JavaScript 条件分支语句if...else
if 语句
只有当指定条件为 true 时，该语句才会执行代码
<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title>
</head>
<body><script>var sum = 300var youhuisum if(sum>200){youhuisum = sum - 10}console.log(sum,youhuisum)</script>
</body>
</html>
if else语句
<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title>
</head>
<body><script>var sum = 150var youhuisum if(sum>200){youhuisum = sum - 10}else{youhuisum = sum}console.log(sum,youhuisum)</script>
</body>
</html> 
if...else if...else if语句
后面可以无限跟else if
总金额sum是100.
如果总金额大于200，youhuisum就减10元，否则就继续判断总金额是否大于100，如果sum大于100，youhuisum就减20元以此类推
<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title>
</head>
<body><script>var sum = 20var youhuisum if(sum>200){youhuisum = sum - 10}else if(sum>100){youhuisum = sum - 20}else if(sum>50){youhuisum = sum - 30}else{youhuisum = sum}console.log(sum,youhuisum)</script>
</body>
</html>