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Sanjar Afaq
Sanjar Afaq

Posted on • Edited on

Conditional middlewares in Express.js

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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.
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```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])
```
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  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());
    

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