[TOC] ### 1.算术计算 > /* +,-,*,/,% */ * * * * * 将先两边的值转为Number再计算 特殊:+运算例外 ~~~ var a="12"; var b = 3; var c ="12px"; var d; // console.log(Number(a)-b); console.log(a-b); // console.log(Number(c)-b) NaN和其他的值做运算,结果还是为NaN console.log(c-b); // console.log(Number(a)-Number(d)); console.log(a-d); ~~~ #### 1.1+特殊 > 算术计算中 `+` 作用:拼接符 只要一边的值为字符串,那么结果就是字符串 ~~~ var a = "10"; var b= "50"; var c = 40; console.log(a+b); console.log(b+c); ~~~ ### 2. 比较运算 ~~~ >,<,<=,>=,==,!=,=== 比较运算中,先将两边的值转换为number,再比较 结果返回boolean ===不仅值相同,类型也要相同 var a=true; var b=1; var c="12"; var d = false; console.log(a===b); console.log(c>b); console.log(b<d); ~~~ #### 2.1特殊-->`==` ~~~ ***(了解) 1.倘若有一边的值为boolean,那么先将boolean-->number再比较 2.倘若一边为字符串,一边为number,那么string-->number再比较 3.null==undefined 4.NaN不能任何值计算,结果都为NaN NaN!=NaN //-->true 5.null和undefined比较之前不能将其转为其它类型 *** 0,"",NaN,undefined,null console.log(Number("")); //0 console.log(Number(null)); //0 console.log(0==""); //true console.log(NaN!=NaN) //true console.log(null===undefined); //false console.log(null==0); //false ~~~ ### 3. 逻辑运算 ~~~ <style> div{ width:100px; height:100px; background: red; } </style> ~~~ ~~~ <div id="tes" class="test"></div> <script> var test = document.getElementById("test") || document.getElementsByClassName("test")[0]; console.log(test); test.onclick = function(){ this.style.backgroundColor = "green" } </script> ~~~ ~~~ &&,||,! 结果返回boolean值 && 两边都为true,结果为true || 只要一遍为true,结果为true !-->取反 var a = 10; var b = 20; var c = 30; var d = 40; console.log(a<b && c>d); console.log(a<b && c<d); console.log(a<b || c>d); console.log(!(a>b)) ~~~ ### 4.赋值运算 * +=,-=,*=,/=,%= > 前++,先自增,后运算 > 后++,先运算,后自增 ~~~ var a =10; var b = 20; b+=++a; console.log(a); //11 /* b=b+a */; c=a++; console.log(c); //11 console.log(a); //12 console.log(b); //31 a++; console.log(a); //13 ~~~ ### 5. 三目运算 ~~~ var a =(10<5)?"大于":"小于"; //判断为真，输出前面的，判断为假，输出后面的 console.log(a); //小于 ~~~ ### 6. 练习 ~~~ console.log(""||"abc"); //"abc" console.log(0 == ''); //true function b(){ } console.log(typeof b);//function ~~~