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 Approaches

they're using cb() to I guess callback and return an error or a value or both in some cases depending on what the callback function sig is?

Answer 1

cb in the context you're describing it is how a vanilla callback function is passed into a (typically) asynchronous function, which is a common pattern in node.js (it's sometimes labelled next, but you can call it bananas if you so desire - it's just an argument).

Typically the first argument is an error object (often false - if all went as planned) and subsequent arguments are data of some form.

For example:

function myAsyncFunction(arg1, arg2, cb) {     // async things     cb(false, { data: 123 }); } 

then using this function:

myAsyncFunction(10, 99, function onComplete(error, data) {     if (!error) {         // hooray, everything went as planned     } else {         // disaster - retry / respond with an error etc     } }); 

Promises are an alternative to this design pattern where you would return a Promise object from myAsyncFunction

For example:

function myAsyncFunction2(arg1, arg2) {     return new Promise(function resolution(resolve, reject, {         // async things         resolve({ data: 123 });     }); } 

then using this function:

myAsyncFunction2(10, 99)     .then(function onSuccess(data) {         // success - send a 200 code etc     })     .catch(function onError(error) {         // oh noes - 500     }); 

They're basically the same thing, just written slightly differently. Promises aren't supported especially widely in a native form, but if put through a transpiler (I'd recommend babel) during a build step they should perform reliably enough in a browser too.

Callbacks will always work in a browser with no shimming / transpilation.