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 of your code can be understood by Flow without annotations. 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-type]
6:11-6:11: Cannot call `concat` with `2` bound to `b` because number [1] is incompatible with string [2]. [incompatible-type]

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