Overwrite `any` in TypeScript when merging interfaces

Question

I\'m using Express and I\'m trying to explicitly define res.locals. In the @types/express package, Express.Response.locals is any, so I can\'t seem to

Answer 1

Unfortunately there is no way to override any using interface merging. You can so some surgery on the type and replace the type using mapped and conditional types:

import * as express from 'express'

type Omit = Pick>

type MyResponse = Omit & { 
  locals: {
    myVar: number
  }
}
function middleware(
  req: express.Request, 
  res: MyResponse, 
  next: express.NextFunction
) {
  res.locals.myVar = '10' // error now
  next()
}