第一种(较复杂)

根据图片路径获取图片数据，转成blob类型

用FileReader读取图片blob数据为dataURL

创建img标签，src属性为dataURL

监听img.onload, 创建canvas,将img对象draw在canvas里

添加水印

使用canvas.toBlob转成最终图像

代码

async/await版

/**

*

* @param {图片路径} imgUrl

* @param {图片宽度} width

* @param {图片高度} height

* @param {水印文字} watermarkText

*/

async function watermark(

imgUrl,

width = 300,

height = 300,

watermarkText = "zpfei.ink"

) {

// 1. 根据图片路径获取图片数据，转成blob类型

const fileBlob = await fetch(imgUrl)

.then((r) => r.blob())

.then((file) => file);

// 2. 用`FileReader`读取图片blob数据为dataURL

const reader = new FileReader();

reader.readAsDataURL(fileBlob);

// 3. 创建img标签，src属性为dataURL

const tempImg = await new Promise((resolve) => {

reader.onload = () => {

const img = document.createElement("img");

img.src = reader.result;

resolve(img);

};

});

// 4. 监听`img.onload`, 创建canvas,将img对象`draw`在canvas里

const canvas = await new Promise((resolve) => {

const canvas = document.createElement("canvas");

canvas.width = width;

canvas.height = height;

tempImg.onload = () => {

const ctx = canvas.getContext("2d");

ctx.drawImage(tempImg, 0, 0);

// 5. 添加水印

ctx.fillStyle = "red";

ctx.textBaseline = "middle";

ctx.fillText(watermarkText, 20, 20);

resolve(canvas);

};

});

// 6. 使用`canvas.toBlob`转成最终图像

const newImg = await new Promise((resolve) => {

canvas.toBlob((canvasBlob) => {

const newImg = document.createElement("img"),

url = URL.createObjectURL(canvasBlob);

newImg.onload = function () {

// 图片加载完成后销毁objectUrl

URL.revokeObjectURL(url);

};

newImg.src = url;

resolve(newImg);

});

});

// document.body.appendChild(newImg);

return newImg;

}

常规回调版

const imgUrl =

"https://cdn.jsdelivr.net/gh/zhangpanfei/static@demo/img/test.jpg";

fetch(imgUrl)

.then((r) => r.blob())

.then((file) => {

const reader = new FileReader();

reader.readAsDataURL(file);

reader.onload = () => {

const canvas = document.createElement("canvas");

canvas.width = 400;

canvas.height = 400;

const img = document.createElement("img");

img.src = reader.result;

img.onload = () => {

const ctx = canvas.getContext("2d");

ctx.drawImage(img, 0, 0);

ctx.fillStyle = "red";

ctx.textBaseline = "middle";

ctx.fillText("zpfei.ink", 20, 20);

canvas.toBlob((canvasBlob) => {

const newImg = document.createElement("img"),

url = URL.createObjectURL(canvasBlob);

newImg.onload = function () {

// 图片加载完成后销毁objectUrl

URL.revokeObjectURL(url);

};

newImg.src = url;

document.body.appendChild(newImg);

});

};

};

});

第二种(简单)

图片路径转成canvas

canvas添加水印

canvas转成img

代码

/**

* 图片路径转成canvas

* @param {图片url} url

*/

async function imgToCanvas(url) {

// 创建img元素

const img = document.createElement("img");

img.src = url;

img.setAttribute("crossOrigin", "anonymous"); // 防止跨域引起的 Failed to execute 'toDataURL' on 'HTMLCanvasElement': Tainted canvases may not be exported.

await new Promise((resolve) => (img.onload = resolve));

// 创建canvas DOM元素，并设置其宽高和图片一样

const canvas = document.createElement("canvas");

canvas.width = img.width;

canvas.height = img.height;

// 坐标(0,0) 表示从此处开始绘制，相当于偏移。

canvas.getContext("2d").drawImage(img, 0, 0);

return canvas;

}

/**

* canvas添加水印

* @param {canvas对象} canvas

* @param {水印文字} text

*/

function addWatermark(canvas, text) {

const ctx = canvas.getContext("2d");

ctx.fillStyle = "red";

ctx.textBaseline = "middle";

ctx.fillText(text, 20, 20);

return canvas;

}

/**

* canvas转成img

* @param {canvas对象} canvas

*/

function convasToImg(canvas) {

// 新建Image对象，可以理解为DOM

var image = new Image();

// canvas.toDataURL 返回的是一串Base64编码的URL

// 指定格式 PNG

image.src = canvas.toDataURL("image/png");

return image;

}

// 运行示例

async function run() {

const imgUrl =

"https://cdn.jsdelivr.net/gh/zhangpanfei/static@demo/img/test.jpg";

// 1.图片路径转成canvas

const tempCanvas = await imgToCanvas(imgUrl);

// 2.canvas添加水印

const canvas = addWatermark(tempCanvas, "zpfei.ink");

// 3.canvas转成img

const img = convasToImg(canvas);

// 查看效果

document.body.appendChild(img);

}