如何使用JavaScript将图像转换为Base64字符串？

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您可以使用FileAPI，但它几乎不受支持。

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最佳答案

您可以使用HTML5<canvas>：

创建一个画布，将图像加载到其中，然后使用toDataURL()获取Base64表示（实际上，它是一个data: URL，但它包含Base64编码的图像）。

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好吧，如果您使用的是Dojo工具包，它为我们提供了一种直接编码或解码为Base64的方法。

试试这个：

要使用dojox.encoding.base64对字节数组进行编码：

var str = dojox.encoding.base64.encode(myByteArray);

要解码Base64编码的字符串：

var bytes = dojox.encoding.base64.decode(str);

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您可以选择多种方法：

1.方法：FileReader

通过XMLHttpRequest相关文档将图像加载为blob，并使用阅读器接口（ReadAsDataURL（）获取数据的方式）将其转换为数据URL：

function toDataURL(url, callback) {
var xhr = new XMLHttpRequest();
xhr.onload = function() {
var reader = new FileReader();
reader.onloadend = function() {
callback(reader.result);
}
reader.readAsDataURL(xhr.response);
};
xhr.open('GET', url);
xhr.responseType = 'blob';
xhr.send();
}


toDataURL('https://www.gravatar.com/avatar/d50c83cc0c6523b4d3f6085295c953e0', function(dataUrl) {
console.log('RESULT:', dataUrl)
})

此代码示例也可以使用WHATWG获取接口实现：

const toDataURL = url => fetch(url)
.then(response => response.blob())
.then(blob => new Promise((resolve, reject) => {
const reader = new FileReader()
reader.onloadend = () => resolve(reader.result)
reader.onerror = reject
reader.readAsDataURL(blob)
}))




toDataURL('https://www.gravatar.com/avatar/d50c83cc0c6523b4d3f6085295c953e0')
.then(dataUrl => {
console.log('RESULT:', dataUrl)
})

这些方法：

有更好的压缩
也适用于其他文件类型

浏览器支持：

2.方法：Canvas（对于旧版浏览器）

将图像加载到Image-Object中，将其绘制到未受污染的画布并将画布转换回dataURL。

function toDataURL(src, callback, outputFormat) {
var img = new Image();
img.crossOrigin = 'Anonymous';
img.onload = function() {
var canvas = document.createElement('CANVAS');
var ctx = canvas.getContext('2d');
var dataURL;
canvas.height = this.naturalHeight;
canvas.width = this.naturalWidth;
ctx.drawImage(this, 0, 0);
dataURL = canvas.toDataURL(outputFormat);
callback(dataURL);
};
img.src = src;
if (img.complete || img.complete === undefined) {
img.src = "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///ywAAAAAAQABAAACAUwAOw==";
img.src = src;
}
}


toDataURL(
'https://www.gravatar.com/avatar/d50c83cc0c6523b4d3f6085295c953e0',
function(dataUrl) {
console.log('RESULT:', dataUrl)
}
)

详细

支持的输入格式：

image/png，image/jpeg，image/jpg，image/gif，image/bmp，image/tiff，image/x-icon，image/svg+xml，image/webp，image/xxx

支持的输出格式：

image/png，image/jpeg，image/webp（铬）

浏览器支持：

http://caniuse.com/#feat=canvas
Internet Explorer 10（Internet Explorer 10仅适用于相同来源的图像）

3.方法：来自本地文件系统的图像

如果您想转换来自用户文件系统的图像，您需要采取不同的方法。使用阅读器接口：

function encodeImageFileAsURL(element) {
var file = element.files[0];
var reader = new FileReader();
reader.onloadend = function() {
console.log('RESULT', reader.result)
}
reader.readAsDataURL(file);
}

<input type="file" onchange="encodeImageFileAsURL(this)" />

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此代码段可以将您的字符串、图像甚至视频文件转换为Base64字符串数据。

<input id="inputFileToLoad" type="file" onchange="encodeImageFileAsURL();" />
<div id="imgTest"></div>
<script type='text/javascript'>
function encodeImageFileAsURL() {


var filesSelected = document.getElementById("inputFileToLoad").files;
if (filesSelected.length > 0) {
var fileToLoad = filesSelected[0];


var fileReader = new FileReader();


fileReader.onload = function(fileLoadedEvent) {
var srcData = fileLoadedEvent.target.result; // <--- data: base64


var newImage = document.createElement('img');
newImage.src = srcData;


document.getElementById("imgTest").innerHTML = newImage.outerHTML;
alert("Converted Base64 version is " + document.getElementById("imgTest").innerHTML);
console.log("Converted Base64 version is " + document.getElementById("imgTest").innerHTML);
}
fileReader.readAsDataURL(fileToLoad);
}
}
</script>

