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);
     })
    }

这是

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

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

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)
);

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);
    

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;
   }
   //...
}

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);
     })
    }