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05 IfAndLoop.js
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88 lines (78 loc) · 1.43 KB
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/// 1.条件判断
var s = "123";
if (s.length) {
// 条件计算结果为3
//
}
// JavaScript把null、undefined、0、NaN和空字符串''视为false，其他值一概视为true，因此上述代码条件判断的结果是true。
// switch
/*
switch (表达式) {
case n:
代码块;
break;
case n:
代码块;
break;
default:
默认代码块;
}
*/
switch (new Date().getDay()) {
case 4:
case 5:
text = "周末快到了：）";
break;
case 0:
case 6:
text = "今天是周末~";
break;
default:
text = "期待周末！";
}
/************************************************************************************************/
/// 2.循环
var arr = ["Apple", "Google", "Microsoft"];
var i, x;
for (i = 0; i < arr.length; i++) {
x = arr[i];
console.log(x);
}
// for(;;)
var x = 0;
for (;;) {
// 将无限循环下去
if (x > 100) {
break; // 通过if判断来退出循环
}
x++;
}
// for...in
var o = {
name: "Jack",
age: 20,
city: "Beijing"
};
for (var key in o) {
console.log(key); // 'name', 'age', 'city'
}
// ** 请注意，for...in对Array的循环得到的（i）是String而不是Number。
var a = ["A", "B", "C"];
for (var i in a) {
console.log(i); // '0', '1', '2'
console.log(a[i]); // 'A', 'B', 'C'
}
// while
var x = 0;
var n = 99;
while (n > 0) {
x = x + n;
n = n - 2;
}
x; // 2500
// do...while
var n = 0;
do {
n = n + 1;
} while (n < 100);
n; // 100