unsound.md

Unsound Flow

An attempt to compile a list of examples of unsound behavior of Flow.

Class property initialization

Flow doesn't check that all class properties are initialized in constructor:

class Foo {
  bar: Bar;

  constructor() {
    // forgot to initialize bar
  }
}

const foo = new Foo();
foo.bar.boom();

Workaround: none

Some array operations are unsound

Array element access is inherently unsound:

const arr: Array<Bar> = [new Bar, new Bar];
arr[100].boom();

The same is applicable to strings and dictionaries.

Workarounds:

Avoid using element access, use array methods and for..of instead;

Declare array element type as optional (this can lead to some unintended behavior).

Implicit any

There are multiple ways to get implicit any (i. e. getting any without typing any explicitly):

• importing untyped module;
• using built-in value that uses any in some way;
• using Object or Function;
• error in try-catch;

Workarounds:

• use custom core libdefs and --no-flowlib flag, update them as necessary;
• use IDE with support for flow coverage to visually identify unexpected any;
• use flow coverage command as a part of build process;
• avoid Object or Function, they are basically variants of any;
• avoid try-catch;

Flow allows to delete mandatory properties (facebook/flow#2207)

const foo = new Foo();
delete foo.bar;
foo.bar.boom();

Workarounds:

• avoid using delete, use delete only with dictionaries;

Flow allows using properties of unsealed objects before they are added

const a = {};

a.bar.boom();

a.bar = { boom() {} };

Workarounds:

• avoid using unsealed objects, if possible;
• avoid accessing properties on unsealed objects, pass them as arguments instead;