What Does the TypeScript 'as' Keyword Do

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#1

What Does the as Keyword Do?

In the following TypeScript code:

if (process.env.NODE_ENV !== ‘production’) { (WithUser as any).displayName = wrapDisplayName(Component, ‘withUser’); }

I’m not sure if as is considered a keyword, but what does the typescript as keyword do in this context?

#2

The typescript as keyword is not part of vanilla JavaScript; it’s specific to TypeScript. In TypeScript, as any instructs the compiler to treat the object as a plain, untyped JavaScript object. The typescript as keyword is used for type assertions, which tells the compiler to treat the object as a different type than what it has inferred.

#3

In TypeScript, not vanilla JavaScript, the typescript as keyword is used for type assertions. It instructs the compiler to treat a value as a specific type.

For example:

var a = 'TEST STRING';
var b = a as string; // Tells the compiler to treat 'a' as a string

This is equivalent to:

var a = 'TEST STRING';
var b = <string>a; // Another way to assert 'a' as a string

However, using the typescript as syntax is preferred in cases involving JSX (JavaScript with HTML tags), as it avoids potential confusion with the angle bracket syntax.

#4

In TypeScript, the typescript as keyword is used for explicit type casting, allowing you to instruct the TypeScript compiler to treat a value as a different type. This means you can tell TypeScript that you have more knowledge about the type of a value than it can infer, and to handle that value as if it were of a different type.

For example:


let myValue: any = "Hello, world!";

let myLength: number = (<string>myValue).length;

In this code, myValue is of type any, meaning TypeScript has no information about its actual type. By using typescript as, you can assert that myValue is a string:


let myValue: any = "Hello, world!";

let myLength: number = (myValue as string).length;

Both methods achieve the same result, but the second example uses the typescript as keyword for the type casting.

Be aware that using typescript as requires you to be confident about the type of the value. Incorrect type assertions can lead to runtime errors. Therefore, it’s crucial to ensure you are casting to the correct type when using the typescript as keyword.