You can use a type assertion to tell the TypeScript compiler that you know the type of a value better than it does. A type assertion is like a type cast in other languages, but it doesn't actually change the type of the value at runtime. Instead, it tells the compiler to treat the value as if it had the specified type.

Here's an example:

```typescript
let value: string | number = "hello";
let length: number = (value as string).length; // type assertion
```

In this example, `value` is declared as a union type of `string` and `number`. However, we know that in this particular case, `value` is a string. We use a type assertion to tell the compiler that we want to treat `value` as a string, so we can access its `length` property.

Note that you should only use type assertions when you're absolutely sure that the value has the specified type. If you're wrong, you could introduce runtime errors or other bugs into your code.
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