Thanks Rambutan for still providing golden nuggets of knowledge 6 years later! I'm new too and while I did solve the problem this is a great explaination/break down on incrementing and loops.

Thanks for this explanation.

Never thought of using the modulo operator that way! Excellent.

Thanks for such a detailed explanation

&& evaluates to the first value if falsy, else the second. This means that null && any will always evaluate to null (not false).
And yes, this is basically a protection against accessing a property of a null or undefined value, because this would lead to a TypeError (not undefined).

重复了

条件？2:1
如果条件为真，值是2。否则，值是1
这是典型的三目运算符

重复了，删除

请问2:1是什么意思？

That is how the stack works, last in first out. Recursion is basically doing node.concat(node.concat(node.concat..., so the inner most concat or the top of the stack executes first.