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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())