Store the filereader output in a variable for future reference

Question

Store the filereader output in a variable for future reference

I am struggling to find a simple answer, even though my code is simple. I attempted the following approach, but every time I try to console.log my testResult, it always returns null. How can I correctly save data from a file?

public getFile(
    sourceFile: File
  ): string {
    let testResult;
    const file = sourceFile[0]
    const fileReader = new FileReader();
    fileReader.readAsText(file, "UTF-8")
    fileReader.onloadend = (e) => { 
      testResult = fileReader.result.toString()
    } 
    console.log(testResult)
    return testResult
  }

This issue ties back to previous topics where I struggled to handle loading a JSON file, translating it, and displaying it to the user. If I could save this file outside of onloadend, I believe I could overcome the remaining challenges (previous methods failed, and this one sets me back at the start).

json typescript file filereader

Answer 1

Answer №1

The issue you're facing is a common one that relates to asynchronous operations. The function assigned to the onloadend request is only called when the loadend event is triggered. However, the rest of the code does not wait for this event and continues executing. This means that the console.log will be executed immediately, returning an empty testResult.

To address this, place the console.log(testResult) line within your onloadend handler:

fileReader.onloadend = (e) => { 
  testResult = fileReader.result.toString();
  console.log(testResult);
}

At this point, testResult will no longer be empty and you can proceed with handling it inside this function. However, if you want the getFile method to be reusable and return the testResult for processing elsewhere, you should wrap it in a Promise like so:

public getFile(
    sourceFile: File
  ): Promise<string> {
    return new Promise((resolve) => {
      const file = sourceFile[0]
      const fileReader = new FileReader();
      fileReader.onloadend = (e) => { 
        const testResult = fileReader.result.toString();
        resolve(testResult);
      }
      fileReader.readAsText(file, "UTF-8");
    });
  }

Now, whenever you need to access a file, you can use the yourInstance.getFile method as shown below:

yourInstance.getFile().then(testResult => {
  // Do whatever you need here
  console.log(testResult);
});

Alternatively, you can use async/await to process the result:

async function processResult() {
  const testResult = await yourInstance.getFile();
  // Do whatever you need
  console.log(testResult);
}

If you're unfamiliar with promises and/or async/await, you can learn more about them here and here.

Answer 2

The issue you're facing is a common one that relates to asynchronous operations. The function assigned to the onloadend request is only called when the loadend event is triggered. However, the rest of the code does not wait for this event and continues executing. This means that the console.log will be executed immediately, returning an empty testResult.

To address this, place the console.log(testResult) line within your onloadend handler:

fileReader.onloadend = (e) => { 
  testResult = fileReader.result.toString();
  console.log(testResult);
}

At this point, testResult will no longer be empty and you can proceed with handling it inside this function. However, if you want the getFile method to be reusable and return the testResult for processing elsewhere, you should wrap it in a Promise like so:

public getFile(
    sourceFile: File
  ): Promise<string> {
    return new Promise((resolve) => {
      const file = sourceFile[0]
      const fileReader = new FileReader();
      fileReader.onloadend = (e) => { 
        const testResult = fileReader.result.toString();
        resolve(testResult);
      }
      fileReader.readAsText(file, "UTF-8");
    });
  }

Now, whenever you need to access a file, you can use the yourInstance.getFile method as shown below:

yourInstance.getFile().then(testResult => {
  // Do whatever you need here
  console.log(testResult);
});

Alternatively, you can use async/await to process the result:

async function processResult() {
  const testResult = await yourInstance.getFile();
  // Do whatever you need
  console.log(testResult);
}

If you're unfamiliar with promises and/or async/await, you can learn more about them here and here.

Store the filereader output in a variable for future reference

Answer №1

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