Why is the first parameter of the callback function of Node.js err

  node.js, question
fs.readFile('/etc/passwd', function (err, data) {
 if (err) throw err;
 console.log(data);
 });

Why is the first argument of the callback function err in node.js?

This is an agreement ~ ~ ~
When using callbacks,
First, check whether error exists, and if so, process the error information;
Otherwise, follow the normal business logic.