通过读取文件流判断文件是否是图片

当不能根据文件扩展名判断文件类型是，可以读取文件的内容来判断文件类型

var fileReader = new FileReader()
fileReader.onload = function () {
  var array = this.result;
  console.log("Array contains", array, isImage(new Uint8Array(array)), "bytes.", String.fromCharCode(...new Uint8Array(array)));
};
function ArrayBufferToString(buf) {
  return String.fromCharCode(...new Uint16Array(buf))
}
var pngMagic = [
  0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a
];
var jpeg_jfif = [
  0x4a, 0x46, 0x49, 0x46
];
var jpeg_exif = [
  0x45, 0x78, 0x69, 0x66
];
var jpegMagic = [
  0xFF, 0xD8, 0xFF, 0xE0
];
var gifMagic0 = [
  0x47, 0x49, 0x46, 0x38, 0x37, 0x61
];
var getGifMagic1 = [
  0x47, 0x49, 0x46, 0x38, 0x39, 0x61
];

function arraycopy(src, index, dist, distIndex, size) {

  for (i = 0; i < size; i++) {
    dist[distIndex + i] = src[index + i]
    // console.log(src[index + i],index+i,src.length,'src')
  }
  console.log(dist, 'dist')
}

function arrayEquals(arr1, arr2) {
  console.log(arr1)
  console.log(arr2)
  if (arr1 == 'undefined' || arr2 == 'undefined') {
    return false
  }
  if (arr1 instanceof Array && arr2 instanceof Array) {
    if (arr1.length != arr2.length) {
      return false
    }
    for (i = 0; i < arr1.length; i++) {
      if (arr1[i] != arr2[i]) {
        return false
      }
    }
    return true
  }
  return false;
}

function isImage(buf) {
  if (buf == null || buf == 'undefined' || buf.length < 8) {
    return null;
  }
  var bytes = [];
  arraycopy(buf, 0, bytes, 0, 6);
  if (isGif(bytes)) {
    return "image/gif";
  }
  bytes = [];
  arraycopy(buf, 6, bytes, 0, 4);
  if (isJpeg(bytes)) {
    return "image/jpeg";
  }
  bytes = [];
  arraycopy(buf, 0, bytes, 0, 8);
  if (isPng(bytes)) {
    return "image/png";
  }
  return null;
}


/**
* @param data first 6 bytes of file
* @return gif image file true,other false
*/
function isGif(data) {
  console.log('GIF')
  return arrayEquals(data, gifMagic0) || arrayEquals(data, getGifMagic1);
}

/**
* @param data first 4 bytes of file
* @return jpeg image file true,other false
*/
function isJpeg(data) {
  console.log('JPEG')
  return arrayEquals(data, jpegMagic) || arrayEquals(data, jpeg_jfif) || arrayEquals(data, jpeg_exif);
}

/**
* @param data first 8 bytes of file
* @return png image file true,other false
*/
function isPng(data) {
  console.log('PNG')
  return arrayEquals(data, pngMagic);
}
/**
 * 通过url获取blob
 * @param url 
 * @param fileName 
 * @returns 
 */
function getFileFromUrl(url, fileName) {
  return new Promise((resolve, reject) => {
    var blob = null;
    var xhr = new XMLHttpRequest();
    xhr.open("GET", url);
    // xhr.setRequestHeader('Accept', 'image/png');
    xhr.responseType = "blob";
    // 加载时处理
    xhr.onload = () => {
      // 获取返回结果
      blob = xhr.response;
      console.log(blob.toString())
      fileReader.readAsArrayBuffer(blob);

      let file = new File([blob], fileName);
      // 返回结果
      resolve(file);
    };
    xhr.onerror = (e) => {
      reject(e)
    };
    // 发送
    xhr.send();
  });
}

getFileFromUrl('http://localhost/xxxxx', 'aa').then(res => {
  console.log(res)
})