Most people advise against using global variables. If you want the same logger class in different modules you can do this
logger.js
module.exports = new logger(customConfig);
foobar.js
var logger = require('./logger');
logger('barfoo');
If you do want a global variable you can do:
global.logger = new logger(customConfig);
Most people advise against using global variables. If you want the same logger class in different modules you can do this
logger.js
module.exports = new logger(customConfig);
foobar.js
var logger = require('./logger');
logger('barfoo');
If you do want a global variable you can do:
global.logger = new logger(customConfig);
global.myNumber; // Declaration of the global variable - undefined
global.myNumber = 5; // Global variable initialized to value 5.
var myNumberSquared = global.myNumber * global.myNumber; // Using the global variable.
Node.js is different from client Side JavaScript when it comes to global variables. Just because you use the word var at the top of your Node.js script does not mean the variable will be accessible by all objects you require such as your 'basic-logger' .
To make something global just put the word global and a dot in front of the variable's name. So if I want company_id to be global I call it global.company_id. But be careful, global.company_id and company_id are the same thing so don't name global variable the same thing as any other variable in any other script - any other script that will be running on your server or any other place within the same code.
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Hi!
I'm working on an API and I need to pass down some header to a low level cache function.
I didn't want to drill this header through many other functions and thought I could do something like we do in React with global context.
How safe and viable it is to declare a global variable on a controller level to be accessed down the line?
Am I being too naive or careless about this approach?
Yes, with some caveats.
You're looking for a module called continuation-local-storage.
This allows you to keep arbitrary data for the remainder of callbacks for the current request, and access it in a global fashion.
I wrote a blog post about it here. But the gist is this:
- Install cls:
npm install --save continuation-local-storage Create a namespace for your app (at the top of the main file for your app)
var createNamespace = require('continuation-local-storage').createNamespace, namespace = createNamespace('myAppNamespace');Create a middleware that runs downstream functions in the cls (continuation-local-storage) namespace
var getNamespace = require('continuation-local-storage').getNamespace, namespace = getNamespace('myAppNamespace'), app.use(function(req, res, next) { // wrap the events from request and response namespace.bindEmitter(req); namespace.bindEmitter(res); // run following middleware in the scope of the namespace we created namespace.run(function() { namespace.set(‘foo’, 'a string data'); next(); }); });Since you ran
nextwithinnamespace.run, any downstream function can access data in the namespacevar getNamespace = require('continuation-local-storage').getNamespace, namespace = getNamespace('myAppNamespace'); // Some downstream function that doesn't have access to req... function doSomething() { var myData = namespace.get('foo'); // myData will be 'a string data' }There is the caveat that certain modules can "lose" the context created by cls. This means that when you go to lookup 'foo' on the namespace, it won't have it. There are a few ways to deal with this, namely using another module like cls-redis, cls-q, or binding to the namespace.
Per request, I think what you're after could be done with ordinary closures. For example, you'd define your custom functions in module that takes a req argument:
util_funcs.js:
module.exports = function( req ){
return {
customFunctionThatRequiresReq: function(){ console.info( req ) },
otherFunctionThatRequiresReq: function(){ console.log( req ) }
};
};
Then wherever you depend on those functions (presumably some middleware elsewhere in the application), you can just require them in context:
var someMiddleWare = function( req, res, next ){
var utils = require( 'util_funcs.js' )( req );
utils.customFunctionThatRequiresReq();
utils.otherFunctionThatRequiresReq();
};
This allows you to avoid littering your function args with req, and no dubious globals.
Here is my suggestion avoid global state like fire.
- It's the number one maintenance problem in Node servers from my experience.
- It makes your code not composable and harder to reuse.
- It creates implicit dependencies in your code - you're never sure which piece depends on which and it's not easy to verify.
You want the parts of code that each piece of an application uses to be as explicit as possible. It's a huge issue.
The issue
We want to synchronize state across multiple requests and act accordingly. This is a very big problem in writing software - some say even the biggest. The importance of the way objects in the application communicate can not be overestimated.
Some solutions
There are several ways to accomplish sharing state across requests or server wide in a Node server. It depends on what you want to do. Here are the two most common imo.
- I want to observe what the requests do.
- I want one request to do things based on what another request did.
1. I want to observe what the requests do
Again, there are many ways to do this. Here are the two I see most.
Using an event emitter
This way requests emit events. The application reads events the requests fire and learns about them accordingly. The application itself could be an event emitter you can observe from the outside.
You can do something like:
request.emit("Client did something silly", theSillyThing);
And then listen to it from the outside if you choose to.
Using an observer pattern
This is like an event emitter but reversed. You keep a list of dependencies on the request and call a handler method on them yourself when something interesting happens on the request.
