例如:fs.writeFile是异步方法
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function test(){ return co(function *() { yield (() => new Promise((resolve, reject) => { fs.writeFile(path, '', (err) => { if (err) return reject(err); resolve(1); }); }))(); return; });}
调用test;
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co(function *() { let res = yield test(); console.log(res);})