输入参数
判断输入的参数个数
However, I suggest that arguments.length, and only that, is OK to keep using for those cases where you need to care about the passed number of arguments. A future version of JS might possibly add a feature that offers the ability to determine the number of arguments passed without consulting arguments.length; if that happens, then we can fully drop usage of arguments!
剩余扩展符处理超出个数的
具名参数不需要管参数的输入顺序了
输出
return 也是 flow control的一种
拥有多return的函数 也就拥有了多个输出出口
Rather than returning from the function early, we used normal flow control (if logic) to determine the retValue's assignment. At the end, we simply return retValue.
I'm not unconditionally saying that you should always have a single return, or that you should never do early returns, but I do think you should be careful about the flow control part of return creating more implicitness in your function definitions. Try to figure out the most explicit way to express the logic; that will often be the best way.
no implicit output
use return to make explicit output
容易产生副作用
JavaScript uses references and reference-copies for arrays, objects, and functions, so we may create an accidental output from our function all too easily.
