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(/\r\n|\n/);
  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(/\r\n|\n/);
      resolve(result);
    };
    // Make sure to handle error states
    reader.onerror = function(e: any) {
      reject(e);
    };
    reader.readAsText(file);
  });
}

Answer 2

Here is the JSBin I have tried and it work like a charm.

function parse(file) {
  const reader = new FileReader();
  reader.readAsText(file);
  reader.onload = function(event) {
    // The file's text will be printed here
  console.log(reader.result)
  }
}

---

Updated:

I write you a Promise version.

async function parse(file) {
  const reader = new FileReader();
  reader.readAsText(file);
  const result = await new Promise((resolve, reject) => {
    reader.onload = function(event) {
    resolve(reader.result)
    }
  })
  console.log(result)
}

Answer 3

To generalize @zero298's answer a tiny bit, here's the generic Promise-based wrapper around FileReader -

// get contents of a file as obtained from an html input type=file element
function getFileContents(file) {
  return new Promise((resolve, reject) => {
    let contents = ""
    const reader = new FileReader()
    reader.onloadend = function (e) {
      contents = e.target.result
      resolve(contents)
    }
    reader.onerror = function (e) {
      reject(e)
    }
    reader.readAsText(file)
  })
}

used like so -

async function parse(file) {
  const contents = await getFileContents(file)
  const result = contents.split(/\r\n|\n/)
  return result
}

or in the general case,

async function show(file) {
  const contents = await getFileContents(file)
  alert(contents)
}