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Showing content from https://developer.mozilla.org/en-US/docs/Web/API/FileReader/result below:

FileReader: result property - Web APIs

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Note: This feature is available in Web Workers.

The result read-only property of the FileReader interface returns the file's contents. This property is only valid after the read operation is complete, and the format of the data depends on which of the methods was used to initiate the read operation.

An appropriate string or ArrayBuffer based on which of the reading methods was used to initiate the read operation. The value is null if the reading is not yet complete or was unsuccessful.

The result types are described below.

This example presents a function reader() which reads a file from a file input. It works by creating a FileReader object and creating a listener for load events, such that when then file is read, the result is obtained and passed to the callback function provided to reader().

The content is handled as raw text data.

// Given this HTMLInputElement of type="file":
// <input id="image" type="file" accept="image/*">

function reader(file, callback) {
  const fr = new FileReader();
  fr.onload = () => callback(null, fr.result);
  fr.onerror = (err) => callback(err);
  fr.readAsDataURL(file);
}

document.querySelector("#image").addEventListener("change", (evt) => {
  // No files, do nothing.
  if (!evt.target.files) {
    return;
  }
  reader(evt.target.files[0], (err, res) => {
    console.log(res); // Base64 `data:image/...` String result.
  });
});

Given the asynchronous nature of FileReader, you could use a Promise-based approach. Here's an example for a file input with attribute multiple that returns a Promise.

// Given this HTMLInputElement:
// <input id="images" type="file" accept="image/*" multiple>

const reader = (file) =>
  new Promise((resolve, reject) => {
    const fr = new FileReader();
    fr.onload = () => resolve(fr);
    fr.onerror = (err) => reject(err);
    fr.readAsDataURL(file);
  });

async function logImagesData(fileList) {
  let fileResults = [];
  const frPromises = fileList.map(reader);

  try {
    fileResults = await Promise.all(frPromises);
  } catch (err) {
    // In this specific case, Promise.all() might be preferred
    // over Promise.allSettled(), since it isn't trivial to modify
    // a FileList to a subset of files of what the user initially
    // selected. Therefore, let's just stash the entire operation.
    console.error(err);
    return;
  }

  fileResults.forEach((fr) => {
    console.log(fr.result); // Base64 `data:image/...` String result.
  });
}

// HTMLInputElement type="file" Event handler:
document.querySelector("#images").addEventListener("change", (evt) => {
  // If no files, do nothing.
  if (!evt.target.files) {
    return;
  }
  logImagesData([...evt.target.files]);
});

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