docs/site/Setting-debug-strings.md
You can specify debug strings when you run an application, as explained below, to display specific log output to the console. You can also redirect the output to a file, if desired. These techniques are often helpful in debugging applications.
LoopBack has a number of built-in debug strings to help with debugging. Specify a string on the command-line via an environment variable as follows:
MacOS and Linux
$ DEBUG=<pattern>[,<pattern>...] npm start
Windows
C:\> set DEBUG=<pattern>[,<pattern>...]
C:\> npm start
where <pattern> is a string-matching pattern specifying debug strings to match. You can specify as many matching patterns as you wish.
For example (MacOS and Linux):
$ DEBUG=loopback:datasource npm start
Or, on Windows:
C:\> set DEBUG=loopback:datasource
C:\> npm start
You'll see output such as (truncated for brevity):
loopback:datasource Settings: {"name":"db","debug":true} -0ms
loopback:datasource Settings: {"name":"geo","connector":"rest",...
You can use an asterisk (*) in the pattern to match any string. For example
the following would match any debug string containing "oracle":
$ DEBUG=*oracle npm start
You can also exclude specific debuggers by prefixing them with a "-" character.
For example, DEBUG=*,-rest-crud:* would include all debuggers except those
starting with "rest-crud:".
These strings have the format
module[:area]:string
Where:
loopback or
loopback-connector-mongodb.cli or repository to identify the
purpose of the moduleapplication.ts.For example:
loopback:cli:model-generator
The debug string model-generator is specified in the source file
generators/model/index.js
of the @loopback/cli module.
As seen before, LoopBack has built-in debug strings to help with debugging.
LoopBack uses the debug package internally. Even if there's no mandate for
LoopBack users to use the same library, you can use this package in your
application to help with debugging.
To do so, you can define your own debug strings like demonstrated in this example:
// Import from debug
import debugFactory from 'debug';
// Define your custom debug string
const debug = debugFactory('example:debug:factory');
// Use it in your code
debug('Oops there was an error!');
To debug parts of your app with this custom debug string, you can run:
DEBUG=example:debug:factory npm start
Refer to the debug documentation for more information.