HTML

 <input style="display: none;" type="file" id="upFile" onchange="imgChange(this)" accept="image/png,image/gif,image/jpeg" />

JS

1.不使用文件服务器中专建议使用 FormData 格式提交

function imgChange(event) {
 //$("#upFile")[0].files[0]    或者  img.files[0]  =》获取文件流
 //window.URL.createObjectURL(img.files[0]) =》 获取本地文件路径

    var formData = new FormData()
    formData.set('image', img.files[0]) //使用 set 便于更新替换图片
}

2.通过 FileReader() 获取base64编码

function imgChange(event) {
    const reader = new FileReader();
    reader.readAsDataURL(img.files[0]);
    reader.onload = function (ev) {
        // base64码
        var imgFileBase64 = this.result
    }
}

3.通过canvas 压缩base64编码

function dealImage(base64, w, callback) {
    var newImage = new Image();
    var quality = 0.6;    //压缩系数0-1之间
    newImage.src = base64;
    newImage.setAttribute("crossOrigin", 'Anonymous');  //url为外域时需要
    var imgWidth, imgHeight;
    newImage.onload = function () {
        imgWidth = this.width;
        imgHeight = this.height;
        var canvas = document.createElement("canvas");
        var ctx = canvas.getContext("2d");
        if (Math.max(imgWidth, imgHeight) > w) {
            if (imgWidth > imgHeight) {
                canvas.width = w;
                canvas.height = w * imgHeight / imgWidth;
            } else {
                canvas.height = w;
                canvas.width = w * imgWidth / imgHeight;
            }
        } else {
            canvas.width = imgWidth;
            canvas.height = imgHeight;
            quality = 0.6;
        }
        ctx.clearRect(0, 0, canvas.width, canvas.height);
        ctx.drawImage(this, 0, 0, canvas.width, canvas.height);
        var base64 = canvas.toDataURL("image/jpeg", quality); //压缩语句
        // 如想确保图片压缩到自己想要的尺寸,如要求在50-150kb之间，请加以下语句，quality初始值根据情况自定
        // while (base64.length / 1024 > 150) {
        //  quality -= 0.01;
        //  base64 = canvas.toDataURL("image/jpeg", quality);
        // }
        // 防止最后一次压缩低于最低尺寸，只要quality递减合理，无需考虑
        // while (base64.length / 1024 < 50) {
        //  quality += 0.001;
        //  base64 = canvas.toDataURL("image/jpeg", quality);
        // }
        callback(base64);//必须通过回调函数返回，否则无法及时拿到该值
    }
}