ts-data-checker

Type checks JSON at runtime. Uses TypeScript language service to validate JSON strings using TypeScript types. I.e. it is TypeScript compiler running at runtime to validate your values.

npm install ts-data-checker

Check JSON (strings) at runtime

// example1.ts
import { checker } from "ts-data-checker";
import { assertTrue, assertFalse } from "./assert";

export type BasicType = {
    foo: number;
    bar: string;
};

// cf. `import { BasicType } from "./example1";`
const { checkJson } = checker("BasicType", "./example1"); 

assertTrue(checkJson(`{ "foo": 1, "bar": "test" }`));
assertFalse(checkJson(`{ "foo": true, "bar": "test" }`)); // foo is not a number, checkJson returns false

Note: since ts-data-checker uses incremental compilation, subsequent calls to checkJson will be faster than the first call.

Use as express middleware

Type check POST payloads against TypeScript types in express middleware:

import express from "express";
import { BlogPost, Comment } from "./model";
import { checkerMiddleware } from "./checkerMiddleware";

const app = express();

app.post(
    '/create',
    express.raw({ type: "application/json" }),
    checkerMiddleware("BlogPost", "./model"),

    (req, res) => {
        // safe assignment since body is parsed and type checked
        const post: BlogPost = req.body;

        console.log("Creating blog post", post.title);

        return res.json({ success: true }).end();
    });

app.post(
    '/comment',
    express.raw({ type: "application/json" }),
    checkerMiddleware("Comment", "./model"),

    (req, res) => {
        // safe assignment since body is parsed and type checked
        const comment: Comment = req.body;

        console.log("Saving comment from", comment.email);

        return res.json({ success: true }).end();
    });

app.listen(3000);

// model.ts

export type BlogPost = {
    title: string;
    body: string;
    tags: string[];
};

export type Comment = {
    email: string;
    text: string;
};

// checkerMiddleware.ts

import { checker } from "ts-data-checker";
import express from "express";

export function checkerMiddleware(typeName: string, module: string) {

    const { checkJson } = checker(typeName, module);

    return function checkerMiddlewareFn(
        req: express.Request, 
        res: express.Response, 
        next: express.NextFunction) {
        
        const s = String(req.body);
        if (checkJson(s)) {
            try {
                req.body = JSON.parse(s);
                return next();
            }
            catch {
                return res.status(400).end();
            }
        }
        return res.status(400).end();
    };
}

Or, check values

// example2.ts
import { checker } from "ts-data-checker";
import { assertTrue, assertFalse } from "./assert";

export type BasicType = {
    foo: number;
    bar: string;
};

// cf. `import { BasicType } from "./example2";`
const { checkValue } = checker("BasicType", "./example2"); 

assertTrue(checkValue({ foo: 1, bar: "test" }));
assertFalse(checkValue({ foo: true, bar: "test" })); // foo is not a number, checkValue returns false

Note: since ts-data-checker uses incremental compilation, subsequent calls to checkValue will be faster than the first call.

checkValue converts to JSON for you, so if you already have a JSON string, use checkJson instead.

What else?

Check the examples dir for other working examples.

Caveats

• Source code: The data model sources need to be available at runtime. Declarations should be enough though.
• Performance: Performance will probably never be as good as custom coded type guards.
• Limitations: Might break for large structures.

Is this a good idea?

I don't know. Is it? I think this functionality is the missing piece for typescript, to be able to check that untrusted data conforms to a certain model before use. I'm not sure running the full TS compiler at runtime is a good idea. Let's find out, or tell me why if it is not.