In a GitHub project I recently saw this function declaration:

function configureStore(onComplete: ?() => void) {

What this question mark is about?

I guess, onComplete is named parameter, getting function calls. And the question mark is stating that this parameter could be optional and will default to "void", which would mean the same like a nil/null pointer what means "no closure" assigned here.

Am I right?

In a GitHub project I recently saw this function declaration:

function configureStore(onComplete: ?() => void) {

What this question mark is about?

I guess, onComplete is named parameter, getting function calls. And the question mark is stating that this parameter could be optional and will default to "void", which would mean the same like a nil/null pointer what means "no closure" assigned here.

Am I right?

• javascript
• flowtype

• No; that's a type. – SLaks Commented May 16, 2016 at 16:54
• Can you explain it a litte more to me pls? (or send a link to docs; i already search at google but did not find anything about) – delete Commented May 16, 2016 at 17:04
• 1 I guess it's either flow or typescript. flowtype, typescriptlang – Felix Kling Commented May 16, 2016 at 17:08
• 5 ? indicates that this is Flow nullable type and not TypeScript. – Estus Flask Commented May 16, 2016 at 17:11
• I just added the URL to the github project. And here also: github./fbsamples/f8app/blob/… – delete Commented May 16, 2016 at 17:19

1 Answer 1

Almost.

() => void is Flow's annotation for a function that returns nothing (undefined, aka void 0).

The leading question mark in ?MyType is Flow's way of expressing a nullable type.

So in this case configureStore accepts one argument called onComplete that must be either null or a function that returns nothing.

Flow will not add a default value for onComplete or coerce it in any way because unlike typescript, Flow does not generate any new JS code. At runtime, all Flow annotations are stripped to get vanilla JS, and that's that.