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Document 'experimental_enhanced' compatibility mode #1630

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Document 'experimental_enhanced' compatibility mode
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25 changes: 23 additions & 2 deletions docs/sources/next/using-k6/javascript-compatibility-mode.md
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Expand Up @@ -20,15 +20,18 @@ To enable ES module support, k6 uses [Babel](https://babeljs.io/) internally to

![Babel transformation in k6](/media/docs/k6-oss/diagram-grafana-k6-babel-pipeline.png)

Some users prefer to bundle their test code outside k6. For this reason, k6 offers two JavaScript compatibility modes:
Additionally, k6 also has experimental support for esbuild, to transpile TypeScript (TS) code and to support most ES6+ features.

Some users prefer to bundle their test code outside k6. For this reason, k6 offers three JavaScript compatibility modes:

- [Extended mode](#extended-mode): The default option, supporting ESM and most ES6+ features.
- [Experimental enhanced mode](#experimental-enhanced-mode): The experimental option, supporting TS and most ES6+ features.
- [Base mode](#base-mode): Limited to CommonJS, excluding the Babel step.

When running tests, you can change the mode by using the `--compatibility-mode` option:

| Env | CLI | Code / Config file | Default |
| ----------------------- | ---------------------- | ------------------ | ------------ |
|-------------------------|------------------------|--------------------|--------------|
| `K6_COMPATIBILITY_MODE` | `--compatibility-mode` | N/A | `"extended"` |

## Extended mode
Expand All @@ -47,6 +50,24 @@ As illustrated in the previous diagram, if k6 detects unsupported ES+ features w

Currently, the k6 Babel transformation only adds ESM support and sets `global` (node's global variable) with the value of `globalThis`.

## Experimental enhanced mode

{{< code >}}

```cli
$ k6 run --compatibility-mode=experimental_enhanced script.js
```

```env
$ K6_COMPATIBILITY_MODE=experimental_enhanced k6 run script.js
```

{{< /code >}}

The experimental enhanced mode is similar to the extended mode, but it uses esbuild instead of Babel to transpile TypeScript (TS) code and to support most ES6+ features.

TypeScript support is partial as it removes the type information but does not provide type safety.

## Base mode

{{< code >}}
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