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据我所知，可以通过FileReader（）将图像转换为Base64字符串或将其存储在画布元素中，然后使用toDataURL（）来获取图像。我有类似的问题，你可以参考这个。

将图像转换为已加载的画布

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以下是我所做的：

// Author James Harrington 2014
function base64(file, callback){
var coolFile = {};
function readerOnload(e){
var base64 = btoa(e.target.result);
coolFile.base64 = base64;
callback(coolFile)
};


var reader = new FileReader();
reader.onload = readerOnload;


var file = file[0].files[0];
coolFile.filetype = file.type;
coolFile.size = file.size;
coolFile.filename = file.name;
reader.readAsBinaryString(file);
}

这就是你如何使用它

base64( $('input[type="file"]'), function(data){
console.log(data.base64)
})

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如果你的形象是

<img id='Img1' src='someurl'>

然后你可以把它转换成

var c = document.createElement('canvas');
var img = document.getElementById('Img1');
c.height = img.naturalHeight;
c.width = img.naturalWidth;
var ctx = c.getContext('2d');


ctx.drawImage(img, 0, 0, c.width, c.height);
var base64String = c.toDataURL();

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试试这个代码：

对于文件上传更改事件，调用此函数：

$("#fileproof").on('change', function () {
readImage($(this)).done(function (base64Data) { $('#<%=hfimgbs64.ClientID%>').val(base64Data); });
});


function readImage(inputElement) {
var deferred = $.Deferred();


var files = inputElement.get(0).files;


if (files && files[0]) {
var fr = new FileReader();
fr.onload = function (e) {
deferred.resolve(e.target.result);
};
fr.readAsDataURL(files[0]);
} else {
deferred.resolve(undefined);
}


return deferred.promise();
}

将Base64数据存储在隐藏文件中以供使用。

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如果你有一个file对象，这个简单的函数将起作用：

function getBase64 (file, callback) {


const reader = new FileReader();


reader.addEventListener('load', () => callback(reader.result));


reader.readAsDataURL(file);
}

使用示例：

getBase64(fileObjectFromInput, function(base64Data){
console.log("Base64 of file is", base64Data); // Here you can have your code which uses Base64 for its operation, // file to Base64 by oneshubh
});

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我发现最安全可靠的方法是使用FileReader()。

演示：图像到Base64

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
</head>
<body>
<input id="myinput" type="file" onchange="encode();" />
<div id="dummy">
</div>
<div>
<textarea style="width:100%;height:500px;" id="txt">
</textarea>
</div>
<script>
function encode() {
var selectedfile = document.getElementById("myinput").files;
if (selectedfile.length > 0) {
var imageFile = selectedfile[0];
var fileReader = new FileReader();
fileReader.onload = function(fileLoadedEvent) {
var srcData = fileLoadedEvent.target.result;
var newImage = document.createElement('img');
newImage.src = srcData;
document.getElementById("dummy").innerHTML = newImage.outerHTML;
document.getElementById("txt").value = document.getElementById("dummy").innerHTML;
}
fileReader.readAsDataURL(imageFile);
}
}
</script>
</body>
</html>

更新-与@AnniekJ请求的评论相同的代码：

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
</head>
<body>
<input id="myinput" type="file" onchange="encode();" />
<div id="dummy">
</div>
<div>
<textarea style="width:100%;height:500px;" id="txt">
</textarea>
</div>
<script>
function encode() {
// Get the file objects that was selected by the user from myinput - a file picker control
var selectedfile = document.getElementById("myinput").files;
// Check that the user actually selected file/s from the "file picker" control
// Note - selectedfile is an array, hence we check it`s length, when length of the array
// is bigger than 0 than it means the array containes file objects
if (selectedfile.length > 0) {
// Set the first file object inside the array to this variable
// Note: if multiple files are selected we can itterate on all of the selectedfile array  using a for loop - BUT in order to not make this example complicated we only take the first file object that was selected
var imageFile = selectedfile[0];
// Set a filereader object to asynchronously read the contents of files (or raw data buffers) stored on the            user's computer, using File or Blob objects to specify the file or data to read.
var fileReader = new FileReader();
// We declare an event of the fileReader class (onload event) and we register an anonimous function that will be executed when the event is raised. it is "trick" we preapare in order for the onload event to be raised after the last line of this code will be executed (fileReader.readAsDataURL(imageFile);) - please read about events in javascript if you are not familiar with "Events"
fileReader.onload = function(fileLoadedEvent) {
// AT THIS STAGE THE EVENT WAS RAISED
// Here we are getting the file contents - basiccaly the base64 mapping
var srcData = fileLoadedEvent.target.result;
// We create an image html element dinamically in order to display the image
var newImage = document.createElement('img');
// We set the source of the image we created
newImage.src = srcData;
// ANOTHER TRICK TO EXTRACT THE BASE64 STRING
// We set the outer html of the new image to the div element
document.getElementById("dummy").innerHTML = newImage.outerHTML;
// Then we take the inner html of the div and we have the base64 string
document.getElementById("txt").value = document.getElementById("dummy").innerHTML;
}
// This line will raise the fileReader.onload event - note we are passing the file object here as an argument to the function of the event
fileReader.readAsDataURL(imageFile);
}
}
</script>
</body>
</html>

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uploadProfile(e) {


let file = e.target.files[0];
let reader = new FileReader();


reader.onloadend = function() {
console.log('RESULT', reader.result)
}
reader.readAsDataURL(file);
}

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这是您可以使用Javascript Promise执行的方式。

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

现在，在事件处理程序中使用它。

const _changeImg = (e) => {
const file = e.target.files[0];
let encoded;
getBase64(file)
.then((result) => {
encoded = result;
})
.catch(e => console.log(e))
}

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document.querySelector('input').onchange = e => {
const fr = new FileReader()
fr.onloadend = () => document.write(fr.result)
fr.readAsDataURL(e.target.files[0])
}

<input type="file">

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需要利用阅读器将blob转换为bas64，更喜欢使用async-wait语法，所以我选择将阅读器逻辑提取到帮助器中，如下所示：

//* Convert resBlob to base64
export const blobToData = (blob: Blob) => {
return new Promise((resolve) => {
const reader = new FileReader()
reader.onloadend = () => resolve(reader.result)
reader.readAsDataURL(blob)
})
}

并在主代码中使用wait调用它：

//* Convert resBlob to dataUrl and resolve
const resData = await blobToData(resBlob)

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我最终使用了返回Promise的function。

const getImg64 = async() => {
const convertImgToBase64URL = (url) => {
console.log(url)
return new Promise((resolve, reject) => {
const img = new Image();
img.crossOrigin = 'Anonymous';
img.onload = () => {
let canvas = document.createElement('CANVAS')
const ctx = canvas.getContext('2d')
canvas.height = img.height;
canvas.width = img.width;
ctx.drawImage(img, 0, 0);
const dataURL = canvas.toDataURL();
canvas = null;
resolve(dataURL)
}
img.src = url;
})
}
//for the demonstration purposes I used proxy server to avoid cross origin error
const proxyUrl = 'https://cors-anywhere.herokuapp.com/'
const image = await convertImgToBase64URL(proxyUrl+'https://image.shutterstock.com/image-vector/vector-line-icon-hello-wave-260nw-1521867944.jpg')
console.log(image)
}
getImg64()

您可以在任何异步函数中使用这种方法。然后您可以为转换后的图像await并继续使用说明。

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如果您面临cors源错误，有一个名为修复的简单代理，它将图像加载到服务器上并将其作为缓冲区数组返回。

因此，我们可以使用提取来获取图像数据并使用filereader将其转换为dataUrl，如@HaNdTriX所述。

  function toDataUrl(url) {
fetch(`https://cors-fix.web.app/v1?url=${url}`)
.then(data => data.blob().then(blob => {
const reader = new FileReader();


reader.onloadend = () =>  {
console.log(reader.result);
};


reader.onerror = () => {
console.log('reader error');
};


reader.readAsDataURL(blob);
}));
}

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您还可以简单地提取Base-64仅URL的一部分，方法是：

var Base64URL = canvas.toDataURL('image/webp')
var Base64 = Base64URL.split(",")[1] //Returns the base64 part

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假设您在浏览器中执行此操作：

等待：

import axios from 'axios'


const response = await axios.get(url, { responseType: 'blob' });
return window.URL.createObjectURL(response.data);

带着承诺

import axios from 'axios'


const response = await axios.get(url, { responseType: 'blob' })
.then((response) => {
const dataUrl = window.URL.createObjectURL(response.data);
// do something with your url
});