Run a maximum of X processes at the same time in a non-async node script

spawn_pool.js

const fs = require('fs'), child_process = require('child_process');

function RunSync(cmd, args)
{
	console.log('Running sync: ' + cmd + ' ' + args.join(' '));
	const proc = child_process.spawnSync(cmd, args, {stdio:[0,1,2]});
	if (proc.status === null) throw 'Error while starting ' + cmd + '. Executable not found at path or no access.';
	if (proc.status !== 0)    throw 'Error while running ' + cmd + '. An error should have been printed above.';
}

function WaitProcs(procs, maxProcs)
{
	for (;;) //clear finished processes and wait while there are more than maxProcs running at the same 
	{
		for (var i = procs.length; i--;)
		{
			if (procs[i].IsRunning()) continue;
			procs[i].AbortIfError();
			procs.splice(i, 1);
		}
		if (procs.length <= (maxProcs|0)) return;
		child_process.spawnSync(process.execPath,['-e','setTimeout(function(){},100)']); //sleep 100 ms
	}
}

function RunAsync(cmd, args, outPath, procs, maxProcs)
{
	WaitProcs(procs, maxProcs - 1);
	console.log('Running async: ' + cmd + ' ' + args.join(' '));
	var proc = child_process.spawn(cmd, args, {stdio:[0,1,2]});
	if (proc.pid === undefined) throw 'Error while starting ' + cmd + '. Executable not found at path or no access.';
	procs.push({
		AbortIfError: () => {if (!fs.existsSync(outPath)) throw 'Error while running ' + cmd + '. An error should have been printed above.'},
		IsRunning:    () => {try{return process.kill(proc.pid,0)}catch{}},
	});
}

// fake program that waits for a while (arg 1) then writes a file (arg 2)
var myCmd = process.execPath;
var myArgs = [ '-e', "setTimeout(function(){require('fs').writeFileSync(process.argv[2], 1)},process.argv[1])" ];

// spawn 10 processes that take 2 seconds async, except the last one which takes .5 seconds and is spawned synchronous
var maxProcs = 4; //run a maximum of 4 processes at the same time
var procs = [];
for (var i = 0, iMax = 10; i != iMax; i++)
{
	var waitTime = (i == iMax - 1 ? 500 : 2000);
	var outPath = "TEMP" + i + ".DAT";
	try { fs.unlinkSync(outPath); } catch (e) {}
	(i == iMax - 1 ? RunSync : RunAsync)(myCmd, myArgs.concat([ waitTime, outPath ]), outPath, procs, maxProcs);
}

console.log("Waiting for remaining processes ...");
WaitProcs(procs);
console.log("Done!");