What is cb() in Node?

Question

Where are people getting cb() from, is this a Node thing or vanilla JS thing?

For example:

Managing Node.js Callback Hell with Promises, Generators and Other Ap

Answer 1

node.js has lots of asynchronous operations that take a completion callback as an argument. This is very common in various node.js APIs.

The node.js convention for this callback is that the first argument passed to the callback is an error code. A falsey value for this first argument means that there is no error.

For example:

fs.readFile("test.txt", function(err, data) {
    if (!err) {
        console.log("file data is: " + data);
    }
});

A function you create yourself may also define it's own callback in order to communicate the end of one or more asynchronous operations.

function getData(id, cb) {
    var fname = "datafile-" + id + ".txt";
    fs.readFile(fname, function(err, data) {
        if (err) {
            cb(err);
        } else if (data.slice(0, 6) !== "Header"){
            // proper header not found at beginning of file data
            cb(new Error("Invalid header"));
        } else {
            cb(0, data);
        }
    });
}

// usage:
getData(29, function(err, data) {
    if (!err) {
        console.log(data);
    }
});