将base64转为二进制

代码如下：

// base64编码表
var map = { "0": 52, "1": 53, "2": 54, "3": 55, "4": 56, "5": 57, "6": 58, "7": 59, "8": 60, "9": 61, "A": 0, "B": 1, "C": 2, "D": 3, "E": 4, "F": 5, "G": 6, "H": 7, "I": 8, "J": 9, "K": 10, "L": 11, "M": 12, "N": 13, "O": 14, "P": 15, "Q": 16, "R": 17, "S": 18, "T": 19, "U": 20, "V": 21, "W": 22, "X": 23, "Y": 24, "Z": 25, "a": 26, "b": 27, "c": 28, "d": 29, "e": 30, "f": 31, "g": 32, "h": 33, "i": 34, "j": 35, "k": 36, "l": 37, "m": 38, "n": 39, "o": 40, "p": 41, "q": 42, "r": 43, "s": 44, "t": 45, "u": 46, "v": 47, "w": 48, "x": 49, "y": 50, "z": 51, "+": 62, "/": 63 }
        
function base64to2(base64) {
  let len = base64.length * .75 // 转换为int8array所需长度
  base64 = base64.replace(/=*$/, '') // 去掉=号（占位的）

  const int8 = new Int8Array(len) //设置int8array视图
  let arr1, arr2, arr3, arr4, p = 0

  for (let i = 0; i < base64.length; i += 4) {
    arr1 = map[base64[i]] // 每次循环 都将base644个字节转换为3个int8array直接
    arr2 = map[base64[i + 1]]
    arr3 = map[base64[i + 2]]
    arr4 = map[base64[i + 3]]
    // 假设数据arr 数据 00101011 00101111 00110011 00110001
    int8[p++] = arr1 << 2 | arr2 >> 4
    // 上面的操作 arr1向左边移动2位 变为10101100
    // arr2 向右移动4位：00000010
    // | 为'与'操作: 10101110
    int8[p++] = arr2 << 4 | arr3 >> 2
    int8[p++] = arr3 << 6 | arr4

  }
  console.log(int8);
  return int8
}
//base64to2(needBase64.num)
console.log(base64to2(needBase64.num))