---
layout: guide
---

> **Warning:** Do not mistake `any` with `mixed`. [Read more](../mixed/)

If you want a way to opt-out of using the type checker, `any` is the way to do
it. **Using `any` is completely unsafe, and should be avoided whenever
possible.**

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

```js
// @flow
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:

```js
// @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.

## Avoid leaking `any` <a class="toc" id="toc-avoid-leaking-any" href="#toc-avoid-leaking-any"></a>

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`.

```js
// @flow
function fn(obj: any) {
  let foo /* (:any) */ = obj.foo;
}
```

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`.

```js
// @flow
function fn(obj: any) {
  let foo /* (:any) */ = obj.foo;
  let bar /* (:any) */ = foo * 2;
}
```

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

```js
// @flow
function fn(obj: any) /* (:any) */ {
  let foo /* (:any) */ = obj.foo;
  let bar /* (:any) */ = foo * 2;
  return bar;
}

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

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

```js
// @flow
function fn(obj: any) {
  let foo: number = obj.foo;
}
```

Now your code will not leak `any`.

```js
// @flow
function fn(obj: any) /* (:number) */ {
  let foo: number = obj.foo;
  let bar /* (:number) */ = foo * 2;
  return bar;
}

let bar /* (:number) */ = fn({ foo: 2 });
let baz /* (:string) */ = "baz:" + bar;
```
