const fs = require('fs');
const path = 'xxx/'+ Date.now() +'.png';
const base64 = data.replace(/^data:image\/\w+;base64,/,""); //去掉图片base64码前面部分data:image/png;base64
// new Buffer 操作权限太大，v6.0后使用Buffer.from()创建构造函数
const dataBuffer = new Buffer(base64, 'base64'); //把base64码转成buffer对象，
fs.writeFile(path, dataBuffer, function(err){//用fs写入文件
 if(err){
  console.log(err);
 }else{
  console.log('写入成功！');
 }
})

const buf = Buffer.from(string, encoding); // 返回一个被 string,编码格式是base64(默认编码格式是utf-8)的值初始化的新的 Buffer 实例
buf.toJSON(); // 转换为JSON对象
buf.write(string, offset, length, encoding) // 写入node缓存区
buf.toString(encoding, start, end) // 从缓存区读取数据

const fs = require("fs");
const util = require("util");
const imageData = await util.promisify(fs.readFileSync(fileUrl)); // 例：xxx/xx/xx.png
const imageBase64 = imageData.toString("base64");
const imagePrefix = "data:image/png;base64,";
console.log(imagePrefix + imageBase64);

const fs = require('fs');
const path = require('path');
const mineType = require('mime-types');
 
let filePath = path.resolve('your/file/path');
 
let data = fs.readFileSync(filePath);
data = new Buffer(data).toString('base64');
 
let base64 = 'data:' + mineType.lookup(filePath) + ';base64,' + data;
 
fs.writeFileSync(path.resolve('your/save/file/path'), base64);