js表达式与运算符代码详解

/**
* Created by Administrator on 2017/12/14.
* 表达式与运算符
*/
//1.基本表达式  加减乘除
var a = 4;
a = 7/6;
var b = (a + 4)/2;
console.log(a);
console.log(b);
console.log("==========");
//end
//2.比较运算符 == != > < >= <=
console.log(a <= b);
console.log(a == b);
console.log(a != b);
console.log("==========");
//end
//3.逻辑表达式 逻辑与(&&)和逻辑或(||)
var ret = (a > 1) && (a < 2);
console.log(ret);
console.log(true && false);
console.log("==========");
//end
//4.字符串
a = 10;
b = 20;
var num_str = "" + a + b + true;
console.log(num_str);   //1020true
num_str = a + b + true + "";
console.log(num_str);   //31
console.log("==========");
//end
//5.简约表达式  += -= *= /=  ++  --
var c = null;
a += c; //a = a + c
a -= c; //a = a - c
a *= c; //a = a * c
a /= c; //a = a / c
a = 10;
a++;    //a = a + 1   先参与运算，最后自增
++a;    //a = a + 1   先自增在参与运算
console.log(a);
a--;    //a = a - 1   先参与运算，最后自减
console.log(a);
--a;    //a = a - 1   先自减在参与运算
a = 4;
console.log(a++);
console.log(++a);
// end