In the realm of JavaScript, what happens when a function yields yet another function while also welcoming another function as an argument?

Question

In the realm of JavaScript, what happens when a function yields yet another function while also welcoming another function as an argument?

After following a Node & Express project tutorial on YouTube, I encountered the following code snippet in an async JavaScript file:

const asyncHWrapper = (fn) => {
  return async (req, res, next) => {
    try {
      await fn(req, res, next);
    } catch (error) {
      next(error);
    }
  };
};
module.exports = asyncHWrapper;

Here is how it is used:

const Task = require("../models/taskModel");
const asyncWrapper = require("../middleware/async");
const { createCustomError } = require("../errors/customErrors");
const getAllTasks = asyncWrapper(async (req, res) => {
  const tasks = await Task.find({});
  res.status(200).json({ tasks });
});

I have some questions that are confusing me:

Is it necessary to return an arrow function in the asyncWrapper? Why not just call the function directly?
Where do the parameters (req, res) in the asyncWrapper function come from? Are they from the "fn" function declaration?
Why should two pairs of async and await be written in the wrapper and when calling it?

Thank you for any help! The tutorial link: Node.js / Express Course - Build 4 Projects

javascript function express wrapper

Answer 1

Answer №1

Before addressing your question, let me take a closer look at the code.

Here is the wrapper code snippet:

const asyncHWrapper = (fn) => {
  return async (req, res, next) => {
    try {
      await fn(req, res, next);
    } catch (error) {
      next(error);
    }
  };
};

and this is how it is implemented:

const getAllTasks = asyncWrapper(async (req, res) => {
  const tasks = await Task.find({});
  res.status(200).json({ tasks });
});

The asyncWrapper function takes an fn parameter which, in this case, is the following function:

async (req, res) => {
  const tasks = await Task.find({});
  res.status(200).json({ tasks });
}

When asyncHWrapper is invoked with the above function, it will generate another function, assigned here as getAllTasks.

Now onto your query:

Is it necessary to return an arrow function within the asyncWrapper? Why not simply call the function?

In essence, you could structure it like so:

const asyncHWrapper = async (fn, req, res, next) => {
    try {
      await fn(req, res, next);
    } catch (error) {
      next(error);
    }
};

And then execute it as follows:

await asyncHWrapper(async (req, res) => {
  const tasks = await Task.find({});
  res.status(200).json({ tasks });
}, req, res, next)

But this would essentially be just a regular function with callbacks and should not be labeled as asyncHWrapper.

Where do the parameters (req,res) in the asyncWrapper function originate from? Are they derived from the "fn" function declaration?

No, these parameters are sourced from getAllTasks; your fn solely consumes the values (req, res), while the next parameter serves error-handling purposes. Therefore, when calling getAllTasks, remember to pass in three arguments: getAllTasks(req, res, next)

Why am I required to include two sets of async and await in the wrapper and during its invocation?

If you are referring to both utilizing await when wiring up getAllTasks and executing fn, it's because both functions are asynchronous in nature.

Answer 2

Before addressing your question, let me take a closer look at the code.

Here is the wrapper code snippet:

const asyncHWrapper = (fn) => {
  return async (req, res, next) => {
    try {
      await fn(req, res, next);
    } catch (error) {
      next(error);
    }
  };
};

and this is how it is implemented:

const getAllTasks = asyncWrapper(async (req, res) => {
  const tasks = await Task.find({});
  res.status(200).json({ tasks });
});

The asyncWrapper function takes an fn parameter which, in this case, is the following function:

async (req, res) => {
  const tasks = await Task.find({});
  res.status(200).json({ tasks });
}

When asyncHWrapper is invoked with the above function, it will generate another function, assigned here as getAllTasks.

Now onto your query:

Is it necessary to return an arrow function within the asyncWrapper? Why not simply call the function?

In essence, you could structure it like so:

const asyncHWrapper = async (fn, req, res, next) => {
    try {
      await fn(req, res, next);
    } catch (error) {
      next(error);
    }
};

And then execute it as follows:

await asyncHWrapper(async (req, res) => {
  const tasks = await Task.find({});
  res.status(200).json({ tasks });
}, req, res, next)

But this would essentially be just a regular function with callbacks and should not be labeled as asyncHWrapper.

