A function is called pure function if it always returns the same result for same argument values and it has no side effects like modifying an argument (or global variable) or outputting something. The only result of calling a pure function is the return value.
https://cn.bing.com/search?q=pure+function&go=Search&qs=n&sp=-1&pq=pure+function&sc=8-13&sk=&cvid=1AFE1068896840E6815D18D1B9DA9B5D&first=7&FORM=PORE
只做计算；
不做赋值；
修改和定义的内容只能是计算结果和内部中间变量