Conditional middlewares in Express.js

Github link to article

Situation

There is a backend app with many middlewares, including two named X and Y. The requirement is that only X runs or Y runs based on a condition.
 

Solution

How do we handle this?
There are a few cases here, with minor variations.

1. Keep it linear - we add the middlewares one after the other. In the body of these middlewares we return early if the condition is not matching.

   • If both are custom middlewares

   function mwX(req, res, next) {
       if(condition)
           return next();
       // do X mw logic
   }
   
   function mwY(req, res, next) {
       if(condition)
           return next();
       // do Y mw logic
   }
   
   app.use([mwX, myY]);
   

- If any of them is library generated. Or if you don't want to litter conditional code into middlewares.

```js
// of course, this can be done using array too
function xConditional(req, res, next) {
    if(condition)
        return X;
    return ((req, res, next) => { return next(); });
}

function yConditional(req, res, next) {
    if(condition)
        return Y;
    return ((req, res, next) => { return next(); });
}


app.use([xConditional, yConditional])
```

1. Ternary - works only if the condition is static (i.e. not dependent on req or res), we can use the ternary operator. app.use(condition ? X : Y);. X and Y could be custom or library generated doesn't matter. Code:
   

   app.use(staticCondition ? X : Y);
   

Note:

• Prefer differentiation based on route, method and router before doing this. They are in-built control mechanisms which are intuitive and easy to use.
• Can be generalized to more than 2 middlewares too.

• Direct wrapper (special case) - if the middlewares are exclusively conditional. Code:
  

  function wrapperMiddleware() {
    if(condition) // based on req or res logic or otherwise
      return X;
    else
      return Y;
  }
  
  app.use(wrapperMiddleware());