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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)