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// Quick a dirty

var start = 0;

do {
	start = start + 3;
	output = mynum.substr(-start, 3) + (output === '' ? '' : ',') + output;
}
while (mynum.length - start > 3);
if (mynum.length > start) {
	output = mynum.substr(0, mynum.length - start) + ',' + output;
}