# Source: https://rspack.dev/api/javascript-api/logger.md

CC 4.0 License> The content of this section is derived from the content of the following links and is subject to the CC BY 4.0 license.
> 
> - [https://webpack.js.org/api/logging/](https://webpack.js.org/api/logging/)
> 
> The following contents can be assumed to be the result of modifications and deletions based on the original contents if not specifically stated.
> 
> 


# Logger

Logging output is an additional way to display messages to the end users.

Rspack logger is available to [loaders](/guide/features/loader.md) and [plugins](/guide/features/plugin.md). Emitting as part of the [Stats](/api/javascript-api/stats-json.md) and configured by the user in [rspack configuration](/config/index.md).

Benefits of custom logging API in Rspack:

- Common place to configure the logging display level
- Logging output exportable as part of the `stats.json`
- Stats presets affect logging output
- Plugins can affect logging capturing and display level
- When using multiple plugins and loaders they use a common logging solution
- CLI, UI tools for Rspack may choose different ways to display logging
- Rspack core can emit logging output, e.g. timing data

By introducing Rspack logging API we hope to unify the way Rspack plugins and loaders emit logs and allow better ways to inspect build problems. Integrated logging solution supports plugins and loaders developers by improving their development experience. Paves the way for non-CLI Rspack solutions like dashboards or other UIs.

:::warning Avoid noise in the log
**Avoid noise in the log!**

Keep in mind that multiple plugins and loaders are used together. Loaders are usually processing multiple files and are invoked for every file. Choose a logging level as low as possible to keep the log output informative.
:::

## Examples

### In plugin

There are two types of logging methods:

1. [compilation.getLogger](/api/javascript-api/compilation.md#getlogger): the content will be stored within the stats, use this when logging is related to the compilation.
2. `compiler.getInfrastructureLogger`: the content will not be stored, used this when logging is outside the compilation cycle.

You can use them in your plugin like below:

```js title=MyPlugin.js
const PLUGIN_NAME = 'my-plugin';
export class MyRspackPlugin {
  apply(compiler) {
    // access Logger from compiler
    const logger = compiler.getInfrastructureLogger(PLUGIN_NAME);
    logger.log('log from compiler');

    compiler.hooks.compilation.tap(PLUGIN_NAME, (compilation) => {
      // access Logger from compilation
      const logger = compilation.getLogger(PLUGIN_NAME);
      logger.info('log from compilation');
    });
  }
}
```

### In loader

You can get the logger from loader context like below:

```js title=MyLoader.js
module.exports = function (source) {
  // access Logger from loader
  const logger = this.getLogger('my-loader');
  logger.info('hello Logger');
  return source;
};
```

## Logger API

### Basic API

**Type:** `(...args: any[]): void;`

Methods are in turn from high to low according to the log level:

- `error`: for error messages.
- `warn`: for warnings.
- `info`: for important information messages. These messages are displayed by default. Only use this for messages that the user really needs to see.
- `log`: for unimportant information messages. These messages are displayed only when user had opted-in to see them.
- `debug`: for debugging information. These messages are displayed only when user had opted-in to see debug logging for specific modules.

While using `compilation.getLogger`, the output level can be controlled by `stats.logging` and `stats.loggingDebug`:

```js title="rspack.config.mjs"
export default {
  plugins: [{
    apply(compiler) {
      compiler.hooks.thisCompilation.tap("test plugin", compilation => {
        const logger = compilation.getLogger("TEST");
        logger.error("I am an error");
        logger.warn("I am a warning");
        logger.info("I am an information");
        logger.log("I am a log");
        logger.debug("I am a debug log");
      });
    }
  }],
  stats: {
    logging: "verbose",
    loggingDebug: true
  },
};
```
```js title=Output
asset main.js 264 bytes [emitted] (name: main)
runtime modules 124 bytes 2 modules
./index.js 15 bytes [built] [code generated]

DEBUG LOG from TEST
<e> I am an error
<w> I am a warning
<i> I am an information
    I am a log
    I am a debug log
```

While using `compiler.getInfrastructureLogger`, the output level can be controlled by `infrastructureLogging.level` and `infrastructureLogging.debug`:

```js title="rspack.config.mjs"
export default {
  plugins: [{
    apply(compiler) {
      compiler.hooks.thisCompilation.tap("test plugin", compilation => {
        const logger = compiler.getInfrastructureLogger("TEST");
        logger.error("I am an error");
        logger.warn("I am a warning");
        logger.info("I am an information");
        logger.log("I am a log");
        logger.debug("I am a debug log");
      });
    }
  }],
  infrastructureLogging: {
    level: "verbose",
    debug: true
  },
};
```
```js title=Output
<e> [TEST] I am an error
<w> [TEST] I am a warning
<i> [TEST] I am an information
    [TEST] I am a log
    [TEST] I am a debug log
Rspack compiled successfully in 49 ms
```

### assert

Display errors when assertion is false.

