TSLint: Expression has type `void`. Put it on its own line as a statement

Question

When I run firebase deploy the CLI app runs the TS compiler and TSLint throws the error shown on the commented line:

express.get("/shopify/callback", async (req: Request, res: Response): Promise<Response|void> => {
 const {shop, code, state} = req.query;

 // parse the cookie string into an array. if state cookie is "", err
 const stateCookie = cookie.parse(req.headers.cookie as string || "").state;
 if (!stateCookie) return res.status(400).send("No state cookie");

 if (state !== stateCookie) return res.status(403).send('Cookie failed verification');

 const {hmac, ...params} = req.query;
 const queryParams = queryString.stringify(params);
 const hash = generateEncryptedHash(queryParams); // ERROR: /home/owner/PhpstormProjects/shopify/projectName/functions/src/index.ts:157:15 - Expression has type `void`. Put it on its own line as a statement.
 if (hash !== hmac) return res.status(400).send("HMAC validation failed");

I don't understand what change it wants, does anyone know how to handle this error? Here is the helper function being run on that line:

const generateEncryptedHash = (params: unknown) => {
 if (typeof  SHOPIFY_API_SECRET === "string") {
  crypto.createHmac("sha256", SHOPIFY_API_SECRET).update(params as DataView).digest('hex');
 } else {
  throw Error("during generateEncryptedHash() SHOPIFY_API_SECRET was not a string")
 }
};

The full terminal error output is below:

$ firebase deploy --only functions

=== Deploying to 'projectName'...

i  deploying functions
Running command: npm --prefix "$RESOURCE_DIR" run lint

> functions@ lint /home/owner/PhpstormProjects/shopify/projectName/functions
> tslint --project tsconfig.json


ERROR: /home/owner/PhpstormProjects/shopify/projectName/functions/src/index.ts:157:15 - Expression has type `void`. Put it on its own line as a statement.

npm ERR! code ELIFECYCLE
npm ERR! errno 2
npm ERR! functions@ lint: `tslint --project tsconfig.json`
npm ERR! Exit status 2
npm ERR! 
npm ERR! Failed at the functions@ lint script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/owner/.npm/_logs/2019-12-01T16_46_05_460Z-debug.log

Error: functions predeploy error: Command terminated with non-zero exit code2

Answer 1

You don't return anything from generateEncryptedHash hence the void return type. To compare the string you have to return something from that function. Like that:

const generateEncryptedHash = (params: unknown): string => {
 if (typeof SHOPIFY_API_SECRET === "string") {
  return crypto.createHmac("sha256", SHOPIFY_API_SECRET).update(params as DataView).digest('hex');
 } else {
  throw new Error("during generateEncryptedHash() SHOPIFY_API_SECRET was not a string")
 }
};

I'm not sure what DataView is but you can update digest only with a string/ Buffer. So you have to use JSON.stringify if the input is an object and that's what you want a hash from, e.g. .update(JSON.stringify(params))

UPD: you should set a type for the argument params