How Typescript generics help you write less

links: TypeScript MOC

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What are TypeScript generics?

TS Generics can be used to bring abstraction to your TS interfaces. Using Generics, you can pass interface as a param to another interface.

Here is an example of a standard API response for both a happy path and an error case.

// successful response ✅
{
	status: 'ok',
	responseCode: 200,
	data: {...}
}
 
// error response ❌
{
	responseCode: 500,
	errorMessage: "Something went wrong ?";
}

Instead of writing an interface for every response and adding these keys, you can simply use Generics to create something like these:

interface ApiResponse<T> {
	errorMessage?: string;
	responseCode?: number;
	data?: T
}

interface UserData {
	name: string;
	email: string;
}
 
const response: ApiResponse<UserData> = {
	name: 'Subramanya',
	email: 'subramanya@fyndx.io'
}

Linking generics with functions

Let’s assume we have a function that we use to make an API request to our backend.

const getRequestTo = (endpoint: string) => {
	return fetch(process.env.BE_HOST + endpoint).then(res => res.json())
}
 
const userResponse = getRequestTo('/user-data')

The type of userResponse would be any. We can have a better typescript implementation here.

const getRequestTo = async <R>(endpoint: string): Promise<ApiResponse<R>> => {
	const request = await fetch(process.env.BE_HOST + endpoint);
	const response = await request.json();
	return response;
}

We can create a similar function for POST requests which also takes JSON as params of type B and the server will send back a JSON response of type R:

const postRequestTo = async <B, R>(endpoint: string, body: B): Promise<ApiResponse<R>> => {
	const request = await fetch(process.env.BE_HOST + endpoint, {
		method: "POST",
		body: JSON.stringify(body),
	});
 
	const response = await request.json();
 
	return response;
}

Similarly, there can be a PATCH request function as well, which handles partial updates of any entity.

const patchRequestTo = async <B, R>(endpoint: string,body: Partial<B>): Promise<ApiResponse> => {
	const request = await fetch(process.env.BE_HOST + endpoint, {
    	method: "PATCH",
	    body: JSON.stringify(body)
    });
	const response = await request.json();
	return response;
};

Here is how to implement something like that:

interface RequestBody {}
interface Response {}
 
const createPost = await postRequestTo<RequestBody, Response>('/post', postData);
 
const updatePost = await patchRequestTo<RequestBody, Response>('/post', {
	title: "new name"
});

Let’s pull it all together with a simple JavaScript class:

class ApiService<T> {
	constructor(entitySlug: string) {
		this.entitySlug = entitySlug;
	}
 
	private entitySlug: string;
 
	getOne = async (id: string): Promise<APIResponse<T>> => {
		const request = await fetch(process.env.BE_HOST + this.entitySlug + '/' + id);
		const response = await request.json();
		return response;
	};
    
	getList = async (): Promise<APIResponse<T[]>> => {
		const request = await fetch(process.env.BE_HOST + this.entitySlug);
		const response = await request.json();
		return response;
	};
 
	create = async (body: Omit<T, 'id' | 'created' | 'updated'>): Promise<APIResponse<T>> => {
		const request = await fetch(process.env.BE_HOST + this.entitySlug, {
			method: 'POST',
			body: JSON.stringify(body)
		});
 
		const response = await request.json();
		return response;
	};
    
	update = async (
		body: Omit<Partial<T>, 'id' | 'created' | 'updated'>
	): Promise<APIResponse<T>> => {
		const request = await fetch(process.env.BE_HOST + this.entitySlug + '/' + body.id, {
			method: 'PATCH',
			body: JSON.stringify(body)
		});
		const response = await request.json();
		return response;
	};
    
	delete = async (id: string): Promise<void> => {
		const request = await fetch(process.env.BE_HOST + this.entitySlug + '/' + id, {
			method: 'DELETE'
		});
		const response = await request.json();
		return response;
	};
};

and you can then create an entity service like this:

export const postService = new ApiService<Post>('/post');

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tags: typescript , generics