js with文法

demo1:

var a, x, y;
var r = 10;

with (Math) {
  a = PI * r * r;
  x = r * cos(PI);
  y = r * sin(PI / 2);
}
           

demo2:

var obj = {
    aa: "xxx",
    bb: [1,2,3,4],
    test: {
        a: "xx"
    }
}
with(obj) {
    console.log(aa);//'xxx'
    console.log(test.a);//'xx'
}