Convert js to es6

[NoahDragon]

Thank you for creating a pull request to contribute to Hexo code! Before you open the request please review the following guidelines and tips to help it be more easily integrated:

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage remained the same at 97.131% when pulling cd8f131 on convert-to-es6 into 4b02684 on master.

[coveralls]

Coverage remained the same at 97.131% when pulling 9a0ef98 on convert-to-es6 into 4b02684 on master.

[coveralls]

Coverage remained the same at 97.131% when pulling 43cb1f8 on convert-to-es6 into 4b02684 on master.

[coveralls]

Coverage remained the same at 97.131% when pulling c9af7f2 on convert-to-es6 into 4b02684 on master.

[coveralls]

Coverage remained the same at 97.131% when pulling c9af7f2 on convert-to-es6 into 4b02684 on master.