[javascript] How do you share constants in NodeJS modules?

Currently I'm doing this:

foo.js

const FOO = 5;

module.exports = {
    FOO: FOO
};

And using it in bar.js:

var foo = require('foo');
foo.FOO; // 5

Is there a better way to do this? It feels awkward to declare the constant in the exports object.

From previous project experience, this is a good way:

In the constants.js:

// constants.js

'use strict';

let constants = {
    key1: "value1",
    key2: "value2",
    key3: {
        subkey1: "subvalue1",
        subkey2: "subvalue2"
    }
};

module.exports =
        Object.freeze(constants); // freeze prevents changes by users

In main.js (or app.js, etc.), use it as below:

// main.js

let constants = require('./constants');

console.log(constants.key1);

console.dir(constants.key3);

I think that const solves the problem for most people looking for this anwwer. If you really need an immutable constant, look into the other answers. To keep everything organized I save all constants on a folder and then require the whole folder.

src/main.js file

const constants = require("./consts_folder");

src/consts_folder/index.js

const deal = require("./deal.js")
const note = require("./note.js")


module.exports = {
  deal,
  note
}

Ps. here the deal and note will be first level on the main.js

src/consts_folder/note.js

exports.obj = {
  type: "object",
  description: "I'm a note object"
}

Ps. obj will be second level on the main.js

src/consts_folder/deal.js

exports.str = "I'm a deal string"

Ps. str will be second level on the main.js

Final result on main.js file:

console.log(constants.deal); Ouput:

{ deal: { str: 'I\'m a deal string' },

console.log(constants.note); Ouput:

note: { obj: { type: 'object', description: 'I\'m a note object' } }

As an alternative, you can group your "constant" values in a local object, and export a function that returns a shallow clone of this object.

var constants = { FOO: "foo" }

module.exports = function() {
  return Object.assign({}, constants)
}

Then it doesn't matter if someone re-assigns FOO because it will only affect their local copy.

In my opinion, utilizing Object.freeze allows for a DRYer and more declarative style. My preferred pattern is:

./lib/constants.js

module.exports = Object.freeze({
    MY_CONSTANT: 'some value',
    ANOTHER_CONSTANT: 'another value'
});

./lib/some-module.js

var constants = require('./constants');

console.log(constants.MY_CONSTANT); // 'some value'

constants.MY_CONSTANT = 'some other value';

console.log(constants.MY_CONSTANT); // 'some value'

Outdated Performance Warning

The following issue was fixed in v8 in Jan 2014 and is no longer relevant to most developers:

Be aware that both setting writable to false and using Object.freeze have a massive performance penalty in v8 - https://bugs.chromium.org/p/v8/issues/detail?id=1858 and http://jsperf.com/performance-frozen-object

Technically, const is not part of the ECMAScript specification. Also, using the "CommonJS Module" pattern you've noted, you can change the value of that "constant" since it's now just an object property. (not sure if that'll cascade any changes to other scripts that require the same module, but it's possible)

To get a real constant that you can also share, check out Object.create, Object.defineProperty, and Object.defineProperties. If you set writable: false, then the value in your "constant" cannot be modified. :)

It's a little verbose, (but even that can be changed with a little JS) but you should only need to do it once for your module of constants. Using these methods, any attribute that you leave out defaults to false. (as opposed to defining properties via assignment, which defaults all the attributes to true)

So, hypothetically, you could just set value and enumerable, leaving out writable and configurable since they'll default to false, I've just included them for clarity.

Update - I've create a new module (node-constants) with helper functions for this very use-case.

constants.js -- Good

Object.defineProperty(exports, "PI", {
    value:        3.14,
    enumerable:   true,
    writable:     false,
    configurable: false
});

constants.js -- Better

function define(name, value) {
    Object.defineProperty(exports, name, {
        value:      value,
        enumerable: true
    });
}

define("PI", 3.14);

script.js

var constants = require("./constants");

console.log(constants.PI); // 3.14
constants.PI = 5;
console.log(constants.PI); // still 3.14

I found the solution Dominic suggested to be the best one, but it still misses one feature of the "const" declaration. When you declare a constant in JS with the "const" keyword, the existence of the constant is checked at parse time, not at runtime. So if you misspelled the name of the constant somewhere later in your code, you'll get an error when you try to start your node.js program. Which is a far more better misspelling check.

