13

I apologize if my question sounds stupid, but I found that I need in many situations global variables such as ones that represent database and redis clients to be used in many files, however these variables themselves need to wait to get their values from promises or async functions that initialize the communication to the database or redis servers.

I want to do something like that

init.js:

export default async () => {
   return await initializeWhatever()
}

db.js:

import init from './init'
let db = null
init().then(val => db = val)
export default db

api.js:

import db from './db'
const doApi = req => {
 db('users').select({username:req.param.username})
}

However the db variable imported in api.js is always null, why doesn't it get updated to the correct value when init() finishes? If my approach to using db is wrong, what is the correct way to export global variables that are computed asynchronously?

ibrahim mahrir
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Ejonas GGgg
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2 Answers2

10

When you export a variable, you're exporting what's stored in the variable, not the variable itself. Assigning to the variable will only change the variable, but will not affect what was exported.

Think of it like this:

let a = null;
let b = a;
a = 'something';
console.log(b);
// null

Note how storing something new in a doesn't change what's stored in b.

To do something like what you want in your question, you need to export an object and update references on that object:

init.js

export default async () => {
    return await initializeWhatever()
}

db-module.js

import init from './init'
const dbModule = { db: null };
init().then(val => dbModule.db = val)
export default dbModule

api.js

import dbModule from './db-module'
const doApi = req => {
    dbModule.db.select({username:req.param.username})
}

Obviously you might want to change some names to make things cleaner. I don't know a lot about your project so I'm not sure how to name them.

Also, you'll have to make sure have some way of making sure that any uses of your dbModule.db object happen after it's actually in place. In other words, you have to make sure there's no way doApi in api.js can be invoked before init in db-module.js finishes.

To do this, you'll need to make the promise chain from init available to api.js:

init.js

export default async () => {
    return await initializeWhatever()
}

db-module.js

import init from './init'
const dbModule = { db: null };
dbModule.promise = init().then(val => dbModule.db = val)
export default dbModule

api.js

import dbModule from './db-module'
const doApi = req => {
    dbModule.db.select({username:req.param.username})
}
dbModule.promise.then(() => {
    // Set up routes and start listening in this promise chain.
    // Ensure there is no way `doApi` can be called before this happens.
});

Why does this work? Because in JS, variables don't store entire copies of objects. They simply store references. If two variables point at the same object, changes to the object will be reflected whether your access it through one variable or another. Think of it like this:

let c = { prop: null };
let d = c;
c.prop = 'something';
console.log(d.prop);
// 'something'
sripberger
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  • It now works, thanks! however I afraid I still don't fully understand what the first statement of your answer really means, and why your example worked while mine didn't since we both changed the exported variable one way or another, you just update a property, is there any articles or doc that elaborates on this matter? – Ejonas GGgg Apr 12 '18 at 14:50
  • It's a simple matter of understanding references in JS. When you import something, you're creating a new variable that references *the exported value*. It does not bind it to a variable that may happen to be referencing that same value at the time of the export. If the exported value was `null`, then the variable created by the import will always be `null`. You may find this question helpful: https://stackoverflow.com/questions/518000/is-javascript-a-pass-by-reference-or-pass-by-value-language – sripberger Apr 12 '18 at 15:10
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    I realized my comments could be better done as edits. So edits have been made. :) – sripberger Apr 12 '18 at 16:02
1

You can make a getter. Something about 

export default function getDB() {
  return db;
}
Kirill Glazunov
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