Setting-debug-strings.md

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Setting debug strings	en	LoopBack 4.0, LoopBack 4, Node.js, TypeScript, OpenAPI, base-connector.js	lb4_sidebar	/doc/en/lb4/Setting-debug-strings.html

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.

Using debug strings

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:".

Debug string format

These strings have the format

module[:area]:string

Where:

• module is the name of the module, for example loopback or loopback-connector-mongodb.
• area is an optional identifier such as cli or repository to identify the purpose of the module
• string is the debug string specified in the target TypeScript/JavaScript source file, such as application.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.

Debug strings reference

Module	Source file	String
@loopback
@loopback/authorization	src/authorize-interceptor.ts	loopback:authorization:interceptor
@loopback/boot	src/bootstrapper.ts	loopback:boot:bootstrapper
	src/booters/interceptor.booter.ts	loopback:boot:interceptor-booter
	src/booters/lifecycle-observer.booter.ts	loopback:boot:lifecycle-observer-booter
	src/booters/model-api.booter.ts	loopback:boot:model-api
	src/booters/service.booter.ts	loopback:boot:service-booter
@loopback/booter-lb3app	src/lb3app.booter.ts	loopback:boot:lb3app
@loopback/cron	src/cron.component.ts	loopback:cron
@loopback/rest-crud	src/crud-rest-builder.ts	loopback:boot:crud-rest
@loopback/cli	please check each generator	loopback:cli:_string_
@loopback/context	src/interceptor.ts	loopback:context:interceptor
	src/binding.ts	loopback:context:binding
	src/context-view.ts	loopback:context:view
	src/invocation.ts	loopback:context:invocation
	src/interceptor-chain.ts	loopback:context:interceptor-chain
@loopback/http-caching-proxy	src/http-caching-proxy.ts	loopback:http-caching-proxy
@loopback/core	src/lifecycle-registry.ts	loopback:core:lifecycle
	src/application.ts	loopback:core:application
@loopback/openapi-v3	src/*	loopback:openapi
@loopback/repository	src/relations/belongs-to/belongs-to.accessor.ts	loopback:repository:relations:belongs-to:accessor
	src/relations/belongs-to/belongs-to.accessor.ts	loopback:repository:relations:belongs-to:accessor
	src/relations/belongs-to/belongs-to.helpers.ts	loopback:repository:relations:belongs-to:helpers
	src/relations/has-many/has-many.helpers.ts	loopback:repository:relations:has-many:helpers
	src/relations/has-many/has-many.inclusion-resolver.ts	loopback:repository:relations:has-many:inclusion-resolver
	src/relations/has-many/has-many.repository-factory.ts	loopback:repository:relations:has-many:repository-factory
	src/relations/has-many/has-many-through.helpers.ts	loopback:repository:relations:has-many-through:helpers
	src/relations/has-many/has-many-through.inclusion-resolver.ts	loopback:repository:relations:has-many-through:inclusion-resolver
	src/relations/has-one/has-one.helpers.ts	loopback:repository:relations:has-one:helpers
	src/relations/has-one/has-one.repository-factory.ts	loopback:repository:relations:has-one:repository-factory
@loopback/repository-json-schema	src/build-schema.ts	loopback:repository-json-schema:build-schema
@loopback/rest	src/rest-server/	loopback:rest:server
	src/sequence.ts	loopback:rest:sequence
loopback-datasource-juggler
	lib/datasource.js	loopback:datasource
Connectors
loopback-connector	lib/connector.js	loopback:connector
loopback-connector-cassandra	lib/cassandra.js	loopback:connector:cassandra
loopback-connector-cloudant	lib/cloudant.js	loopback:connector:cloudant
loopback-connector-couchdb2	lib/couchdb2.js	loopback:connector:couchdb2
loopback-connector-dashdb	lib/dashdb.js	loopback:connector:dashdb
loopback-connector-db2	lib/db2.js	loopback:connector:db2
loopback-connector-ibmi	lib/ibmiconnector.js	loopback:connector:ibmiconnector
loopback-connector-kv-redis	lib/kv-redis.js	loopback:connector:kv-redis
loopback-connector-mongodb	lib/mongodb.js	loopback:connector:mongodb
loopback-connector-mssql	lib/mssql.js	loopback:connector:mssql
loopback-connector-mysql	lib/mysql.js	loopback:connector:mysql
loopback-connector-oracle	lib/oracle.js	loopback:connector:oracle
loopback-connector-postgresql	lib/postgresql.js	loopback:connector:postgresql
loopback-connector-rest	lib/rest-builder.js	loopback:connector:rest
	lib/rest-connector.js	loopback:connector:rest
	lib/rest-model.js	loopback:connector:rest
	lib/swagger-client.js	loopback:connector:rest:swagger
loopback-connector-soap	lib/soap-connector.js	loopback:connector:soap

Adding debugs

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.