If you define the constant with the define() function like Dominic suggested, you won't get an error if you misspelled the constant, and the value of the misspelled constant will be undefined (which can lead to debugging headaches).

But I guess this is the best we can get.

Additionally, here's a kind of improvement of Dominic's function, in constans.js:

global.define = function ( name, value, exportsObject )
{
    if ( !exportsObject )
    {
        if ( exports.exportsObject )
            exportsObject = exports.exportsObject;
        else 
            exportsObject = exports;        
    }

    Object.defineProperty( exportsObject, name, {
        'value': value,
        'enumerable': true,
        'writable': false,
    });
}

exports.exportObject = null;

In this way you can use the define() function in other modules, and it allows you to define constants both inside the constants.js module and constants inside your module from which you called the function. Declaring module constants can then be done in two ways (in script.js).

First:

require( './constants.js' );

define( 'SOME_LOCAL_CONSTANT', "const value 1", this ); // constant in script.js
define( 'SOME_OTHER_LOCAL_CONSTANT', "const value 2", this ); // constant in script.js

define( 'CONSTANT_IN_CONSTANTS_MODULE', "const value x" ); // this is a constant in constants.js module

Second:

constants = require( './constants.js' );

// More convenient for setting a lot of constants inside the module
constants.exportsObject = this;
define( 'SOME_CONSTANT', "const value 1" ); // constant in script.js
define( 'SOME_OTHER_CONSTANT', "const value 2" ); // constant in script.js

Also, if you want the define() function to be called only from the constants module (not to bloat the global object), you define it like this in constants.js:

exports.define = function ( name, value, exportsObject )

and use it like this in script.js:

constants.define( 'SOME_CONSTANT', "const value 1" );

import and export (prob need something like babel as of 2018 to use import)

types.js

export const BLUE = 'BLUE'
export const RED = 'RED'

myApp.js

import * as types from './types.js'

const MyApp = () => {
  let colour = types.RED
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import

I don't think is a good practice to invade the GLOBAL space from modules, but in scenarios where could be strictly necessary to implement it:

Object.defineProperty(global,'MYCONSTANT',{value:'foo',writable:false,configurable:false});

It has to be considered the impact of this resource. Without proper naming of those constants, the risk of OVERWRITTING already defined global variables, is something real.

I ended up doing this by exporting a frozen object with anonymous getter functions, rather than the constants themselves. This reduces the risk of nasty bugs introduced due to a simple typo of the const name, as a runtime error will be thrown in case of a typo. Here's a full example that also uses ES6 Symbols for the constants, ensuring uniqueness, and ES6 arrow functions. Would appreciate feedback if anything in this approach seems problematic.

'use strict';
const DIRECTORY = Symbol('the directory of all sheets');
const SHEET = Symbol('an individual sheet');
const COMPOSER = Symbol('the sheet composer');

module.exports = Object.freeze({
  getDirectory: () => DIRECTORY,
  getSheet: () => SHEET,
  getComposer: () => COMPOSER
});

I recommend doing it with webpack (assumes you're using webpack).

Defining constants is as simple as setting the webpack config file:

var webpack = require('webpack');
module.exports = {
    plugins: [
        new webpack.DefinePlugin({
            'APP_ENV': '"dev"',
            'process.env': {
                'NODE_ENV': '"development"'
            }
        })
    ],    
};

This way you define them outside your source, and they will be available in all your files.

Since Node.js is using the CommonJS patterns, you can only share variables between modules with module.exports or by setting a global var like you would in the browser, but instead of using window you use global.your_var = value;.

ES6 way.

export in foo.js

const FOO = 'bar';
module.exports = {
  FOO
}

import in bar.js

const {FOO} = require('foo');