Using TypeScript to Implement Content Security Policy Nonce

Question

Using TypeScript to Implement Content Security Policy Nonce

I encountered an issue with my TypeScript Express project while attempting to implement a CSP Nonce using Helmet.

app.use(helmet.contentSecurityPolicy({
    useDefaults: true,
    directives: {
        scriptSrc: ["'self'", (req, res) => `'nonce-${res.locals.cspNonce}'`],
    }
}));

Upon running the program, I received the following error message:

./index.ts:820
return new TSError(diagnosticText, diagnosticCodes);
       ^
TSError: ⨯ Unable to compile TypeScript:
src/app.ts:17:59 - error TS2339: Property 'locals' does not exist on type 'ServerResponse'.

17         scriptSrc: ["'self'", (req, res) => `'nonce-${res.locals.nonce}'`],
                                                         ~~~~~~

at createTSError (/home/frdiolin/WebstormProjects/Calender2.0/node_modules/ts-node/src/index.ts:820:12)
...

The interesting part is that the same code runs without any issues in JavaScript. Can someone shed some light on what might be causing this discrepancy?

typescript express helmet.js

Answer 1

Answer №1

To put it simply: convert res to an Express Response. Check out the code snippet provided below.

This issue is specific to TypeScript only. While res.locals is a feature of Express, Helmet is intended to function independently of Express, which means res.locals are not included in Helmet's types. Essentially, res does not recognize a .locals property because Helmet does not presume it exists.

You can resolve this by specifying to TypeScript that this is an Express response object. Use res as Response.

Here is an illustration:

import express, { Response } from "express";

// ...

app.use(
  helmet.contentSecurityPolicy({
    useDefaults: true,
    directives: {
      scriptSrc: [
        "'self'",
        // The `res as Response` is crucial here.
        (_req, res) => `'nonce-${(res as Response).locals.cspNonce}'`,
      ],
    },
  })
);

Answer 2

To put it simply: convert res to an Express Response. Check out the code snippet provided below.

This issue is specific to TypeScript only. While res.locals is a feature of Express, Helmet is intended to function independently of Express, which means res.locals are not included in Helmet's types. Essentially, res does not recognize a .locals property because Helmet does not presume it exists.

You can resolve this by specifying to TypeScript that this is an Express response object. Use res as Response.

Here is an illustration:

import express, { Response } from "express";

// ...

app.use(
  helmet.contentSecurityPolicy({
    useDefaults: true,
    directives: {
      scriptSrc: [
        "'self'",
        // The `res as Response` is crucial here.
        (_req, res) => `'nonce-${(res as Response).locals.cspNonce}'`,
      ],
    },
  })
);

Using TypeScript to Implement Content Security Policy Nonce

Answer №1

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