Detailed explanation of common forms of JS closures

//作用域链
  var a = 1;
  function test() {
    var b =2;
    return a;
  }
  alert(test());//弹出1；
  alert(b);//不能获取b
//scope chain
  var a = 1;
  function test() {
    var b = 2;
    function test1() {
      var c = 3;
      alert(a);
      alert(b);
      alert(c);
    }
    test1();
  }
  test();//弹出1，弹出2，弹出3；

//词法作用域；
  function f1() {
    var a = 12;
    return f2();
  }
  function f2() {
    return a;
  }
  alert(f1());//并不能获取a，a在f2()中并未定义；
function f1() {
    var a = 1;
    return f2();
  }
  function f2() {
    var b = 3;
    alert(b);
    return a;
  }
  alert(f1());//弹出3，a在f2()中未定义

function f1() {
    var a = 1;
    return f2();
  }
  function f2() {
    var b = 3;
    alert(b);
    return a;
  }
  alert(f1());//弹出3，a在f2()中未定义,undefined
  var a=55;
  alert(f1());//弹出3，弹出55

//通过闭包突破全局作用域链
  function f() {
    var a = "sun";
    return function () {
      return a;
    }
  }
  var test = f();
  alert(test());//弹出sun
var n;
function f() {
  var a = "sun";
  n = function () {
    return a;
  }
}
f();
alert(n());//弹出sun
  function f(param) {
    var n =function () {
      return param;
    };
    param++;
    return n;
  }
  var test = f(45);
  alert(test());//弹出46；