const fileInput = document.querySelector('input');

fileInput.addEventListener('change', (e) => {

// get a reference to the file
const file = e.target.files[0];

// encode the file using the FileReader API
const reader = new FileReader();
reader.onloadend = () => {

    // use a regex to remove data url part
    const base64String = reader.result
        .replace('data:', '')
        .replace(/^.+,/, '');

    // log to console
    // logs wL2dvYWwgbW9yZ...
    console.log(base64String);
};
reader.readAsDataURL(file);});

我用过这个简单的方法，而且效果很好

 function  uploadImage(e) {
  var file = e.target.files[0];
    let reader = new FileReader();
    reader.onload = (e) => {
    let image = e.target.result;
    console.log(image);
    };
  reader.readAsDataURL(file);
  
}

JavaScript btoa()函数可用于将数据转换为base64编码的字符串

<div>
    <div>
        <label for="filePicker">Choose or drag a file:</label><br>
        <input type="file" id="filePicker">
    </div>
    <br>
    <div>
        <h1>Base64 encoded version</h1>
        <textarea id="base64textarea" 
                  placeholder="Base64 will appear here" 
                  cols="50" rows="15"></textarea>
    </div>
</div>

var handleFileSelect = function(evt) {
    var files = evt.target.files;
    var file = files[0];

    if (files && file) {
        var reader = new FileReader();

        reader.onload = function(readerEvt) {
            var binaryString = readerEvt.target.result;
            document.getElementById("base64textarea").value = btoa(binaryString);
        };

        reader.readAsBinaryString(file);
    }
};

if (window.File && window.FileReader && window.FileList && window.Blob) {
    document.getElementById('filePicker')
            .addEventListener('change', handleFileSelect, false);
} else {
    alert('The File APIs are not fully supported in this browser.');
}

const fileInput = document.querySelector('input');

fileInput.addEventListener('change', (e) => {

// get a reference to the file
const file = e.target.files[0];

// encode the file using the FileReader API
const reader = new FileReader();
reader.onloadend = () => {

    // use a regex to remove data url part
    const base64String = reader.result
        .replace('data:', '')
        .replace(/^.+,/, '');

    // log to console
    // logs wL2dvYWwgbW9yZ...
    console.log(base64String);
};
reader.readAsDataURL(file);});

打印稿版本

const file2Base64 = (file:File):Promise<string> => {
    return new Promise<string> ((resolve,reject)=> {
         const reader = new FileReader();
         reader.readAsDataURL(file);
         reader.onload = () => resolve(reader.result?.toString() || '');
         reader.onerror = error => reject(error);
     })
    }

在搜索如何将文件对象转换为字符串时，此页是第一个匹配项。如果你不关心base64，这个问题的答案很简单:

    str = await file.text()