Personally, I usually prefer an event emitter because I think they usually solve the case better.
2. I want one request to do things based on what another request did.
This is a lot tricker than just listening. again, there are several approaches here. What they have in common is that we put the sharing in a service
Instead of having global state - each request gets access to a service - for example when you read a file you notify the service and when you want a list of read files - you ask the service. Everything is explicit in the dependency.
The service is not global, only dependencies of it. For example, it can coordinate resources and the data, being some form of Repository).
Nice theory! Now what about my use case?
Here are two options for what I would do in your case. It's far from the only solution.
First option:
- Each of the modules are an event emitter, whenever they read a file they emit an event.
- A service listens to all their events and keeps count.
- Requests have access to that service explicitly and can query it for a list of files.
- Requests perform writes through the modules themselves and not the added service.
Second option:
- Create a service that owns a copy of module1, module2 and module3. (composition)
- The service delegates actions to the modules based on what is required from it.
- The service keeps the list of files accessed since the requests were made through it.
- The request stops using the modules directly - uses the service instead.
Both these approaches have advantages and disadvantages. A more complicated solution might be required (those two are in practice pretty simple to do) where the services are abstracted further but I think this is a good start.
One simple way is storing data on the request object.
Here is an example (using Express):
app.get('/hello.txt', function(req, res){
req.transaction = req.transaction || [];
if (req.transaction.length) {
// something else has already written to this array
}
});
However, I don't really see how you can need this. When you call moduleA or moduleB, you just have to pass an object as argument, and it solves your issue. Maybe you're looking for dependency injection?
I'm storing User info in a global variable and accessed that I was told that it's a bad way and doesn't permit multiple users when there is high traffic. What do you think of it and does it really limit the users?
You can use global like so:
global._ = require('underscore')
In Node.js, you can set global variables via the "global" or "GLOBAL" object:
GLOBAL._ = require('underscore'); // But you "shouldn't" do this! (see note below)
or more usefully...
GLOBAL.window = GLOBAL; // Like in the browser
From the Node.js source, you can see that these are aliased to each other:
node-v0.6.6/src/node.js:
28: global = this;
128: global.GLOBAL = global;
In the code above, "this" is the global context. With the CommonJS module system (which Node.js uses), the "this" object inside of a module (i.e., "your code") is not the global context. For proof of this, see below where I spew the "this" object and then the giant "GLOBAL" object.
console.log("\nTHIS:");
console.log(this);
console.log("\nGLOBAL:");
console.log(global);
/* Outputs ...
THIS:
{}
GLOBAL:
{ ArrayBuffer: [Function: ArrayBuffer],
Int8Array: { [Function] BYTES_PER_ELEMENT: 1 },
Uint8Array: { [Function] BYTES_PER_ELEMENT: 1 },
Int16Array: { [Function] BYTES_PER_ELEMENT: 2 },
Uint16Array: { [Function] BYTES_PER_ELEMENT: 2 },
Int32Array: { [Function] BYTES_PER_ELEMENT: 4 },
Uint32Array: { [Function] BYTES_PER_ELEMENT: 4 },
Float32Array: { [Function] BYTES_PER_ELEMENT: 4 },
Float64Array: { [Function] BYTES_PER_ELEMENT: 8 },
DataView: [Function: DataView],
global: [Circular],
process:
{ EventEmitter: [Function: EventEmitter],
title: 'node',
assert: [Function],
version: 'v0.6.5',
_tickCallback: [Function],
moduleLoadList:
[ 'Binding evals',
'Binding natives',
'NativeModule events',
'NativeModule buffer',
'Binding buffer',
'NativeModule assert',
'NativeModule util',
'NativeModule path',
'NativeModule module',
'NativeModule fs',
'Binding fs',
'Binding constants',
'NativeModule stream',
'NativeModule console',
'Binding tty_wrap',
'NativeModule tty',
'NativeModule net',
'NativeModule timers',
'Binding timer_wrap',
'NativeModule _linklist' ],
versions:
{ node: '0.6.5',
v8: '3.6.6.11',
ares: '1.7.5-DEV',
uv: '0.6',
openssl: '0.9.8n' },
nextTick: [Function],
stdout: [Getter],
arch: 'x64',
stderr: [Getter],
platform: 'darwin',
argv: [ 'node', '/workspace/zd/zgap/darwin-js/index.js' ],
