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docs: document deno eval CJS/ESM auto-detection (2.8) #3091

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15 changes: 13 additions & 2 deletions runtime/reference/cli/eval.md
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Expand Up @@ -24,14 +24,25 @@ TypeScript works out of the box:
deno eval "const greeting: string = 'Hello'; console.log(greeting)"
```

## CommonJS support
## CommonJS and ESM auto-detection

CommonJS modules are automatically recognized and supported:
Starting in Deno 2.8, `deno eval` inspects the snippet for `import` /
`export` declarations and treats it as ESM if it finds any. Otherwise the
snippet runs as CommonJS — meaning `require`, `module.exports`, and friends
work without any flag.

```sh
# Detected as CommonJS — no --ext=cjs needed
deno eval "const path = require('path'); console.log(path.join('a', 'b'))"

# Detected as ESM because of the static import
deno eval "import { ok } from 'node:assert'; ok(true); console.log('ok')"
```

If you need to override the heuristic — for example when a snippet has no
imports but you still want it parsed as ESM — pass `--ext=mjs` (or
`--ext=cjs` to force CommonJS).

## Printing expression results

Use `--print` (or `-p`) to evaluate an expression and print its result, similar
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