javascript - FileReader readAsText() async issues?

Question

I have implemented the following code to parse a CSV via a <input type="file" /> selection:

export async function parse(file: File) {
  let content = '';
  const reader = new FileReader();
  reader.onload = function(e: any) {
    content = e.target.result;
  };
  await reader.readAsText(file);
  const result = content.split(/
|
/);
  return result;
}

If I run this code and put a breakpoint on the line where I declare result, it retrieves the contents of the file successfully. If I do not put any breakpoint, the content is empty. As you can see, I added await to the line where the reader reads the file as text, but it's still not working.

Answer 1

await doesn't help here. readAsText() doesn't return a Promise.

You need to wrap the whole process in a Promise:

export function parse(file: File) {
  // Always return a Promise
  return new Promise((resolve, reject) => {
    let content = '';
    const reader = new FileReader();
    // Wait till complete
    reader.onloadend = function(e: any) {
      content = e.target.result;
      const result = content.split(/
|
/);
      resolve(result);
    };
    // Make sure to handle error states
    reader.onerror = function(e: any) {
      reject(e);
    };
    reader.readAsText(file);
  });
}