javascript - ES6 generate problem

Question

While learning ES6 generate, why can’t the following example be printed and an error will be reported?
How to modify?

Should I execute gen() or gen.next()

var fs = require('fs');

var readFile = function (fileName) {
  return new Promise(function (resolve, reject) {
    fs.readFile(fileName, function(error, data) {
      if (error) reject(error);
      resolve(data);
    });
  });
};

var gen = function* () {
  var f1 = yield readFile('../test/1.json');
  var f2 = yield readFile('../test/2.json');
  console.log(JSON.stringify(f1));
  console.log(JSON.stringify(f1));
};

//gen()
console.log(gen.next())

Answer 1

What the generate function returns after calling is a generate object
And generate.next also returns an object, which contains two attributes: value and done
So it should be called like this

let g = gen()
console.log(g.next()) //{ value: ..., done: false }

Answer 2

//返回一个Iterator指针，还没开始执行
const g = gen();
//执行 readFile('../test/1.json');
g.next();
//执行 readFile('../test/2.json');
//下面没有yield，所以会一直执行下去
g.next();