doc: fix module.md headings · nodejs/node@b667cc4

Register a module that exports [hooks][] that customize Node.js module

resolution and loading behavior. See [Customization hooks][].

## `module.stripTypeScriptTypes(code[, options])`

### `module.stripTypeScriptTypes(code[, options])`

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});

```

## Module compile cache

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The module compile cache can be enabled either using the [`module.enableCompileCache()`][]

method or the [`NODE_COMPILE_CACHE=dir`][] environment variable. After it is enabled,

whenever Node.js compiles a CommonJS or a ECMAScript Module, it will use on-disk

[V8 code cache][] persisted in the specified directory to speed up the compilation.

This may slow down the first load of a module graph, but subsequent loads of the same module

graph may get a significant speedup if the contents of the modules do not change.

To clean up the generated compile cache on disk, simply remove the cache directory. The cache

directory will be recreated the next time the same directory is used for for compile cache

storage. To avoid filling up the disk with stale cache, it is recommended to use a directory

under the [`os.tmpdir()`][]. If the compile cache is enabled by a call to

[`module.enableCompileCache()`][] without specifying the directory, Node.js will use

the [`NODE_COMPILE_CACHE=dir`][] environment variable if it's set, or defaults

to `path.join(os.tmpdir(), 'node-compile-cache')` otherwise. To locate the compile cache

directory used by a running Node.js instance, use [`module.getCompileCacheDir()`][].

Currently when using the compile cache with [V8 JavaScript code coverage][], the

coverage being collected by V8 may be less precise in functions that are

deserialized from the code cache. It's recommended to turn this off when

running tests to generate precise coverage.

The enabled module compile cache can be disabled by the [`NODE_DISABLE_COMPILE_CACHE=1`][]

environment variable. This can be useful when the compile cache leads to unexpected or

undesired behaviors (e.g. less precise test coverage).

Compilation cache generated by one version of Node.js can not be reused by a different

version of Node.js. Cache generated by different versions of Node.js will be stored

separately if the same base directory is used to persist the cache, so they can co-exist.

At the moment, when the compile cache is enabled and a module is loaded afresh, the

code cache is generated from the compiled code immediately, but will only be written

to disk when the Node.js instance is about to exit. This is subject to change. The

[`module.flushCompileCache()`][] method can be used to ensure the accumulated code cache

is flushed to disk in case the application wants to spawn other Node.js instances

and let them share the cache long before the parent exits.

### `module.constants.compileCacheStatus`

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The following constants are returned as the `status` field in the object returned by

[`module.enableCompileCache()`][] to indicate the result of the attempt to enable the

[module compile cache][].

<table>

<tr>

<th>Constant</th>

<th>Description</th>

</tr>

<tr>

<td><code>ENABLED</code></td>

<td>

Node.js has enabled the compile cache successfully. The directory used to store the

compile cache will be returned in the <code>directory</code> field in the

returned object.

</td>

</tr>

<tr>

<td><code>ALREADY_ENABLED</code></td>

<td>

The compile cache has already been enabled before, either by a previous call to

<code>module.enableCompileCache()</code>, or by the <code>NODE_COMPILE_CACHE=dir</code>

environment variable. The directory used to store the

compile cache will be returned in the <code>directory</code> field in the

returned object.

</td>

</tr>

<tr>

<td><code>FAILED</code></td>

<td>

Node.js fails to enable the compile cache. This can be caused by the lack of

permission to use the specified directory, or various kinds of file system errors.

The detail of the failure will be returned in the <code>message</code> field in the

returned object.

</td>

</tr>

<tr>

<td><code>DISABLED</code></td>

<td>

Node.js cannot enable the compile cache because the environment variable

<code>NODE_DISABLE_COMPILE_CACHE=1</code> has been set.

</td>

</tr>

</table>

### `module.enableCompileCache([cacheDir])`

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* `cacheDir` {string|undefined} Optional path to specify the directory where the compile cache

will be stored/retrieved.

* Returns: {Object}

* `status` {integer} One of the [`module.constants.compileCacheStatus`][]

* `message` {string|undefined} If Node.js cannot enable the compile cache, this contains

the error message. Only set if `status` is `module.constants.compileCacheStatus.FAILED`.

* `directory` {string|undefined} If the compile cache is enabled, this contains the directory

where the compile cache is stored. Only set if `status` is

`module.constants.compileCacheStatus.ENABLED` or

`module.constants.compileCacheStatus.ALREADY_ENABLED`.

Enable [module compile cache][] in the current Node.js instance.

If `cacheDir` is not specified, Node.js will either use the directory specified by the

[`NODE_COMPILE_CACHE=dir`][] environment variable if it's set, or use

`path.join(os.tmpdir(), 'node-compile-cache')` otherwise. For general use cases, it's

recommended to call `module.enableCompileCache()` without specifying the `cacheDir`,

so that the directory can be overridden by the `NODE_COMPILE_CACHE` environment

variable when necessary.

Since compile cache is supposed to be a quiet optimization that is not required for the

application to be functional, this method is designed to not throw any exception when the

compile cache cannot be enabled. Instead, it will return an object containing an error

message in the `message` field to aid debugging.

If compile cache is enabled successfully, the `directory` field in the returned object

contains the path to the directory where the compile cache is stored. The `status`

field in the returned object would be one of the `module.constants.compileCacheStatus`

values to indicate the result of the attempt to enable the [module compile cache][].

This method only affects the current Node.js instance. To enable it in child worker threads,

either call this method in child worker threads too, or set the

`process.env.NODE_COMPILE_CACHE` value to compile cache directory so the behavior can

be inherited into the child workers. The directory can be obtained either from the

`directory` field returned by this method, or with [`module.getCompileCacheDir()`][].

### `module.flushCompileCache()`

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Flush the [module compile cache][] accumulated from modules already loaded

in the current Node.js instance to disk. This returns after all the flushing

file system operations come to an end, no matter they succeed or not. If there

are any errors, this will fail silently, since compile cache misses should not

interfere with the actual operation of the application.

### `module.getCompileCacheDir()`

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* Returns: {string|undefined} Path to the [module compile cache][] directory if it is enabled,

or `undefined` otherwise.

<i id="module_customization_hooks"></i>

## Customization Hooks

`path` is the resolved path for the file for which a corresponding source map

should be fetched.

### `module.flushCompileCache()`

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> Stability: 1.1 - Active Development

Flush the [module compile cache][] accumulated from modules already loaded

in the current Node.js instance to disk. This returns after all the flushing

file system operations come to an end, no matter they succeed or not. If there

are any errors, this will fail silently, since compile cache misses should not

interfer with the actual operation of the application.

### Class: `module.SourceMap`

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