callbacks" here and you would not use one unless you wrapped it in a Promise can did the same type of error handling anyway.
But if you have an "error" then throw
the error. This works fine in a promise chain, but a "callback" which does not return as a Promise is simply not part of that chain and can never be "caught". So simply don't use the callback when you don't need to.
Just for kicks, wrapping a callback with a Promise would be done like:
function doCount() {
return new Promise((resolve,reject) =>
Character.count().exec((err,count) => {
if (count > 3)
reject(new Error("too many charaters"));
resolve(count);
})
);
};
But it's noted not to be necessary considering the native methods return something you can resolve as a Promise anyway.
推荐阅读
- laravel - 网址重定向到 xampp 页面而不是主站点 | 拉拉维尔
- visual-studio-code - VS Code更改在侧边/滚动条区域中查找所有匹配项的颜色
- npm - 即使在安装了对等依赖项之后,也会出现未满足的对等依赖项警告
- android - 尝试加载 System.loadLibrary("loader-jni") 时应用程序崩溃
- sql - 如何使用另一个表的新创建的 PK 更新 1 个表中的值
- java - 无法使用 Spring Webflux Webclient 映射二级 json 值
- angular - Angular - 在模块定义中使用动态变量
- c++ - C++ 中使用什么类型来定义数组大小?
- winston - 为什么winston 从 Logger 转移到 createLogger
- c# - 如何使用现有结果预测结果