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### Definition

In the standard library it is implemented like this:

```javascript
type NonNullable<T> = T extends null | undefined ? never : T;
```

That is: if `T` is `null` or `undefined`, the result is `never`; otherwise `T` itself is returned.

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### Usage examples

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### Practical use

1. **Safely accessing values after a check:**

```javascript
  function getLength(value: string | null) {
    if (value) {
      let safe: NonNullable<typeof value> = value; // now definitely a string
      return safe.length;
    }
    return 0;
  }
  ```
2. **Typing API responses after filtering out null:**

```javascript
  const users = [ "Tom", null, "Bob" ];
  const valid = users.filter(Boolean) as NonNullable<typeof users[number]>[];
  // valid: string[]
  ```
3. **Combined with other utility types:**

```javascript
  type User = { name?: string | null };
  type SafeUser = NonNullable<User["name"]>; // string
  ```

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### In short

| Utility | Purpose |
|---|---|
| `NonNullable<T>` | Removes `null` and `undefined` |
| `Partial<T>` | Makes all fields optional |
| `Required<T>` | Makes all fields required |
| `Readonly<T>` | Makes all fields read-only |
| `Extract<T, U>` | Keeps the intersection of types |
| `Exclude<T, U>` | Removes the intersection of types |

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