1function getBase64(file) {
2  return new Promise((resolve, reject) => {
3    const reader = new FileReader();
4    reader.readAsDataURL(file);
5    reader.onload = () => resolve(reader.result);
6    reader.onerror = error => reject(error);
7  });
8}
9
10var file = document.querySelector('#files > input[type="file"]').files[0];
11getBase64(file).then(
12  data => console.log(data)
13);

1// Convert file to base64 string
2export const fileToBase64 = (filename, filepath) => { 
3return new Promise(resolve => {    
4var file = new File([filename], filepath);    
5  var reader = new FileReader();    // Read file content on file loaded event
6  reader.onload = function(event) { 
7    resolve(event.target.result);    };        // Convert data to base64
8  reader.readAsDataURL(file);  
9});
10};
11// Example call:
12fileToBase64("test.pdf", "../files/test.pdf").then(result => {  console.log(result);});