var a = 10;
var b = 10;
var c = a + b;
var d = "hello ";
var e = "world";
alert(c);
alert(a + d);
alert(d + e);

var f = "10";
　　f = f + 2;
　　f += 2;
　　alert(f);

 var g = 5;
 g++;
 g = g + 1;
 g += 1;
 alert(g);
 var h = g++;
 h = g;(先执行)
 g++;(再执行)
 alert(h);
 var k = ++g;
 g++;(先执行)
 k = g;(再执行)
 alert(k);

var l = 3 >= 2;
var p = 123;
var q = "123";
alert(l);
alert(p != q);
alert(p !== q );
alert(p == q);
alert(p === q );

var aa = 2;
var bb = 3;
var cc = !(aa >= bb);
alert(cc);

var dd = 2 >= 3; /*flase*/
var ee = 4 >= 1; /*/true*/
alert(dd && ee);

var dd = 2 >= 3; /*flase*/
var ee = 4 >= 1; /*/true*/
alert(dd || ee);

var bl = 2>=1?"对":"错";
alert(bl);

var spmc = prompt("请输入商品名称");
var spmc = spmc == "鸡块"?"请给我10元":spmc == "汉堡"?"请给我15元":spmc == "可乐"?"请给我5元":"请输入正确的商品";
alert(spmc);