变量输出

var b = 1;

function outer() {
  var b = 2;

  function inner() {
    b++;

    var b = 3;

    console.log(b);
  }

  inner();
}

outer();

function inner() {
  var b; // b is undefined

  b++; // b is NaN

  b = 3; // b is 3

  console.log(b); // output "3"
}