Lets add zod to add solid input validation and return erroneous status codes if the input is incorrect

• Install zod

npm install zod

• Update index.ts

import express from "express";
import { z } from 'zod'

export const app = express();
app.use(express.json());

const zodSchemaForSum = z.object({
    a: z.number(),
    b: z.number()
})

app.post("/sum", (req, res) => {
    const parsedData = zodSchemaForSum.safeParse(req.body)

    if (!parsedData.success) {
        return res.status(411).json({
            message: "Incorrect inputs"
        })
    }

    const answer = parsedData.data.a + parsedData.data.b

    res.json({
        answer
    })
});

app.get("/sum", (req, res) => {
    const parsedResponse = zodSchemaForSum.safeParse({
        a: Number(req.headers["a"]),
        b: Number(req.headers["b"])
    })
    
    if (!parsedResponse.success) {
        return res.status(411).json({
            message: "Incorrect inputs"
        })
    }

    const answer = parsedResponse.data.a + parsedResponse.data.b;

    res.json({
        answer
    })
});

• Update sum.test.ts

import { describe, expect, test, it } from '@jest/globals';
import request from "supertest";
import { app } from "../src/index.js"

describe("POST /sum", () => {
  it("should return the sum of two numbers", async () => {
    const res = await request(app).post("/sum").send({
      a: 1,
      b: 2
    });
    expect(res.statusCode).toBe(200);
    expect(res.body.answer).toBe(3);
  });

  it("should return 411 if no inputs are provided", async () => {
    const res = await request(app).post("/sum").send({});
    expect(res.statusCode).toBe(411);
    expect(res.body.message).toBe("Incorrect inputs");
  });

});

describe("GET /sum", () => {
  it("should return the sum of two numbers", async () => {
    const res = await request(app)
      .get("/sum")
      .set({
        a: "1",
        b: "2"
      })
      .send();
    expect(res.statusCode).toBe(200);
    expect(res.body.answer).toBe(3);
  });

  it("should return 411 if no inputs are provided", async () => {
    const res = await request(app)
      .get("/sum").send();
    expect(res.statusCode).toBe(411);
  });

});