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.');
}

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);
  
}

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);});

现代ES6路(异步/await)

const toBase64 = file => new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = () => resolve(reader.result);
    reader.onerror = error => reject(error);
});

async function Main() {
   const file = document.querySelector('#myfile').files[0];
   console.log(await toBase64(file));
}

Main();

UPD:

如果你想捕捉错误

async function Main() {
   const file = document.querySelector('#myfile').files[0];
   try {
      const result = await toBase64(file);
      return result
   } catch(error) {
      console.error(error);
      return;
   }
   //...
}

尝试使用FileReader类的解决方案:

function getBase64(file) {
   var reader = new FileReader();
   reader.readAsDataURL(file);
   reader.onload = function () {
     console.log(reader.result);
   };
   reader.onerror = function (error) {
     console.log('Error: ', error);
   };
}

var file = document.querySelector('#files > input[type="file"]').files[0];
getBase64(file); // prints the base64 string

注意，.files[0]是一个File类型，它是Blob的子类型。因此它可以与FileReader一起使用。 请参阅完整的工作示例。

下面是我写的几个函数，用于获得json格式的文件，可以轻松传递:

    //takes an array of JavaScript File objects
    function getFiles(files) {
        return Promise.all(files.map(getFile));
    }

    //take a single JavaScript File object
    function getFile(file) {
        const reader = new FileReader();
        return new Promise((resolve, reject) => {
            reader.onerror = () => { reader.abort(); reject(new Error("Error parsing file"));}
            reader.onload = function () {

                //This will result in an array that will be recognized by C#.NET WebApi as a byte[]
                let bytes = Array.from(new Uint8Array(this.result));

                //if you want the base64encoded file you would use the below line:
                let base64StringFile = btoa(bytes.map((item) => String.fromCharCode(item)).join(""));

                //Resolve the promise with your custom file structure
                resolve({ 
                    bytes,
                    base64StringFile,
                    fileName: file.name, 
                    fileType: file.type
                });
            }
            reader.readAsArrayBuffer(file);
        });
    }

    //using the functions with your file:

    file = document.querySelector('#files > input[type="file"]').files[0]
    getFile(file).then((customJsonFile) => {
         //customJsonFile is your newly constructed file.
         console.log(customJsonFile);
    });

    //if you are in an environment where async/await is supported

    files = document.querySelector('#files > input[type="file"]').files
    let customJsonFiles = await getFiles(files);
    //customJsonFiles is an array of your custom files
    console.log(customJsonFiles);