Sometimes you'll end up in a situation where you'll know a little better than TypeScript about the type of something.

Usually, this will happen when you know the type of a value is more specific than its current type.

*Type assertions* are a way to tell the compiler "trust me, I know what I'm doing."

A type assertion is like a type cast in other languages, but where some languages might perform some special checking or restructuring of data, a type assertion has no runtime impact.

The assumption is that you, the programmer, have performed any special checks that you need.

Type assertions have two forms.

One is the "angle-bracket" syntax:

let someValue: any;

let strLength: number = (<string>someValue).length;

And the other is the `as`-syntax:

let someValue: any;

let strLength: number = (someValue as string).length;

The two samples are equivalent.

Using one over the other is mostly a choice of preference; however, when using TypeScript with JSX, only `as`-style assertions are allowed.

We'll discuss the details later, but many common problems in JavaScript are alleviated by using `let`, so it's a best practice to adopt it over using `var` whenever possible.