【Javascript Hoisting】
hoisting teaches that variable and function declarations are physically moved to the top your coding, but this is not what happens at all. What does happen is the variable and function declarations are put into memory during the compile phase, but stays exactly where you typed it in your coding.
JavaScript only hoists declarations, not initializations. If you are using a variable that is declared and initialized after using it, the value will be undefined. The below two examples demonstrate the same behavior.
参考：https://developer.mozilla.org/en-US/docs/Glossary/Hoisting