How is it that a callback function can successfully execute with fewer arguments provided?

Question

How is it that a callback function can successfully execute with fewer arguments provided?

I'm really intrigued by how this code functions. In the verifyUser function, it passes a callback function as the fourth argument to the verifyToken function. However, upon examining the verifyToken function itself, I noticed that it only has three parameters/arguments. How is this possible?

import jwt from "jsonwebtoken";
import { createError } from "../utils/error.js";

export const verifyToken = (req, res, next) => {
    const token = req.cookies.access_token;
    if (!token) {
        return next(createError(401, "You are not authenticated!"));
    }
    
    jwt.verify(token, process.env.JWT, (err, user) => {
        if (err) return next(createError(403, "Token is not valid!"));
        req.user = user;
        next();
    });
};

export const verifyUser = (req, res, next) => {
    verifyToken(req, res, next, () => {
        if (req.user.id === req.params.id || req.user.isAdmin) {
            next();
        } else {
            return next(createError(403, "You are not authorized!"));
        }
    });
};

javascript express web callback backend

Answer 1

Answer №1

After delving into the depths of Express's source code, I unearthed this insightful documentation

Upon invoking the next() function, it behaves like middleware - proceeding to execute the route or any subsequent parameter functions.

Source

It appears that next() first attempts to call a callback argument if one is provided, before moving on to invoke the next middleware.

This design choice seems intended to allow for chaining a call to verifyToken before verifyUser, maintaining the flow without altering the core implementation of next() which would otherwise jump to the next middleware when called within verifyToken, bypassing verifyUser.

I crafted a sample script that demonstrates this concept:

function printResult(object, next) {
  console.log("The final result is: " + object.result);
  next();
}

function printLog(object, next){
  console.log('Running printLog')
  console.log('The current result is: ' + object.result)
  next();
}

function add5(object, next) {
  // Chain printLog before add5 so that it always runs before it
  printLog(object, next, () => {
    console.log('Now running add5')
    object.result = object.result + 5;
    next();
  })
}


function addOne(object, next) {
  console.log('Running addOne')
  object.result = object.result + 1;
  next();
}

function callStack(stack, object) {

  function next(index, object) {
    return function runNext() {
       let caller = runNext.caller
       if(caller.name !== '' && caller.arguments[2]) {
          caller.arguments[2]()
       } else if (stack[index + 1]){
          stack[index + 1](object, next(index + 1, object));
       }
     }  
   }
   
  stack[0](object, next(0, object)); 
}

const stack = [addOne, add5, printResult]
const object = {result: 0}

callStack(stack, object)

It's worth noting that the presence of the anonymous function check caller.name !== '' is necessary due to an error thrown when checking the arguments property of an anonymous function.

A more in-depth explanation of why this feature matters.

If your app follows this sequence, everything works as expected:

Middleware 1
- next() proceeds to next middleware
verifyToken (Middleware 2)
- next() moves to next middleware
Middleware 3

However, in a scenario where `next()` doesn't trigger the parameter function:

Middleware 1
- next() advances to next middleware
verifyUser (Middleware 2)
- invokes verifyToken
  - next() inside verifyToken skips ahead to next middleware <- This causes an issue
- The execution flow never reaches verifyUser <- Due to this skipped step
- next() continues to next middleware
Middleware 3

To address this, they enable `next()` to call a parameter function:

Middleware 1
- next() progresses to next middleware
verifyUser (Middleware 2)
- triggers verifyToken with verifyUser's logic embedded as a callback verifyCB (merely an example alias)
  - next() in verifyToken locates and invokes the callback, namely verifyCB
- next() in verifyCB proceeds to next middleware
Middleware 3

Answer 2

After delving into the depths of Express's source code, I unearthed this insightful documentation

Upon invoking the next() function, it behaves like middleware - proceeding to execute the route or any subsequent parameter functions.

Source

It appears that next() first attempts to call a callback argument if one is provided, before moving on to invoke the next middleware.

This design choice seems intended to allow for chaining a call to verifyToken before verifyUser, maintaining the flow without altering the core implementation of next() which would otherwise jump to the next middleware when called within verifyToken, bypassing verifyUser.

I crafted a sample script that demonstrates this concept:

function printResult(object, next) {
  console.log("The final result is: " + object.result);
  next();
}

function printLog(object, next){
  console.log('Running printLog')
  console.log('The current result is: ' + object.result)
  next();
}

function add5(object, next) {
  // Chain printLog before add5 so that it always runs before it
  printLog(object, next, () => {
    console.log('Now running add5')
    object.result = object.result + 5;
    next();
  })
}


function addOne(object, next) {
  console.log('Running addOne')
  object.result = object.result + 1;
  next();
}

function callStack(stack, object) {

  function next(index, object) {
    return function runNext() {
       let caller = runNext.caller
       if(caller.name !== '' && caller.arguments[2]) {
          caller.arguments[2]()
       } else if (stack[index + 1]){
          stack[index + 1](object, next(index + 1, object));
       }
     }  
   }
   
  stack[0](object, next(0, object)); 
}

const stack = [addOne, add5, printResult]
const object = {result: 0}

callStack(stack, object)

It's worth noting that the presence of the anonymous function check caller.name !== '' is necessary due to an error thrown when checking the arguments property of an anonymous function.

A more in-depth explanation of why this feature matters.

If your app follows this sequence, everything works as expected:

Middleware 1
- next() proceeds to next middleware
verifyToken (Middleware 2)
- next() moves to next middleware
Middleware 3

However, in a scenario where `next()` doesn't trigger the parameter function:

Middleware 1
- next() advances to next middleware
verifyUser (Middleware 2)
- invokes verifyToken
  - next() inside verifyToken skips ahead to next middleware <- This causes an issue
- The execution flow never reaches verifyUser <- Due to this skipped step
- next() continues to next middleware
Middleware 3

To address this, they enable `next()` to call a parameter function:

Middleware 1
- next() progresses to next middleware
verifyUser (Middleware 2)
- triggers verifyToken with verifyUser's logic embedded as a callback verifyCB (merely an example alias)
  - next() in verifyToken locates and invokes the callback, namely verifyCB
- next() in verifyCB proceeds to next middleware
Middleware 3

How is it that a callback function can successfully execute with fewer arguments provided?

Answer №1

A more in-depth explanation of why this feature matters.

If your app follows this sequence, everything works as expected:

However, in a scenario where `next()` doesn't trigger the parameter function:

To address this, they enable `next()` to call a parameter function:

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How is it that a callback function can successfully execute with fewer arguments provided?

Answer №1

A more in-depth explanation of why this feature matters.

If your app follows this sequence, everything works as expected:

However, in a scenario where next() doesn't trigger the parameter function:

To address this, they enable next() to call a parameter function:

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However, in a scenario where `next()` doesn't trigger the parameter function:

To address this, they enable `next()` to call a parameter function: