如何在JavaScript中转换文件到base64 ?

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.');
}

使用FileReader 类尝试解决方案:

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

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(file => getFile(file)));
}


//take a single JavaScript File object
function getFile(file) {
var 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: bytes,
base64StringFile: 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);

在德米特里·帕夫卢汀和乔舒亚的基础上。这是一个扩展版本，它提取base64内容(在开始时删除元数据)，并确保填充是正确的。

function getBase64(file) {
return new Promise((resolve, reject) => {
const reader = new FileReader();
reader.readAsDataURL(file);
reader.onload = () => {
let encoded = reader.result.toString().replace(/^data:(.*,)?/, '');
if ((encoded.length % 4) > 0) {
encoded += '='.repeat(4 - (encoded.length % 4));
}
resolve(encoded);
};
reader.onerror = error => reject(error);
});
}

现代ES6方式(异步/等待)

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();

乌利希期刊指南:

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]);
}
}

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);
  

}

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);
})
}

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);});

使用这种方法将任何文件转换为base64

_fileToBase64(file: File) {
return new Promise((resolve, reject) => {
const reader = new FileReader();
reader.readAsDataURL(file);
reader.onload = () => resolve(reader.result.toString().substr(reader.result.toString().indexOf(',') + 1));
reader.onerror = error => reject(error);
});
}

这是

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


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

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

    str = await file.text()

通过添加扩展上述解决方案，在我需要的用例 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
}