JavaScript-关系运算符
关系运算符
- >
- <
- >=
- <=
- ==
- !=
- ===
- !==
关系运算符的返回值
只有两个, 要么是 true, 要么是 false，如果关系成立, 就返回 true，如果关系不成立, 就返回 false
关系运算符的注意点
- 对于非数值类型的数据, 会先转换成数值类型, 再进行判断
- 对于关系运算符来说, 任何数据和 NaN 进行比较, 返回值都是
false - 如果参与比较的都是字符串类型, 那么不会转换成数值类型再比较, 而是直接比较字符对应的
Unicode编码
示例如下
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Demo</title>
<script>
let res = 10 != 5;
console.log(res);
</script>
</head>
<body>
</body>
</html>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Demo</title>
<script>
// let resOne = 1 > 1;
let resOne = 1 > true;
// let resTwo = 1 > 0;
let resTwo = 1 > false;
// let resThree = 1 > 0;
let resThree = 1 > null;
// let resFour = 1 > 10;
let resFour = 1 > "10";
console.log(resOne);
console.log(resTwo);
console.log(resThree);
console.log(resFour);
</script>
</head>
<body>
</body>
</html>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Demo</title>
<script>
let resOne = 666 > NaN;
// let res = 666 > NaN;
let resTwo = 666 > undefined;
console.log(resOne);
console.log(resTwo);
</script>
</head>
<body>
</body>
</html>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Demo</title>
<script>
// let resOne = 0061 > 0062;
let resOne = "a" > "b";
// let resTwo = 0062 > 0061;
let resTwo = "b" > "a";
// 如果字符串中有多个字符, 那么会从左至右的依次比较, 直到条件不满足为止
let resThree = "ab" > "ac";
console.log(resOne);
console.log(resTwo);
console.log(resThree);
</script>
</head>
<body>
</body>
</html>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Demo</title>
<script>
// false
let resOne = null == 0;
// false
let resTwo = undefined == 0;
// true
let resThree = null == undefined;
// 在企业开发中千万不要通过 == 来判断某一个数据是否是 NaN
// 如果想判断某一个数据是否是 NaN 那么可以通过 isNaN 来进行判断
// false
let resFour = NaN == NaN;
let resFive = NaN;
console.log(resOne);
console.log(resTwo);
console.log(resThree);
console.log(resFour);
console.log(isNaN(resFive));
</script>
</head>
<body>
</body>
</html>
=== 与 !==
会同时判断取值和数据类型，而 == 与 != 只会判断取值
关系运算符的结合性和优先级
- 关系运算符都是左结合性(从左至右的运算)
- 关系运算符中 > < >= <= 的优先级高于 == != === !==
示例如下
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Demo</title>
<script>
// == 只会判断取值是否相等, 不会判断数据类型是否相等
// let resOne = 123 == 123;true
let resOne = 123 == "123";
// === 不仅会判断取值是否相等, 并且还会判断数据类型是否相等
// 只有取值和数据类型都相等, 才会返回true
// let res = 123 == 123;false
let resTwo = 123 === "123";
console.log(resOne);
console.log(resTwo);
</script>
</head>
<body>
</body>
</html>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Demo</title>
<script>
// let resOne = 123 != 123;false
let resOne = 123 != "123";
// let resTwo = 123 !== "123";true
let resTwo = 123 !== "123";
console.log(resOne);
console.log(resTwo);
</script>
</head>
<body>
</body>
</html>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Demo</title>
<script>
// 注意点: 正是因为关系运算符是 `左结合性`, 所以不能利用关系运算符来判断区间
// let resOne = true > 3;let resOne = 1 > 3;
let resOne = 10 > 5 > 3;
// let resTwo = true <= 20;let resTwo = 1 <= 20;
let resTwo = 10 <= 25 <= 20;
console.log(resOne);
console.log(resTwo);
// let resThree = 10 == true;let resThree = 10 == 1;
let resThree = 10 == 10 > 0;
console.log(resThree);
</script>
</head>
<body>
</body>
</html>
