Type Annotations

Adding type annotations is an important part of your interaction with Flow.

Flow has a powerful ability to infer the types of your programs. The majority For example, you don't have to produce annotations for common patterns like Array.map:

1["foo", "bar"].map(s => ( // s is inferred to have type string2  s.length3));

Still, there are places where you'll want to add types.

Imagine the following concat function for concatenating two strings together.

1function concat(a, b) {
                 
                    2  return a + b;3}

1:17-1:17: Missing an annotation on `a`. [missing-local-annot]
1:20-1:20: Missing an annotation on `b`. [missing-local-annot]

You need to add annotations on parameters of concat, so that Flow can type check its body. Now you'll get a warning from Flow if you are calling this function with unexpected types.

1function concat(a: string, b: string) {2  return a + b;3}4
5concat("A", "B"); // Works!6concat(1, 2); // Error!
        
           

6:8-6:8: Cannot call `concat` with `1` bound to `a` because number [1] is incompatible with string [2]. [incompatible-call]
6:11-6:11: Cannot call `concat` with `2` bound to `b` because number [1] is incompatible with string [2]. [incompatible-call]

This guide will teach you the syntax and semantics of all the different types you can have in Flow.