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Flow Shenanigans

note

Not every example here is a bug. Sometimes it's just unexpected behavior resulting from the type-system design or from type system theory in general.

Dangerous array access​

None of the typing systems can handle this correctly, all show no error during static analysis (but could be runtime error).

Flow (pr):

let a = [1, 2, 3];
let b: number = a[10]; // undefined
let c = b * 2;

Typescript (issue):

let a = [1, 2, 3];
let b: number = a[10]; // undefined
let c = b * 2;

ReasonML:

let a: array(int) = [|1, 2, 3|];
let b: int = a[10]  // undefined
let c = b * 2

boolean is incompatible with true | false​

declare function foo(true | false): void;
declare function bar(): boolean;

foo(bar());
4: foo(bar())
       ^ Cannot call `foo` with `bar()` bound to the first parameter because: Either boolean [1] is incompatible with boolean literal `true` [2]. Or boolean [1] is incompatible with boolean literal `false` [3].
References:
2: declare function bar(): boolean                           ^ [1]
1: declare function foo(true | false): void
                        ^ [2]
1: declare function foo(true | false): void
                               ^ [3]

https://github.com/facebook/flow/issues/4196

mixed type cannot be exhausted​

declare var flowDebugPrint: $Flow$DebugPrint;

function test(x: mixed) {
  if (typeof x === 'string') {
    return true;
  }
  flowDebugPrint(x); // still 'mixed', see the output 🤔
}

flow.org/try

Debug output:

{
  "reason": {
    "pos": {
      "source": "-",
      "type": "SourceFile",
      "start": { "line": 7, "column": 18 },
      "end": { "line": 7, "column": 18 }
    },
    "desc": "mixed"
  },
  "kind": "MixedT"
}

One solution is to manually define your custom mixed type which can be exhausted.

Incorrect array destructing​

const [w, a, t] = { p: '' }; // no error

flow.org/try

On the other hand, TS is OK with this code (while Flow throws an error correctly):

const foo: {} = '';

https://www.typescriptlang.org/play/index.html#code/MYewdgzgLgBAZiEAuGBvAvjAvDA5LgbiA

Defaults for non-existent properties are allowed​

const React = require('react');

function Component({ defaultProp = 'string' }) {
  return null;
}

<Component />;

This is allowed even though some people would expect something like "Error, defaultProp is missing in props", it's a feature: https://github.com/facebook/flow/commit/6dec7d5dbbd12a6f210f7c3ae21841a932eb71a8 (from 0.109.0)