- **Level:** `error`
- **Type:**: `assert(assertion: any, ...args: any[]): void;`

```js
logger.assert(false, "I am an assert error");
logger.assert(true, "Never displayed");
```
```js title=Output
LOG from TEST
<e> I am an assert error
```

### status

Display progress status information, use `console.status` if exists, otherwise fallback to \`console.info.

- **Level:** `info`
- **Type:** `status(...args: any[]): void`

```js
logger.status("status info");
```
```js title=Output
[TEST] status info
```

### trace

Display a stack trace, only available while using compilation logger and also need to enable the `stats.loggingTrace`.

- **Level:** `debug`
- **Type:** `trace(): void`

```js
logger.trace();
```
```js title=Output
DEBUG LOG from TEST
    Trace
|     at Object.fn
|     at SyncHook.callAsyncStageRange
```

### clear

Clean all logs, just like `console.clear()`.

- **Level:** `log`
- **Type:** `clear(): void;`

```js
logger.debug("not displayed");
logger.clear();
logger.debug("will displayed");
```
```js title=Output
[TEST] will displayed
```

### Group API

Includes these methods:

- `group(...args: any[]): void`: to group messages. Displayed collapsed like `logger.log`.
- `groupEnd(...args: any[]): void`: to end a logging group.
- `groupCollapsed(...args: any[]): void`: to group messages together. Displayed collapsed like `logger.log`. Displayed expanded when logging level is set to `'verbose'` or `'debug'`.

```js
logger.group("Group");
logger.info("Info");
logger.log("Log");
logger.debug("Debug");
logger.groupCollapsed("Collapsed group");
logger.log("Log inside collapsed group");
logger.group("Inner group");
logger.log("Inner inner message");
logger.groupEnd();
logger.groupEnd();
logger.log("Log");
logger.groupEnd();
logger.log("End");
```
```js title=Output
<-> [TEST] Group
  <i> [TEST] Info
      [TEST] Log
      [TEST] Debug
  <-> [TEST] Collapsed group
        [TEST] Log inside collapsed group
    <-> [TEST] Inner group
          [TEST] Inner inner message
      [TEST] Log
    [TEST] End
```

### Time API

Includes these methods:

- `time(label: any): void`: to start a timer.
- `timeLog(label: any): void`: record time difference without ending the timer.
- `timeEnd(label: any): void`: to end the timer and record the time difference.
- `timeAggregate(label: any): void`: to aggregate capture the time difference.
- `timeAggregateEnd(label: any): void`: to end the aggregate capturing and record the total difference.

```js
const wait = time => new Promise(resolve => setTimeout(resolve, time))
logger.time("normal");
await wait(100);
logger.timeLog("normal");
await wait(100);
logger.timeEnd("normal");

for (let i = 10;i--;) {
logger.time("aggregate")
await wait(i \* 10);
logger.timeAggregate("aggregate")
}
logger.timeAggregateEnd("aggregate")

```
```js title=Output
<t> [TEST] normal: 101.091167 ms
<t> [TEST] normal: 202.565 ms
<t> [TEST] aggregate: 460.416124 ms
```

### Profile API

Includes these methods:

- `profile(label: any): void`: to start capturing a profile. Delegated to `console.profile` when supported.
- `profileEnd(label: any): void`: to end capturing a profile. Delegated to `console.profileEnd` when supported.

### Child logger

You can also create a child logger with `logger.getChildLogger()`. Child logger has same methods.

```js
const logger = compiler.getInfrastructureLogger("TEST");
logger.info("logger info");
const childLogger = logger.getChildLogger("CHILD");
childLogger.info("child logger info");
```
```js title=Output
<i> [TEST] logger info
<i> [TEST/CHILD] child logger info
```