---
title: "Late Initialization"
description: "Learn about running Sentry in an ESM or CJS application, in scenarios where you cannot run init early."
url: https://docs.sentry.io/platforms/javascript/guides/flue/install/late-initialization/
---

# Late Initialization | Sentry for Flue

Are you unsure if you should use this installation method? Review our [installation methods](https://docs.sentry.io/platforms/javascript/guides/flue/install.md).

For auto-instrumentation to work, `Sentry.init()` normally has to run before anything else is imported in your application.

In some cases that isn't possible, for example when you fetch your DSN from an external source. For these cases, `--import` the SDK itself instead of your own instrument file. The SDK then wraps the modules your application imports, and you call `Sentry.init()` later:

```bash
node --import @sentry/node/import main.mjs
```

This works for CommonJS applications as well. `--require` is no longer supported, so use `--import` there too:

```bash
node --import @sentry/node/import app.js
```

With the flag in place, call `Sentry.init()` at the point of your choosing:

```javascript
import * as Sentry from "@sentry/node";
import startApp from "./app.mjs";
import fetchDsn from "./utils/fetchDsn.mjs";

startApp();

const dsn = await fetchDsn();
Sentry.init({
  dsn,

  // Add Tracing by setting tracesSampleRate
  // We recommend adjusting this value in production
  tracesSampleRate: 1.0,
});

// From now on, Sentry is initialized,
// and the app is still auto-instrumented
```

Use this method only if it's strictly necessary. Anything that happens before `Sentry.init()` runs isn't captured, so an error thrown during startup goes unreported. In most cases it's better to find a way to run `Sentry.init()` early, as described in the [quickstart](https://docs.sentry.io/platforms/javascript/guides/flue.md).

## [What Wrapping Means](https://docs.sentry.io/platforms/javascript/guides/flue/install/late-initialization.md#what-wrapping-means)

`--import @sentry/node/import` wraps the modules your application imports before your code runs. At that point the modules are wrapped but inactive: nothing is emitted or captured from them.

Once you call `Sentry.init()`, the wrapped modules start emitting data to Sentry.