HTML5 实现图片上传预处理功能

var input = document.createElement('input');input.type = 'file';input.addEventListener('change', function() {  var file = this.files[0];});input.click();

var img = document.createElement('img');img.src = window.URL.createObjectURL(file);var img = document.createElement("img");var reader = new FileReader();reader.onload = function(e) {  img.src = e.target.result;}reader.readAsDataURL(file);

var canvas = document.createElement("canvas");var ctx = canvas.getContext("2d");var MAX_WIDTH = 800;var MAX_HEIGHT = 600;var width = img.width;var height = img.height;if (width > height) {  if (width > MAX_WIDTH) {    height *= MAX_WIDTH / width;    width = MAX_WIDTH;  }} else {  if (height > MAX_HEIGHT) {    width *= MAX_HEIGHT / height;    height = MAX_HEIGHT;  }}canvas.width = width;canvas.height = height;ctx.drawImage(img, 0, 0, width, height);

canvas.toBlob(function(blob) {  var form = new FormData();  form.append('file', blob);  fetch('/api/upload', {method: 'POST', body: form});});