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const file = document.querySelector('#select-file').files[0];
const reader = new FileReader();

reader.onload = res => {
  console.log(res.target.result); 
};
reader.onerror = err => console.log(err);

reader.readAsText(file);

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function readFile(file) {
  return new Promise((resolve, reject) => {
    const reader = new FileReader();

    reader.onload = res => {
      resolve(res.target.result);
    };
    reader.onerror = err => reject(err);

    reader.readAsText(file);
  });
}

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async function onSubmit() {
  const file = document.querySelector('#select-file').files[0];

  const contents = await readFile(file);
}