When I get the following error:

events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: spawn ENOENT
    at errnoException (child_process.js:1000:11)
    at Process.ChildProcess._handle.onexit (child_process.js:791:34)

What procedure can I follow to fix it?

Author note: Lots of issues with this error encouraged me to post this question for future references.

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• https://stackoverflow.com/questions/27603713/nodejs-spawn-enoent-error-on-travis-calling-global-npm-package
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How to research the spawn call raising the error:

• Use NODE_DEBUG=child_process, Credits to @karl-richter. Simple, quick, October 2019
• Use a wrapper to decorate child_process.spawn, Credits to @jiaji-zhou. Simple, quick, January 2015
• Long procedure, credits to @laconbass. Complex, time-cost, December 2014

Known, usual causes

1. Environment issues

   • The command executable does not exist within the system (dependency not being installed). see prominc's answer
   • The command executable does not exist within a directory of those specified by PATH environment variable.
   • The executable binary was compiled with uncompatible libraries. see danilo-ramirez answer

2. Windows-only bugs/quirks

   • '.cmd' extension / shell: true. see li-zheng answer
   • Administrator permisions. see steve's answer

3. Wrong spawn('command', ['--argument', 'list'], { cwd, env, ...opts }) usage

   • Specified working directory (opts.cwd) does not exist · see leeroy-brun's answer
   • Argument list within command String spawn('command --wrong --argument list')
   • Env vars within command string spawn('ENV_VAR=WRONG command')
   • Argument list Array specified as String spawn('cmd', '--argument list')
   • Unset PATH env variable spawn('cmd', [], { env: { variable } } => spawn('cmd', [], { env: { ...process.env, variable } }

There are 2 posible origins for ENOENT:

1. Code you are writing
2. Code you depend on

When origin is code you depend on, usual cause is an Environment Issue (or windows quirk)