function x(y) {
console.log(y);
if (y==0) return;
x(y-1);
}

引擎首先解析函数声明,获知定义了一个名为x的函数对象
在解析执行函数顺序
1)console.log(y);
2) 如果y=0 退出函数执行
3) 如果不是,执行函数对象x
...

js里的递归靠栈来实现的：

==> y = 5 , x(5) , console.log(5) x(5)| ==> y = 4 , x(4) , console.log(4) x(4)|x(5)| ==> y = 3 , x(3) , console.log(3) x(3)|x(4)|x(5)| ==> y = 2 , x(2) , console.log(2) x(2)|x(3)|x(4)|x(5)| == y = 1 , x(1) , console.log(1) x(1)|x(2)|x(3)|x(4)|x(5)| ==> y = 0 , x(0) , console.log(0) x(0)|x(1)|x(2)|x(3)|x(4)|x(5)| ==> out stack x(1)|x(2)|x(3)|x(4)|x(5)| ==> out stack x(2)|x(3)|x(4)|x(5)| ==> out stack x(3)|x(4)|x(5)| ==> out stack x(4)|x(5)| ==> out stack x(5)| ==> out stack empty stack|