Repository: TypeScript
Issue: tsc includes previous output in result when allowJs is enabled and "exclude" is non-empty
Labels: Suggestion,In Discussion

**TypeScript Version:** 2.7.0-dev.20171222

**Code**

For easy access, clone this gist: https://gist.github.com/WasabiFan/6922d6eb6224945ac809c216fdc37089

**test.ts:**
```ts
// Source code here
```

**tsconfig.json:**
```json
{
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",
    "outDir": "out",
    "allowJs": true
  },
  "exclude": [
    "node_modules"
  ]
}
```

Removing the `exclude` block or disabling `allowJs` avoids the issue.

Note that this _is_ an easy state to get into: in my case, I generated a VSCode extension with their official templates and then enabled `allowJs`. I didn't realize what was going on until my PC ran out of RAM, as it automatically ran the watch task and the depth grew every time I saved.

**Expected behavior:**

I can compile the project with `tsc` as many times as I'd like.

**Actual behavior:**
```
PS D:\...> git clone https://gist.github.com/WasabiFan/6922d6eb6224945ac809c216fdc37089 tsc-recursion-demo-gist
Cloning into 'tsc-recursion-demo-gist'...
remote: Counting objects: 7, done.
remote: Compressing objects: 100% (5/5), done.
remote: Total 7 (delta 1), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (7/7), done.
PS D:\...> cd .\tsc-recursion-demo-gist\
PS D:\...\tsc-recursion-demo-gist> tsc -p .
PS D:\...\tsc-recursion-demo-gist> tsc -p .
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/test.js' because it would overwrite input file.
PS D:\...\tsc-recursion-demo-gist> tsc -p .
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/out/test.js' because it would overwrite input file.
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/test.js' because it would overwrite input file.
PS D:\...\tsc-recursion-demo-gist> tsc -p .
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/out/out/test.js' because it would overwrite input file.
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/out/test.js' because it would overwrite input file.
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/test.js' because it would overwrite input file.
PS D:\...\tsc-recursion-demo-gist> tsc -p .
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/out/out/out/test.js' because it would overwrite input file.
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/out/out/test.js' because it would overwrite input file.
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/out/test.js' because it would overwrite input file.
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/test.js' because it would overwrite input file.
PS D:\...\tsc-recursion-demo-gist> tsc -p .
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/out/out/out/out/test.js' because it would overwrite input file.
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/out/out/out/test.js' because it would overwrite input file.
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/out/out/test.js' because it would overwrite input file.
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/out/test.js' because it would overwrite input file.
error TS5055: Cannot write file 'D:/.../tsc-recursion-demo-gist/out/test.js' because it would overwrite input file.
```

<img src="https://user-images.githubusercontent.com/3310349/34297314-513d601a-e6cc-11e7-895c-e158930e2936.png" width="400" />

The fix (and the best option) is to exclude the output directory. But given that existing templates _don't_ do that, I think this is a significant issue. I don't understand why this doesn't happen when the `exclude` block is removed; I assume there's an internal safety to prevent this problem that's overridden.