Catching Fatal Errors with Node.js child_process

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I'm relatively new to hardcore Node.js hacking so I'm seeing all sorts of lovely new errors that I have no clue how to solve when I initially see them.  To this point I've managed to keep a smile on my face while trying to fix these errors, a quality I quite enjoy about myself.  One of the recent errors I encountered was with child_process, whereby an error would occur within an execSync command and the entire app would brick; not even  a try/catch would save me.  I did find a solution, however.

The JavaScript

The best way to catch errors without letting your app brick is by using the process' spawn (or in this case spawnSync) method:

var childProcess = require('child_process');

var commitMessage = (function() {
	var spawn = childProcess.spawnSync('git', ['log', '--format=%B', '-n', '1']);
	var errorText = spawn.stderr.toString().trim();

	if (errorText) {
	  console.log('Fatal error from `git log`.  You must have one commit before deploying.');
	  throw new Error(errorText);
	}
	else {
	  return spawn.stdout.toString().trim();
	}
})();

With this method you can check the stderr buffer first; if there's a String from it you know it errored out, if no error text then the process was clean!

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Discussion

  1. there is also a spawn.status property that holds the exit code. A check for spawn.status !== 0 might be more reliable in cases when there no error message…

  2. eliranmal

    also, spawn.stderr.toString() is not safe, as stderr may be undefined.
    you’re better off just wrapping it with the String constructor:

    String(spawn.stderr)
  3. caoyy

    Perhaps it is more elegant to use status to judge whether the execution was successful or not.

    const childProcess = require('child_process');
    
    var result = (() => {
      const { stderr, stdout, status } = childProcess.spawnSync('npm', ['install']);
    
      if (status !== 0) {
        const errorText = stderr.toString();
        console.log('Fatal error from npm install.');
    
        throw new Error(errorText);
      }
      return stdout.toString();
    })();
    

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