Add assertions when implicitly returning from a function

[](https://oss.issuehunt.io/r/gajus/flow-runtime/issues/2)

Currently if a function implicitly returns undefined (by leaving the function without an explicit `return ` statement) we won't generate a check for it, e.g:

```js
function foo (): string {
// no return
}
```
currently compiles to:
```js
function foo () {
const _returnType = t.return(t.string());
// no return
}
```

We should insert a call to `_returnType.assert(undefined)` at these points.

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IssueHunt Summary

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### Submitted pull Requests
- [#249 For #2: Add explicit return (with assetion) if last function statement is not return one](https://oss.issuehunt.io/r/gajus/flow-runtime/pull/249)
- [#301 fix: enforce implicit return assertion for non-empty function bodies](https://oss.issuehunt.io/r/gajus/flow-runtime/pull/301)
- [#306 Fix implicit return assertions after fallthrough statements](https://oss.issuehunt.io/r/gajus/flow-runtime/pull/306)
---

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The issue provides a JavaScript function example and the expected generated assertion, but names no files or tests. Start by tracing the Babel plugin's function-return handling for fallthrough paths. Done means implicitly returning undefined produces a _returnType.assert(undefined) call, with coverage for the shown case and other fallthrough statements.

Written by the indexing model from the issue text.