TypeScript + Express : req.params

Question

I have the following express route :

app.get('/:id', function (req, res) {
  var idNum: number = Number(req.params.id);
  var idCast: number = +req.params.id;
  var id: number = req.params.id;

  console.log('typeof idNum ', typeof idNum , '  ', 'idNum== 0 :  ', idNum== 0  , '  ', 'idNum=== 0 :  ', idNum=== 0);
  console.log('typeof idCast', typeof idCast, '  ', 'idCast == 0 :', idCast == 0, '  ', 'idCast === 0 :', idCast === 0);
  console.log('typeof id    ', typeof id    , '  ', 'id == 0 :    ', id == 0    , '  ', 'id === 0 :'    , id === 0);

  res.json({});
});

this returns :

typeof idNum  number    idNum== 0   : true    idNum=== 0   : true
typeof idCast number    idCast == 0 : true    idCast === 0 : true
typeof id     string    id == 0     : true    id === 0     : false

I understand that typescript provide only compile-time type-checking, and I guess it means it's not aware that req.params provide arguments as strings.

Is there any way I can automatically cast my params to Number ? or at least raise an error of I didn't done it manually ? Else, it seems that tympescript, unless used in a full-typescript environnement is useless.

Endly, is there any "big" open source project using TypeScript with ExpressJS that I could read the source from ?

Sators · Accepted Answer · 2022-01-13 12:48:37Z

32

You can type the req object by extending the Request type provided by express with the following syntax:

Request<Params, ResBody, ReqBody, ReqQuery>

Therefore, in your example, you could do something like the following to explicitly declare that your property is an incoming string to ensure you convert it to a Number type:

import { Request } from "express"

...

app.get('/:id', function (req: Request<{ id: string}>, res) {
  
...

answered Jan 13, 2022 at 12:48

Sators

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Comments

John · Accepted Answer · 2021-09-12 10:09:14Z

9

It seems that the req.params is any so the compiler has no way of knowing that the value for id is string, BUT of course it's a string as it comes as a path param which are always strings.

You should be able to deal with it using a router middleware, something like:

router.use(function (req, res, next) {
    if (req.params && req.params.id && typeof req.params.id === "string") {
        let num = Number(req.params.id);
        if (!isNaN(num)) {
            req.params.id = Number(req.params.id);
        }
    }
    next();
});

That should convert all params named id (which are strings) into numbers.

edited Sep 12, 2021 at 10:09

John

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answered Sep 6, 2016 at 12:41

Nitzan Tomer

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3 Comments

IggY Over a year ago

Thanks for the answer, but it looks like really dirty to me to do that :/

Nitzan Tomer Over a year ago

I doubt that you'll find a more "automatic" way of doing that. The middleware feature is there for cases just like this one.

Shiv Over a year ago

If you know that id is coming as number from the client url or in api request, No need to check is it string or not, we can type cast directly. Like const id = Number(req.params.id)

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