nix-language-atlas javascript

21 feb 2018


in the nix-language-atlas series on i want to discuss how well programming languages, for which i’m familiar with, integrate with nix. let’s revisit javascript, as there had been major improvements since i wrote about it last time. we won’t look into the emscripten toolchain.

DSL-PM in general

are ‘domain specific language package manager(s)’ (DSL-PM) as npm or yarn a good thing?

integrate DSL-PM(s) into nix/nixpkgs

DSL-PM properties nix requires:

  • reproducible dependency calculations/downloads
  • reproducible configuration, build & installation into the store of each dependency
  • reproducible configuration, build & installation into the store of main target

DSL-PM evolution (javascript)

note: read this if you are interested in npm/yarn differences. it seems that yarn forced many of its concepts into npm.

yarn2nix workflow

let’s see a simple example how to integrate a yarn based project into your nix codebase:

  1. we clone an example:

    git clone
    cd example-yarn-package
    mkdir bin
    touch bin/myapp
    chmod u+x bin/myapp
  2. let’s add a default.nix

    { pkgs ? import <nixpkgs> {} }:
      yarn2nixSrc = pkgs.fetchFromGitHub {
        owner = "moretea";
        repo = "yarn2nix";
        rev = "0472167f2fa329ee4673cedec79a659d23b02e06";
        sha256 = "10gmyrz07y4baq1a1rkip6h2k4fy9is6sjv487fndbc931lwmdaf";
      yarn2nixRepo = pkgs.callPackage yarn2nixSrc {};
      inherit (yarn2nixRepo) mkYarnPackage;
      mkYarnPackage {
        src = ./.;

    and add contents to bin/myapp

    #!/usr/bin/env node
    'use strict';
    // For package depenency demonstration purposes only
    var multiply = require('lodash/multiply');
    console.log("h" + multiply(2,0.5) + ", it works!")

    add a “bin” to package.json, see this patch:

    diff --git a/package.json b/package.json
    index 36952fd..a38ceaf 100644
    --- a/package.json
    +++ b/package.json
    @@ -18,6 +18,7 @@
       "devDependencies": {
         "jest-cli": "15.1.1"
    +  "bin" : { "myapp" : "bin/myapp" },
       "jest": {
         "testEnvironment": "node"
  3. now build the source

    nix-build -Q
    building path(s) ‘/nix/store/gkdb8hsykw3idp4xpv6by618wad5s052-offline’
    building path(s) ‘/nix/store/b6kiqlcaacxrlq4nnjgk69mwjnrlvkpv-yarn2nix-modules-0.1.0’
    yarn config v1.3.2
    success Set "yarn-offline-mirror" to "/nix/store/gkdb8hsykw3idp4xpv6by618wad5s052-offline".
    Done in 0.10s.
    yarn install v1.3.2
    [1/4] Resolving packages...
    [2/4] Fetching packages...
    [3/4] Linking dependencies...
    [4/4] Building fresh packages...
    warning Ignored scripts due to flag.
    Done in 4.99s.
    <skipped many lines>>
    building path(s) ‘/nix/store/7yg1mah963w35dhxjcylw5q0r62bkk83-yarn.nix’
    these derivations will be built:
    <skipped many lines>>
    building path(s) ‘/nix/store/chsx89ifr7dk7mc00d6789indh0rn41x-to-fast-properties-1.0.2.tgz’
    building path(s) ‘/nix/store/sqpsd9y77kkq1vvfpfcfzk9q347dwg14-walker-1.0.7.tgz’
    building path(s) ‘/nix/store/j7fp6iz0wj6dcp8ibsqyph2vy4p007wk-watch-0.10.0.tgz’
    building path(s) ‘/nix/store/nv5p8ijp15mc2kbpp26mvd8pn7vqlwyy-webidl-conversions-3.0.1.tgz’
    building path(s) ‘/nix/store/a3amdrmhzrvd3v1lsy1ih5rfpdamyj46-whatwg-url-3.0.0.tgz’
    building path(s) ‘/nix/store/vhwym48w1pvgzli9i57fnzqw0pjp3a83-window-size-0.2.0.tgz’
    building path(s) ‘/nix/store/2dg59ikzfrdwbcpkwg2xwjxs9rhpilb8-worker-farm-1.3.1.tgz’
    building path(s) ‘/nix/store/497p7kg9iyb09ah0vvqx88dpq3bn843p-yargs-5.0.0.tgz’
    building path(s) ‘/nix/store/w3lmwwq4cac653nhajnvyl1k2vhivaws-yargs-parser-3.2.0.tgz’
    building path(s) ‘/nix/store/ywn6ysmnvrkyzjwpdiwr4iivk9z9kx2y-offline’
    building path(s) ‘/nix/store/pnbb86igrhhfw3bdv9kcqnyz4hxgfkmd-example-yarn-package-modules-1.0.0’
    building path(s) ‘/nix/store/r9s6cmq5ik47c0s1hd48xz3r6pixmm1r-example-yarn-package-1.0.0’

