In JavaScript, you can also use type assertions to tell the type system that a value has a certain type. Type assertions in JavaScript are known as "type casting."

You can use the as keyword or the angle bracket syntax to cast a value to a specific type. For example, if you have a variable x that you know is a string, you can cast it to the string type using either of the following syntaxes:

```js
const x = "hello";
const y = x as string; // Using 'as' keyword
const z = <string> x; // Using angle brackets
```

However, it is important to note that type casting does not actually change the type of the value. It is just a way to tell the type system that you know the type of the value. If the value is not actually of the specified type, you may get runtime errors or unexpected behavior.

In addition to type casting, you can also use type guards to narrow the type of a value based on runtime checks. For example, if you have a variable x that can be either a string or a number, you can use a type guard like the following to narrow its type to just a string:

```js
function isString(value: any): value is string {
  return typeof value === "string";
}

let x: string | number = "hello";
if (isString(x)) {
  // x is now narrowed to type 'string'
  console.log(x.toUpperCase());
}
```

In this example, the isString function is a type guard that checks if the value parameter is a string. The value is string syntax is a type predicate that tells the type system that the function returns a boolean value that indicates whether the value is of a certain type. When you use the if (isString(x)) statement, TypeScript knows that x is now narrowed to type string, so you can safely call toUpperCase() on it.
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