Any

any opts out of type checking entirely. It is unsafe and should be avoided. Do not confuse it with unknown (safe) or empty (the bottom type).

let x: any = 42;
x.foo.bar; // No error — type checking is disabled

When to use this​

Almost never. Prefer unknown for values of arbitrary type — it accepts any value while remaining type-safe by requiring refinement before use. Reserve any for incremental migration of untyped code or the rare case where correct code cannot be expressed in Flow's type system. You can enforce this with the unclear-type lint rule.

How any bypasses type checking​

For example, the following code will not report any errors:

function add(one: any, two: any): number {
  return one + two;
}


add(1, 2);     // Works.
add("1", "2"); // Works.
add({}, []);   // Works.

Even code that will cause runtime errors will not be caught by Flow:

function getNestedProperty(obj: any) {
  return obj.foo.bar.baz;
}


getNestedProperty({});

There are only a couple of scenarios where you might consider using any:

1. When you are in the process of converting existing code to using Flow types and you are currently blocked on having the code type checked (maybe other code needs to be converted first).
2. When you are certain your code works and for some reason Flow is unable to type check it correctly. There are a (decreasing) number of idioms in JavaScript that Flow is unable to statically type.

You can ban any by enabling the unclear-type lint rule.

You can use the coverage command to identify code typed as any.

Avoid leaking any​

When you have a value with the type any, you can cause Flow to infer any for the results of all of the operations you perform.

For example, if you get a property on an object typed any, the resulting value will also have the type any.

function fn(obj: any) {
  let foo = obj.foo; // Results in `any` type
}

You could then use the resulting value in another operation, such as adding it as if it were a number and the result will also be any.

function fn(obj: any) {
  let foo = obj.foo; // Results in `any` type
  let bar = foo * 2; // Results in `any` type
}

You could continue this process until any has leaked all over your code.

function fn(obj: any) {
  let foo = obj.foo;
  let bar = foo * 2;
  return bar; // Results in `any` type
}


let bar = fn({ foo: 2 }); // Results in `any` type
let baz = "baz:" + bar; // Results in `any` type

Prevent this from happening by cutting any off as soon as possible by casting it to another type.

function fn(obj: any) {
  let foo: number = obj.foo;
}

Now your code will not leak any.

See Also​

• Unknown — the safe supertype of all types, requiring refinement before use
• Empty — the bottom type, the subtype of all types
• Type Hierarchy — how any, unknown, and empty relate in the type system
• Coverage — identifying code typed as any
• Lint Rule Reference — the unclear-type lint to ban any usage