How to Create Dynamic Type Based on Other Values Using TypeScript?

TypeScript is a powerful tool that helps you improve the quality of your code by adding static type checking. One of the most exciting features of TypeScript is its ability to create dynamic types based on other values. In this article, we’ll explore how to achieve this and take your TypeScript skills to the next level!

What are Dynamic Types?

Before we dive into the nitty-gritty, let’s quickly cover what dynamic types are. In TypeScript, dynamic types are types that are determined at runtime, rather than at compile-time. This means that the type of a value can change based on certain conditions or inputs. Dynamic types are super powerful, as they allow you to create more flexible and adaptable code.

Why Do We Need Dynamic Types?

So, why do we need dynamic types? Well, there are several scenarios where dynamic types come in handy:

• When working with APIs that return different data structures based on user input
• When creating generic functions that need to adapt to different types of data
• When using libraries or frameworks that require dynamic type checking

Creating Dynamic Types Using the `as` Keyword

One of the simplest ways to create dynamic types in TypeScript is by using the `as` keyword. The `as` keyword allows you to explicitly specify the type of a value. Here’s an example:

let dynamicType: string | number = Math.random() < 0.5 ? 'hello' : 42;
console.log(dynamicType as string); // TypeScript will not complain

In this example, we’re using the `as` keyword to tell TypeScript that `dynamicType` is a string, even though it could be a number. This is useful when you know that the type of a value will change based on certain conditions.

Using Conditional Types

Conditional types are a more advanced way to create dynamic types in TypeScript. A conditional type is a type that is based on a condition. The condition can be a type predicate, which is a function that returns a type. Here’s an example:

type isString = T extends string ? true : false;

let dynamicType: string | number = Math.random() < 0.5 ? 'hello' : 42;

if (isString) {
  console.log(dynamicType.toUpperCase()); // TypeScript will not complain
} else {
  console.log(dynamicType.toFixed(2)); // TypeScript will not complain
}

In this example, we’re using a conditional type to determine whether `dynamicType` is a string or a number. If it’s a string, we can call the `toUpperCase()` method, and if it’s a number, we can call the `toFixed()` method.

Inferential Conditional Types

Inferential conditional types are a type of conditional type that infers the type of a value based on a condition. Here’s an example:

type inferType = T extends { name: string } ? { name: string } : { id: number };

let dynamicType: inferType<{ name: string }> = { name: 'John' };
console.log(dynamicType.name); // TypeScript will not complain

let dynamicType2: inferType<{ id: number }> = { id: 42 };
console.log(dynamicType2.id); // TypeScript will not complain

In this example, we’re using an inferential conditional type to infer the type of `dynamicType` and `dynamicType2` based on the shape of the object. If the object has a `name` property, the type is inferred to be `{ name: string }`, and if it has an `id` property, the type is inferred to be `{ id: number }`.

Using the `typeof` Operator

The `typeof` operator is a built-in operator in TypeScript that returns the type of a value as a string. You can use the `typeof` operator to create dynamic types. Here’s an example:

let dynamicType: string | number = Math.random() < 0.5 ? 'hello' : 42;

if (typeof dynamicType === 'string') {
  console.log(dynamicType.toUpperCase()); // TypeScript will not complain
} else {
  console.log(dynamicType.toFixed(2)); // TypeScript will not complain
}

In this example, we’re using the `typeof` operator to determine the type of `dynamicType` at runtime. If it’s a string, we can call the `toUpperCase()` method, and if it’s a number, we can call the `toFixed()` method.

Creating Dynamic Types Using Mapped Types

Mapped types are a powerful feature in TypeScript that allows you to create new types by transforming existing types. You can use mapped types to create dynamic types. Here’s an example:

type DynamicType = {
  [P in keyof T]: T[P] extends string ? string : T[P] extends number ? number : unknown;
};

let dynamicType: DynamicType<{ name: string, age: number }> = { name: 'John', age: 42 };
console.log(dynamicType.name); // TypeScript will not complain
console.log(dynamicType.age); // TypeScript will not complain

In this example, we’re using a mapped type to create a dynamic type that transforms the properties of an object based on their types. If a property is a string, the type is mapped to `string`, and if it’s a number, the type is mapped to `number`.

Best Practices for Creating Dynamic Types

When creating dynamic types, it’s essential to keep the following best practices in mind:

• Use the `as` keyword sparingly, as it can lead to type errors if used incorrectly
• Avoid using dynamic types for complex logic, as it can lead to type errors and make your code harder to maintain
• Use conditional types and inferential conditional types to create more robust and flexible dynamic types
• Use mapped types to transform existing types into dynamic types

Conclusion

Creating dynamic types in TypeScript is a powerful way to add flexibility and adaptability to your code. By using the `as` keyword, conditional types, inferential conditional types, the `typeof` operator, and mapped types, you can create dynamic types that adapt to different scenarios and inputs. Remember to follow best practices and use dynamic types judiciously to avoid type errors and make your code more maintainable.

Type	Description
as keyword	Explicitly specifies the type of a value
Conditional types	Creates a type based on a condition
Inferential conditional types	Infers the type of a value based on a condition
typeof operator	Returns the type of a value as a string
Mapped types	Transforms existing types into dynamic types

I hope this article has helped you understand how to create dynamic types in TypeScript. Remember to practice and experiment with different scenarios to become a master of dynamic types!

Frequently Asked Question

Get ready to unleash the power of TypeScript and create dynamic types based on other values like a pro!