What does npm mean by 'Skipping failed optional dependency'?

Question

Latest version of node and npm causing problems in running karma. When I try to install karma-cli npm i -g karma karma-cli I get following warning:

npm WARN optional Skipping failed optional dependency /chokidar/fsevents:
npm WARN notsup Not compatible with your operating system or architecture: [email protected]
npm WARN optional Skipping failed optional dependency /gulp-karma/chokidar/fsevents:
npm WARN notsup Not compatible with your operating system or architecture: [email protected]

And when I try to run karma test, I get following error:

"C:\Program Files\nodejs\node.exe" "C:\Program Files (x86)\JetBrains\WebStorm 11.0.3\plugins\js-karma\js_reporter\karma-intellij\lib\intellijServer.js" --karmaPackageDir=C:\Users\xxx\AppData\Roaming\npm\node_modules\karma --configFile=C:\Users\xxx\Documents\project\karma.conf.js
26 02 2016 11:24:07.871:WARN [plugin]: Cannot find plugin "karma-chrome-launcher".
  Did you forget to install it ?
  npm install karma-chrome-launcher --save-dev
26 02 2016 11:24:07.877:WARN [plugin]: Cannot find plugin "karma-firefox-launcher".
  Did you forget to install it ?
  npm install karma-firefox-launcher --save-dev
26 02 2016 11:24:07.878:WARN [plugin]: Cannot find plugin "karma-phantomjs-launcher".
  Did you forget to install it ?
  npm install karma-phantomjs-launcher --save-dev
26 02 2016 11:24:07.879:WARN [plugin]: Cannot find plugin "karma-jasmine".
  Did you forget to install it ?
  npm install karma-jasmine --save-dev
26 02 2016 11:24:07.880:WARN [plugin]: Cannot find plugin "karma-junit-reporter".
  Did you forget to install it ?
  npm install karma-junit-reporter --save-dev
26 02 2016 11:24:07.885:WARN [plugin]: Cannot find plugin "karma-teamcity-reporter".
  Did you forget to install it ?
  npm install karma-teamcity-reporter --save-dev
26 02 2016 11:24:07.887:WARN [plugin]: Cannot find plugin "karma-coverage".
  Did you forget to install it ?
  npm install karma-coverage --save-dev
26 02 2016 11:24:07.888:WARN [plugin]: Cannot find plugin "karma-jasmine-html-reporter".
  Did you forget to install it ?
  npm install karma-jasmine-html-reporter --save-dev
26 02 2016 11:24:07.889:WARN [plugin]: Cannot find plugin "karma-ng-html2js-preprocessor".
  Did you forget to install it ?
  npm install karma-ng-html2js-preprocessor --save-dev
26 02 2016 11:24:07.890:WARN [plugin]: Cannot find plugin "karma-browserify".
  Did you forget to install it ?
  npm install karma-browserify --save-dev
26 02 2016 11:24:07.893:WARN [preprocess]: Can not load "coverage", it is not registered!
  Perhaps you are missing some plugin?
26 02 2016 11:24:07.893:WARN [preprocess]: Can not load "ng-html2js", it is not registered!
  Perhaps you are missing some plugin?
26 02 2016 11:24:07.893:WARN [preprocess]: Can not load "browserify", it is not registered!
  Perhaps you are missing some plugin?
26 02 2016 11:24:07.901:WARN [reporter]: Can not load "coverage", it is not registered!
  Perhaps you are missing some plugin?
26 02 2016 11:24:07.901:WARN [reporter]: Can not load "junit", it is not registered!
  Perhaps you are missing some plugin?
C:\Users\xxx\AppData\Roaming\npm\node_modules\karma\node_modules\di\lib\injector.js:9
      throw error('No provider for "' + name + '"!');
      ^

Error: No provider for "framework:jasmine"! (Resolving: framework:jasmine)
    at error (C:\Users\xxx\AppData\Roaming\npm\node_modules\karma\node_modules\di\lib\injector.js:22:12)
    at Object.parent.get (C:\Users\xxx\AppData\Roaming\npm\node_modules\karma\node_modules\di\lib\injector.js:9:13)
    at get (C:\Users\xxx\AppData\Roaming\npm\node_modules\karma\node_modules\di\lib\injector.js:54:19)
    at C:\Users\xxx\AppData\Roaming\npm\node_modules\karma\lib\server.js:138:20
    at Array.forEach (native)
    at Server._start (C:\Users\xxx\AppData\Roaming\npm\node_modules\karma\lib\server.js:137:21)
    at invoke (C:\Users\xxx\AppData\Roaming\npm\node_modules\karma\node_modules\di\lib\injector.js:75:15)
    at Server.start (C:\Users\xxx\AppData\Roaming\npm\node_modules\karma\lib\server.js:102:18)
    at Object.<anonymous> (C:\Program Files (x86)\JetBrains\WebStorm 11.0.3\plugins\js-karma\js_reporter\karma-intellij\lib\intellijServer.js:12:13)
    at Module._compile (module.js:410:26)
    at Object.Module._extensions..js (module.js:417:10)
    at Module.load (module.js:344:32)
    at Function.Module._load (module.js:301:12)
    at Function.Module.runMain (module.js:442:10)
    at startup (node.js:136:18)
    at node.js:966:3

Process finished with exit code 1

My configuration specification as follows:

Node version: v4.3.0
NPM version: 3.7.3

I am not able to get karma version installed, I think it's not installed properly in my machine.

Solution

This is not an error. It is a warning that fseventsd, which is Mac OS specific, cannot be installed on Linux.

There is no need to be alarmed, and the package that needs fsevents will still work - that's why it's an optional dependency.

Since many people are confused about this (particularly since this used to be a real error, not a warning) there's an open bug to make the current warning into an INFO instead

Additionally yarn already does this.