How do you share constants in NodeJS modules

Question

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

User · Answer

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

User · Answer

You can explicitly export it to the global scope with global FOO   5  Then you simply need to require the file  and not even save your return value   But really  you shouldn t do that   Keeping things properly encapsulated is a good thing  You have the right idea already  so keep doing what you re doing

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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

User · Answer

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        gt      let colour   types RED     https   developer mozilla org en-US docs Web JavaScript Reference Statements import

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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

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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

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ES6 way   export in foo js  const FOO    bar   module exports       FOO     import in bar js  const  FOO    require  foo

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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

User · Answer

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       gt  DIRECTORY    getSheet       gt  SHEET    getComposer       gt  COMPOSER

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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

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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

User · Answer

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

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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

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

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