stdin: [Getter],
env:
{ TERM_PROGRAM: 'iTerm.app',
'COM_GOOGLE_CHROME_FRAMEWORK_SERVICE_PROCESS/USERS/DDOPSON/LIBRARY/APPLICATION_SUPPORT/GOOGLE/CHROME_SOCKET': '/tmp/launch-nNl1vo/ServiceProcessSocket',
TERM: 'xterm',
SHELL: '/bin/bash',
TMPDIR: '/var/folders/2h/2hQmtmXlFT4yVGtr5DBpdl9LAiQ/-Tmp-/',
Apple_PubSub_Socket_Render: '/tmp/launch-9Ga0PT/Render',
USER: 'ddopson',
COMMAND_MODE: 'unix2003',
SSH_AUTH_SOCK: '/tmp/launch-sD905b/Listeners',
__CF_USER_TEXT_ENCODING: '0x12D732E7:0:0',
PATH: '/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:~/bin:/usr/X11/bin',
PWD: '/workspace/zd/zgap/darwin-js',
LANG: 'en_US.UTF-8',
ITERM_PROFILE: 'Default',
SHLVL: '1',
COLORFGBG: '7;0',
HOME: '/Users/ddopson',
ITERM_SESSION_ID: 'w0t0p0',
LOGNAME: 'ddopson',
DISPLAY: '/tmp/launch-l9RQXI/org.x:0',
OLDPWD: '/workspace/zd/zgap/darwin-js/external',
_: './index.js' },
openStdin: [Function],
exit: [Function],
pid: 10321,
features:
{ debug: false,
uv: true,
ipv6: true,
tls_npn: false,
tls_sni: true,
tls: true },
kill: [Function],
execPath: '/usr/local/bin/node',
addListener: [Function],
_needTickCallback: [Function],
on: [Function],
removeListener: [Function],
reallyExit: [Function],
chdir: [Function],
debug: [Function],
error: [Function],
cwd: [Function],
watchFile: [Function],
umask: [Function],
getuid: [Function],
unwatchFile: [Function],
mixin: [Function],
setuid: [Function],
setgid: [Function],
createChildProcess: [Function],
getgid: [Function],
inherits: [Function],
_kill: [Function],
_byteLength: [Function],
mainModule:
{ id: '.',
exports: {},
parent: null,
filename: '/workspace/zd/zgap/darwin-js/index.js',
loaded: false,
exited: false,
children: [],
paths: [Object] },
_debugProcess: [Function],
dlopen: [Function],
uptime: [Function],
memoryUsage: [Function],
uvCounters: [Function],
binding: [Function] },
GLOBAL: [Circular],
root: [Circular],
Buffer:
{ [Function: Buffer]
poolSize: 8192,
isBuffer: [Function: isBuffer],
byteLength: [Function],
_charsWritten: 8 },
setTimeout: [Function],
setInterval: [Function],
clearTimeout: [Function],
clearInterval: [Function],
console: [Getter],
window: [Circular],
navigator: {} }
*/** Note: regarding setting "GLOBAL._", in general you should just do var _ = require('underscore');. Yes, you do that in every single file that uses Underscore.js, just like how in Java you do import com.foo.bar;. This makes it easier to figure out what your code is doing because the linkages between files are 'explicit'. It is mildly annoying, but a good thing. .... That's the preaching.
There is an exception to every rule. I have had precisely exactly one instance where I needed to set "GLOBAL._". I was creating a system for defining "configuration" files which were basically JSON, but were "written in JavaScript" to allow a bit more flexibility. Such configuration files had no 'require' statements, but I wanted them to have access to Underscore.js (the entire system was predicated on Underscore.js and Underscore.js templates), so before evaluating the "configuration", I would set "GLOBAL._". So yeah, for every rule, there's an exception somewhere. But you had better have a darn good reason and not just "I get tired of typing 'require', so I want to break with the convention".
It is actually very easy to do this using the "set" and "get" methods available on an express object.
Example as follows, say you have a variable called config with your configuration related stuff that you want to be available in other places:
In app.js:
var config = require('./config');
app.configure(function() {
...
app.set('config', config);
...
}
In routes/index.js
exports.index = function(req, res){
var config = req.app.get('config');
// config is now available
...
}
A neat way to do this is to use app.locals provided by Express itself.
Here is the documentation.
// In app.js:
app.locals.variable_you_need = 42;
// In index.js
exports.route = function(req, res){
var variable_i_needed = req.app.locals.variable_you_need;
}
This should work call your function in your request callback.
var request = require('request');
var variable1;
request('https://api.coindesk.com/v1/bpi/currentprice/EUR.json', function (error, response, body){
var btceurpricejson = (body);
var obj = JSON.parse(btceurpricejson);
variable1 = (obj.bpi.EUR.rate_float);
getBTCItemPrice();
});
function getBTCItemPrice() {
console.log(variable1);
};
Its a bad practice to use global. But as said above calling getBTCItemPrice() inside the callback will serve your purpose.