Mastering Existential Types in TypeScript

Working with Existential Types in TypeScript

Existential types, sometimes referred to as existential quantification, is a type system construct that allows for the definition of types in which the type of certain variables is unknown until runtime. This type system construct is particularly useful when building applications that involve dynamic or unknown data. In this article we will explore how to work with existential types in TypeScript.

Using the Any Type

One of the most common ways to use existential types is to employ the any type. The any type is used to declaratively declare a type of a variable without explicitly defining it. This is especially useful when dealing with variables whose type is unknown at compile time. Let’s look at an example:

let x: any;

x = 1; // x is now of type number
x = 'hello'; // x is now of type string

As you can see in the example above, the type of the variable x is determined at runtime as a result of assigning it a value. This is an example of existential typing in action.

Using the Existential Operator

In addition to the any type, TypeScript also provides the existential operator (?) as a way to express existential types in a more explicit manner. The existential operator can be used to explicitly tell the compiler that a given variable may or may not have a certain type. Let’s look at an example:

let x: number | undefined;

x = 1; // x is of type number
x = undefined; // x is of type undefined
x =? 'hello'; // x may or may not be of type string

As you can see in the example above, the existential operator allows us to declare a type for the variable x that is determined at runtime. This means that the type of x may be either a number or a string depending on what value is assigned to it at runtime.

Conclusion

Existential types are a powerful and useful tool to have in your TypeScript toolbox. By leveraging the any type or the existential operator, you can create powerful and reusable types that are only determined at runtime. This allows you to build applications that are more robust and flexible when dealing with dynamic data.