Avoid having types in files where annotation is missingJS-0489

It is not recommended to use flow type imports, aliases, and annotations in files missing a flow file declaration or @flow annotation. It can be fixed by using @flow annotation if the file has flow types instead, use @noflow annotation.

By default, Flow monitors all flow files. It is required to add the flow annotation i.e // @flow. Without this, Flow will simply ignore the file. Flow gathers all the files with this flag and uses the type information available from them to ensure consistency and error-free programming. So if a file is having flow types, annotations, aliases and imports in it but no // @flow annotation before any code in a JavaScript file, those types wont do anything as flow will ignore the file.

Alternatively, developers can use flow check --all or add a property all=true in the .flowconfig file to tell flow to type-check all files.

[options]
all=true

Bad Practice

const x: number = 42;

type FooType = number;

import type A from "a"

export type {A} from "a"

function t<T>(): T{}

Recommended

// @flow
const x: number = 42;

/* @flow weak */
type FooType = number;

/* @noflow */
type FooType = number;

/* @noflow */
import type A from "a"

/* @noflow */
import {type A} from "a"

/* @noflow */
export type {A} from "a"

// an unrelated comment
// @flow
export type {A} from "a"

// @flow
function t<T>(): T{}

References

• Flow Documentation