DeprecationWarning: Calling an asynchronous function without callback is deprecated. - how to find where the "function:" is?

Question

I recently updated my node to 7.2.1 and noticed that there is a warning coming:

(node:4346) DeprecationWarning: Calling an asynchronous function without callback is deprecated.

What is this 4346 for? I only have 2000 lines in the js file, so it can't be a line-number. Where can I find the code?

Answer 1

You can use either the --trace-deprecation or --throw-deprecation options.

For example:

node --trace-deprecation app.js 

or:

node --throw-deprecation app.js 

The first option will log a stack trace and the second will throw an error (which, if not caught, will also log a stack trace).

Also, 4346 is most likely the process ID.

Answer 2

You need to include a callback function for the Asynchronous method (writeFile in your case).

For example

var fs = require('fs'); fs.writeFile('writeMe.txt',data,'utf8',(error)=>{     // your code goes here }); 

where

(error) => { }); 

is the callback function.

From Version: v7.0.0
The callback parameter is no longer optional. Not passing it will emit a deprecation warning.

Please refer: https://nodejs.org/api/fs.html#fs_fs_writefile_file_data_options_callback for more info.