    note: the last output line of nix-build is what we are interested in!

  4. checking the result

    ls -lathr result/node_modules/
    <skipped many lines>>
    lrwxrwxrwx 1 root root  102  1. Jan 1970  align-text -> /nix/store/pnbb86igrhhfw3bdv9kcqnyz4hxgfkmd-example-yarn-package-modules-1.0.0/node_modules/align-text
    lrwxrwxrwx 1 root root  105  1. Jan 1970  acorn-globals -> /nix/store/pnbb86igrhhfw3bdv9kcqnyz4hxgfkmd-example-yarn-package-modules-1.0.0/node_modules/acorn-globals
    lrwxrwxrwx 1 root root   97  1. Jan 1970  acorn -> /nix/store/pnbb86igrhhfw3bdv9kcqnyz4hxgfkmd-example-yarn-package-modules-1.0.0/node_modules/acorn
    lrwxrwxrwx 1 root root   98  1. Jan 1970  abbrev -> /nix/store/pnbb86igrhhfw3bdv9kcqnyz4hxgfkmd-example-yarn-package-modules-1.0.0/node_modules/abbrev
    lrwxrwxrwx 1 root root   96  1. Jan 1970  abab -> /nix/store/pnbb86igrhhfw3bdv9kcqnyz4hxgfkmd-example-yarn-package-modules-1.0.0/node_modules/abab
  5. execute the binary

    h1, it works!

using nix-build we declaratively built the node package!

yarn2nix internals

yarn (imperative)

  1. yarn -> read yarn.lock

  2. make / build all the downloads

  3. come up with node_packages folder

  4. yarn install

    note: in general we skip step (4.)

yarn (declarative)

  1. yarn2nix -> read yarn.lock

  2. translate each dependency into a mkDerivation

    each is a mkDerivation residing in /nix/store/…

  3. evaluate each mkDerivation, gain store path

  4. call yarn with the list of all mkDerivation(s), yarn comes up with node_modules

    a single mkDerivation also residing in /nix/store/… but encapsulating other /nix/store entries

  5. final mkDerivation uses this node_modules and creates the store path we are interested in


  • take notice, and i can’t stress this enough, we map yarn.lock entries into single pkgs.stdenv.mkDerivation (by calling mkYarnPackage) but later pass all these into yarn which creates the node_modules contents from this. this is exactly how a DSL-PM must be designed to be easily integrated
  • yarn.lock obsoletes the requirement of manually creating a dependency file per project:
    • shown in the example above where i only added a default.nix
    • i’d wish dep’s Gopkg.lock used with go would also contain a hash already but they are using GIT and we have to call nix-prefetch-git to generate a sha256 hash manually
  • yarn2nix might not be as advanced as node2nix but we at use it for all projects. yarn/yarn2nix is really fast, in dependency calculation and deployment, compared what we had before
  • i’d wish that npm’s sha1 would be replaced by something more recent but in comparison to dep they at least have a hash of some sort
  • yarn2nix is not a part of nixpkgs yet, see
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