Where do the parameters (req,res) in the asyncWrapper function originate from? Are they derived from the "fn" function declaration?

No, these parameters are sourced from getAllTasks; your fn solely consumes the values (req, res), while the next parameter serves error-handling purposes. Therefore, when calling getAllTasks, remember to pass in three arguments: getAllTasks(req, res, next)

Why am I required to include two sets of async and await in the wrapper and during its invocation?

If you are referring to both utilizing await when wiring up getAllTasks and executing fn, it's because both functions are asynchronous in nature.

Answer 3

Answer №2

Exploring the concept of a "wrapper" can enhance your understanding. Consider a function called divide:

const divide = (a,b) => a/b;

You can easily use this in regular code:

x = divide(10,5); // sets x to 2.

If you want to account for potential errors like division by zero, you could add error handling code within the divide function itself. Alternatively, you can choose to "wrap" divide with an error handler to separate the error handling logic from the main division logic. This is where defining a safeDivide function becomes valuable:

const safeDivide = catchErrors(divide);

Just like divide, safeDivide also needs to take two arguments. Therefore, the catchErrors wrapper must return a function. Here's a starting point:

const catchErrors = (fn) => {
    return (p,q) => fn(p,q);
}

This initial setup doesn't change much. However, we can now enhance it by introducing try/catch blocks gradually:

const catchErrors = (fn) => {
    return (p,q) => {
        try {
            return fn(p,q);
        } catch (e) {
            console.log("An error occurred. Proceeding with null result.");
            return null;
        }
    }
}

The catchErrors function now returns null if an exception occurs during the execution of the passed function. Next, let’s revisit how the catchErrors wrapper function is utilized:

const safeDivide = catchErrors(divide);

When used with the divide function, the catchErrors wrapper prepares a new function that performs the division while handling any exceptions. This response addresses your initial inquiry.

Your second query pertains to the origins of 'req' and 'res'. These are argument names meant for passing into functions. They are handed over to both the wrapped function and the inner unnamed function that contains Task.find and res.status(200) calls. The Express framework will supply these arguments.

For details on the async/await elements of the wrapper, I'll address that separately.

Answer 4

Exploring the concept of a "wrapper" can enhance your understanding. Consider a function called divide:

const divide = (a,b) => a/b;

You can easily use this in regular code:

x = divide(10,5); // sets x to 2.

If you want to account for potential errors like division by zero, you could add error handling code within the divide function itself. Alternatively, you can choose to "wrap" divide with an error handler to separate the error handling logic from the main division logic. This is where defining a safeDivide function becomes valuable:

const safeDivide = catchErrors(divide);

Just like divide, safeDivide also needs to take two arguments. Therefore, the catchErrors wrapper must return a function. Here's a starting point:

const catchErrors = (fn) => {
    return (p,q) => fn(p,q);
}

This initial setup doesn't change much. However, we can now enhance it by introducing try/catch blocks gradually:

const catchErrors = (fn) => {
    return (p,q) => {
        try {
            return fn(p,q);
        } catch (e) {
            console.log("An error occurred. Proceeding with null result.");
            return null;
        }
    }
}

The catchErrors function now returns null if an exception occurs during the execution of the passed function. Next, let’s revisit how the catchErrors wrapper function is utilized:

const safeDivide = catchErrors(divide);

When used with the divide function, the catchErrors wrapper prepares a new function that performs the division while handling any exceptions. This response addresses your initial inquiry.

Your second query pertains to the origins of 'req' and 'res'. These are argument names meant for passing into functions. They are handed over to both the wrapped function and the inner unnamed function that contains Task.find and res.status(200) calls. The Express framework will supply these arguments.

For details on the async/await elements of the wrapper, I'll address that separately.

In the realm of JavaScript, what happens when a function yields yet another function while also welcoming another function as an argument?

Answer №1

Answer №2

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