This function asyncHandler is a higher-order function that takes another function requestHandler as an argument. It returns a new function that handles asynchronous operations within Express.js middleware.

Here's how it works:

1. It takes requestHandler as an argument, which is a function that handles a request, response, and next middleware function.
2. It returns a new function that takes req, res, and next as arguments (typical Express.js middleware signature).
3. Within this new function, Promise.resolve() is used to ensure that the requestHandler function always returns a promise. This is important for handling both synchronous and asynchronous code.
4. The requestHandler function is called with req, res, and next arguments, and its return value (which is expected to be a promise) is handled by Promise.resolve().
5. If the promise resolves successfully, the .then() chain doesn't do anything because there's no additional logic specified. If it fails (i.e., the promise is rejected), .catch() is triggered, and it passes the error to the next() function, which sends it to Express's error handling middleware.

src\\utils\\asyncHandler.js

const asyncHandler = (requestHandler) => {
    return (req, res, next) => {
        Promise.resolve(requestHandler(req, res, next)).catch((err) => next(err))
    }
}

export {asyncHandler}

src>>>utils>>>ApiError.js

class ApiError extends Error {
    constructor(
        statusCode,
        message= "Something went wrong",
        errors = [],
        stack = ""
    ){
        super(message)
        this.statusCode = statusCode
        this.data = null
        this.message = message
        this.success = false;
        this.errors = errors

        if (stack) {
            this.stack = stack
        } else{
            Error.captureStackTrace(this, this.constructor)
        }

    }
}

export {ApiError}

**src\\utils\\ApiResponse.js**

class ApiResponse {
    constructor(statusCode, data, message = "Success"){
        this.statusCode = statusCode
        this.data = data
        this.message = message
        this.success = statusCode < 400
    }
}