title: Cannot find module 'commander'
body: <p>When I try and run my mocha tests I get </p>
  
  <pre><code>$ npm t
  
  &gt; digitalAlarmClock@1.0.0 test /home/durrantm/Dropnot/code/js/mochaChai/digitalAlarmClock
  &gt;  mocha *.test.js
  
  internal/modules/cjs/loader.js:979
    throw err;
    ^
  
  Error: Cannot find module 'commander'
  Require stack:
  - /usr/lib/nodejs/mocha/bin/_mocha
      at Function.Module._resolveFilename (internal/modules/cjs/loader.js:976:15)
      at Function.Module._load (internal/modules/cjs/loader.js:859:27)
      at Module.require (internal/modules/cjs/loader.js:1036:19)
      at require (internal/modules/cjs/helpers.js:72:18)
      at Object.&lt;anonymous&gt; (/usr/lib/nodejs/mocha/bin/_mocha:10:17)
      at Module._compile (internal/modules/cjs/loader.js:1147:30)
      at Object.Module._extensions..js (internal/modules/cjs/loader.js:1167:10)
      at Module.load (internal/modules/cjs/loader.js:996:32)
      at Function.Module._load (internal/modules/cjs/loader.js:896:14)
      at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12) {
    code: 'MODULE_NOT_FOUND',
    requireStack: [ '/usr/lib/nodejs/mocha/bin/_mocha' ]
  }
  npm ERR! Test failed.  See above for more details.
  </code></pre>
  
  <p>How to fix?</p>