Question 204909 in Json Question posted in
Our archive of expertly curated questions and answers provides insights and solutions to common problems related to this popular data interchange format. From parsing and manipulating JSON data to integrating it with various programming languages and web services, our archive has got you covered. Start exploring today and take your JSON skills to the next level

JSON reading function returning Undefined instead of expected value – Typescript

I am working on a function to read a JSON file based on what is past to it and return the value. Inside the actual function, I can output to the console exactly what I expect, but when I return it then output to the console I get undefined instead.

The JSON file I’m working with looks like this:

{
    "ECMadrid": "tbd",
    "ECParis": "tbd2"
}

The function I use to read the file looks like this:

    getReferenceNumber(productName){
        fs.readFile('./filepath.json', 'utf-8', (err, jsonString) => {
            if (err) {
                console.log(err)
            } else {
                try {
                    const data = JSON.parse(jsonString);
                    console.log("The reference number before returning is: ", data[productName]);
                    return data[productName];
                } catch (err) {
                    console.log("Error parsing JSON:", err);
                }
            }
        });
    }

And I call the function inside a test file like this:

let referenceNumber = await jsonHelper.getReferenceNumber("ECMadrid");
await console.log("I got sent: ", referenceNumber);

I would expect both of those console logs to output the same result but instead my output is like this (and notably, it outputs the console log that comes after the function call first, which I thought was strange and worth pointing out):

I got sent: undefined
The reference number before returning is: tbd

I’m trying to figure out why the getReferenceNumber function is correctly assigning what I want to data[productName] but returning undefined to the class where I’m actually calling it. Thanks in advance for any feedback

Tags:

2

Answers


  1. 0

    getReferenceNumber is not async, and does not return a promise. The only return statement is in a nested function, which returns from that function, and not from getReferenceNumber. Which means getReferenceNumber returns no value, which is why you get `undefined.

    You probably want fsPromises.readFile instead.

    I believe this is equivalent.

    async getReferenceNumber(productName: string) {
  try {
    const jsonString = await fs.promises.readFile('./filepath.json', 'utf-8')

    try {
      const data = JSON.parse(jsonString);
      console.log("The reference number before returning is: ", data[productName]);
      return data[productName];
    }
    catch (err) {
      console.log("Error parsing JSON:", err);
    }
  } catch (err) {
    console.log(err);
  }
}
    Login or Signup to reply.
  2. 0

    You can swap readFile for readFileSync to use a synchronous call (if you are okay with it being blocking code).
    You can then remove your await‘s in your test file.

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search