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.');
}

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;
   }
   //...
}

onInputChange(evt) {
    var tgt = evt.target || window.event.srcElement,
    files = tgt.files;
    if (FileReader && files && files.length) {
        var fr = new FileReader();
        fr.onload = function () {
            var base64 = fr.result;
            debugger;
        }
        fr.readAsDataURL(files[0]);
    }
}

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.');
}

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);
     })
    }

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。