三种条件表达的写法的特点及用处进行了总结归纳,简述如下:
1. if...else结构
// Set r to 0 or 1 var r= Math.floor(2*Math.random()) // Set a, b and c to "small" if r==0 an else set them to "big" // using three different techniques // Method 1: If else var a; if (r==0){a = "small"} else {a = "big"}; // Method 2: Conditional operator var b = r==0 ? "small" : "big"; // Method 3: And/or operators var c = r==0 && "small" || "big"; // Check the values of our variables alert(r+" "+a+" "+b+" "+c);
2. if...else if...else结构
// Set r to 0,1,2 or 3 var r= Math.floor(4*Math.random()) // Set a, b and c to "nada","small","big" and "huge" // depending on the value or r using three different techniques // Method 1: If.. else if... else var a; if (r==0){a="nada"} else if (r==1){a="small"} else if (r==2){a="big"} else {a="huge"}; // Method 2: Conditional operators var b = r==0 ? "nada" : r==1 ? "small" : r==2 ? "big" : "huge"; // Method 3: And/or operators var c = r==0 && "nada" || r==1 && "small" || r==2 && "big" || "huge"; // Check the values of our variables alert(r+" "+a+" "+b+" "+c);
3. 执行函数
// Set r to 0,1,2 or 3 var r= Math.floor(4*Math.random()) // The global variable x and our four functions var x=""; nada=function(){x+="Nada! "}; small=function(){x+="Small! "}; big=function(){x+="Big! "}; huge=function(){x+="Huge! "}; // Call a specific function depending on the value of r // using three different techniques // Method 1: If.. else if... else if (r==0){nada()} else if (r==1){small()} else if (r==2){big()} else {huge()}; // Method 2: Conditional operators r==0 ? nada() : r==1 ? small() : r==2 ? big() : huge(); // Method 3: And/or operators r==0 && (nada() || true) //nada()函数不一定返回true,为了保证后续的逻辑或||判断不被执行,需要返回true值,下同 || r==1 && (small() || true) || r==2 && (big() || true) || huge(); // Check the values of our variables alert(r+" "+x);
4. 执行代码
// Set r to 0,1,2 or 3 var r= Math.floor(4*Math.random()) // The global variable x var x=""; // Executing different code depending on the value of r // using three different techniques // Method 1: If.. else if... else if (r==0){x+="Nada! "} else if (r==1){x+="Small! "} else if (r==2){x+="Big! "} else {x+="Huge! "}; // Method 2: Conditional operators r==0 ? function(){x+="Nada! "}() : r==1 ? function(){x+="Small! "}() : r==2 ? function(){x+="Big! "}() : function(){x+="Huge! "}(); // Method 3: And/or operators r==0 && (function(){x+="Nada! "}() || true) //有人在评论中指出这里的匿名函数是不必需的,在只有一条可执行代码时是这样的,但是如果有多条代码需要执行,匿名函数还是不错的 || r==1 && (function(){x+="Small! "}() || true) || r==2 && (function(){x+="Big! "}() || true) || function(){x+="Huge! "}(); // Check the values of our variables alert(r+" "+x);
在只存在两种条件的判断中,用if...else或?:都是相当直白,而&&和||的运算方式就稍嫌复杂。但是其实只要明白以下两个基本原则,所有问题都会迎刃而解了:
其一、当用逻辑与&&和逻辑或||运算符运算时,方向都是自左向右的,&&运算到第一个值为false的条件(或可转换为false的值,如null/undefined/0/""/NaN等)时停止,而运算到第一个值为true的条件(或可转换为true的值)时停止;整个条件返回的值是最后检测的条件的值,不一定只是true/false。
其二、逻辑与&&运算符较逻辑或运算符相比,前者有更高的优先级。
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