如何在不同模块中使用嵌套承诺正确处理错误

Question

I'm new to Node JS, and I'm struggling to handle error properly when using promises. Here is what I currently have:

I have a module call db.js with an init function used to set up and start the db connection:

function initDb(){
return new Promise((resolve, reject) => {
    if(database){
        console.warn("Trying to init DB again !")
    }else{
        client.connect(config.db.url, {useNewUrlParser: true, useUnifiedTopology: true})
        .then((client) => {
            console.log("DB initialized - connected to database: " + config.db.name)
            database = client.db(config.db.name)
        })
        .catch((err) => {
            reject(err)
        })
    }
    resolve(database)
})}

This function will return a promise, and it will be called into the index.js:

initDb()
.then(() => {
    app.listen(8081, function () {
        console.log('App is running on port 8081')
    })
})
.catch((error) => {
    console.log(error)
})

As you can see I have two catch. One in the db module and the other one in the index.js

It seams weird to catch error in two places... What is the good pattern to use to handle error in this case ?

Answer 1

You'll want to avoid the Promise constructor antipattern . Also don't store the database itself in a variable, store the promise for it instead.

let databasePromise;
function initDb() {
    if (databasePromise) {
        console.debug("DB already initialised");
        return databasePromise;
    }
    databasePromise = client.connect(config.db.url, {useNewUrlParser: true, useUnifiedTopology: true})
    .then((client) => {
        console.log("DB initialized - connected to database: " + config.db.name)
        return client.db(config.db.name)
    });
    return databasePromise;
}

Answer 2

If you don't write catch in the initDb function in db.js module, you can catch that error in the calling function, so it's okay if you don't write .catch((err) => { reject(err)}) in db.js module, the error will go to calling function in index.js and you can directly handle it there.

It is not weird to catch the error at both the place, in fact in coming versions of node this will be a recommended practice to write catch(err) for handling errors at all promises.