Declaration Files

What’s a Declaration File?

Let’s look at a more general, and sometimes more convenient way to declare types for modules: .flow files.

There are two possible use cases, depending on whether an implementation file exists or not.

In the first case, the exported types of a module are declared in a declaration file <FILENAME>.flow, that is located in the same directory as the corresponding implementation file <FILENAME>. The declaration file completely shadows the colocated implementation. In other words, Flow will completely ignore <FILENAME> and just read <FILENAME>.flow instead.

In the second case, the implementation file is missing entirely. <FILENAME>.flow is treated as if it is named <FILENAME>.

Note that the .flow extension applies both to .js files as well as .json ones. The corresponding declaration files have extensions .js.flow and .json.flow, respectively.

Now let’s see an example of the first case documented above. Suppose we have the following code in a file src/LookBeforeYouLeap.js:

// @flow
import { isLeapYear } from "./Misc";
if (isLeapYear("2020")) console.log("Yay!");

and suppose that src/Misc.js has an incompatible implementation of isLeapYear:

// @flow
export function isLeapYear(year: number): boolean {
  return year % 4 == 0; // yeah, this is approximate

If we now create a declaration file src/Misc.js.flow, the declarations in it will be used instead of the code in src/Misc.js. Let’s say we have the following declarations in src/Misc.js.flow.

NOTE: The syntax for declarations in a declaration file is the same as we’ve seen in Creating Library Definitions section.

// @flow
declare export function isLeapYear(year: string): boolean;

What do you think will happen?

Right, the isLeapYear call in src/LookBeforeYouLeap.js will typecheck, since the year parameter expects a string in the declaration file.

As this example shows, declaration files must be written with care: it is up to the programmer to ensure they are correct, otherwise they may hide type errors.

Inlining declarations in regular code

Sometimes it is useful to make declarations inline, as part of the source of an implementation file.

In the following example, say you want to finish writing the function fooList without bothering to mock up its dependencies first: a function foo that takes a number and returns a string, and a class List that has a map method. You can do this by including declarations for List and foo:

declare class List<T> {
  map<U>(f: (x: T) => U): List<U>;
declare function foo(n: number): string;

function fooList(ns: List<number>): List<string> {
  return ns.map(foo);

Just don’t forget to replace the declarations with proper implementations.

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