如果你在寻找一个基于承诺的解决方案，这是@Dmitri的代码适应:

function getBase64(file) {
  return new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = () => resolve(reader.result);
    reader.onerror = error => reject(error);
  });
}

var file = document.querySelector('#files > input[type="file"]').files[0];
getBase64(file).then(
  data => console.log(data)
);

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。

这是

// fileObj: File
const base64 = window.URL.createObjectURL(fileObj);

// You can use it with <img src={base64} />

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);
    

通过添加扩展上述解决方案，并在需要的地方添加用例 能够遍历表单上的多个字段并获取它们的值 其中一个是文件，它导致了异步需求的问题

解决方法如下:

    async collectFormData() {
        // Create the file parsing promise
        const toBase64 = file => new Promise((resolve, reject) => {
            const reader = new FileReader();
            reader.readAsDataURL(file);
            reader.onload = () => resolve(reader.result);
            reader.onerror = error => reject(error);
        });
    
        
        let form_vals = []
        let els = [] // Form elements collection

        // This is separate because wrapping the await in a callback
        // doesn't work 
        $(`.form-field`).each(function (e) {
            els.push(this) // add to the collection of form fields
        })

        // Loop through the fields to collect information 
        for (let elKey in els) {
            let el = els[elKey]

            // If the field is input of type file. call the base64 parser
            if ($(el).attr('type') == 'file') {
                // Get a reference to the file
                const file = el.files[0];

                form_vals.push({
                    "key": el.id,
                    "value": await toBase64(file)
                })

        }


        // TODO: The rest of your code here form_vals will now be 
        // populated in time for a server post
    }

这纯粹是为了解决处理多个字段的问题 以一种更流畅的方式

如果你在寻找一个基于承诺的解决方案，这是@Dmitri的代码适应:

function getBase64(file) {
  return new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = () => resolve(reader.result);
    reader.onerror = error => reject(error);
  });
}

var file = document.querySelector('#files > input[type="file"]').files[0];
getBase64(file).then(
  data => console.log(data)
);