Type assertions

TypeScript is a strongly typed language that allows you to define the types of your variables and functions. However, there may be times when the compiler can’t determine the type of a variable on its own. That’s where type assertions come in.

Type assertions allow you to tell the compiler what type a variable is, even if it can’t be determined automatically. This can be useful when working with external libraries or when you’re working with a variable that has a more general type than you need.

Here’s how you can use type assertions in TypeScript:

// Using angle bracket syntax
let myVariable: any = 'hello world';
let myString: string = (<string>myVariable).toUpperCase();

// Using the "as" keyword
let myVariable: any = 'hello world';
let myString: string = (myVariable as string).toUpperCase();

In the first example, we use angle bracket syntax to assert that myVariable is a string. We then assign the uppercase version of that string to myString.

In the second example, we use the as keyword to do the same thing.

Type assertions can also be used with complex types, such as arrays or objects:

let myVariable: any = [1, 2, 3];
let myArray: number[] = myVariable as number[];

In this example, we assert that myVariable is an array of numbers. We then assign that array to myArray.

Type assertions can be a powerful tool in your TypeScript arsenal, but they should be used with caution. If you assert the wrong type for a variable, you may run into runtime errors.

In general, it’s best to let TypeScript infer types whenever possible. But when you do need to use a type assertion, make sure you understand what you’re doing and test your code thoroughly.

I hope this tutorial has been helpful in explaining TypeScript type assertions. With this knowledge, you’ll be able to use type assertions to work more effectively with TypeScript and JavaScript.