时间：2021-07-01 10:21:17 帮助过：21人阅读
1.字符串的不可变性
字符串定义了后，会一直占据内存空间，企鹅该处内存空间（栈）不可被重新赋值。
2.短路运算
||、&& 二元运算符，返回参与运算的操作数的原值（原数据类型和原数据），
运算结束后，返回导致运算结束的那个操作数。
3.三元运算符
code1？code2：code3; 与if-else 不同：
返回code2或code3的值----code2，code3 都可以空的{}代替；
不能写break，continue。
4.NaN
NaN ！=NaN，
任何NaN参与的数学运算，其结果都是NaN
有NaN参与的条件表达式: 比较运算符 >/>=/</<=/==/=== 运算结果为false
！==/！= 运算结果为true
<script>
var a;
console.log(Boolean(NaN>=4));
console.log(Boolean(NaN<4));
console.log(Boolean(NaN=4));
console.log(Boolean(NaN==4));
console.log(Boolean(a=4));
console.log(NaN);
console.log(a);
if(NaN==NaN){
a = "NaN==NaN";
}
var b;
if(NaN!==NaN){
b = "NaN!=NaN";
}
console.log(a+"\n"+b);
</script>
5. JS 简单数据类型的转换---特殊情况演示
数据：0，“”，false，null，undefined,"123abc"等
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title></title>
<style>
div {
line-height: 24px;
margin: 0;
padding: 0;
}
.one {
width: 920px;
position: absolute;
left: 50%;
top: 50%;
margin-left: -460px;
margin-top: -240px;
}
.all {
float: left;
border: 2px solid #000000;
}
.all-top {
font-size: 20px;
font-weight: bold;
}
.all-bottom {
line-height: 48px;
font-size: 16px;
}
.details {
float: left;
border: 2px solid #000000;
line-height: 24px;
margin-left: -2px;
}
.details:hover {
position: relative;
border: 2px solid #ff0000;
}
.line-long {
border-top: 2px solid #000000;
height: 0;
width: 908px;
}
.line-short {
border-top: 2px dashed #000000;
height: 0;
width: 742px;
margin-left: 166px;
}
</style>
<script>
document.write("<div class='one'>");
function f1() {
return typeof res[res.length - 1];
}
var arr = [0, "", false, null, undefined, NaN, 6.66, -9, "abc124", "-12.23abc23", "qwer", "s s"];
document.write("<div class='all'><div class='all-top'>" + " 原数据及类型" + "</br>" + "转换方法 </div>" + "<div class='all-bottom'>" + "+" + "</br>" + "Number()" + "</br>" + "parseInt()" + "</br>" + "parseFloat()" + "</br>" + "\"\"" + "</br>" + ".toString" + "</br>" + "String()" + "</br>" + "!!" + "</br>" + "Boolean()" + "</br>" + "</div></div>")
for (var i = 0; i < arr.length; i++) {
switch (true) {
case arr[i] === "":
{
var res = ['""'];
break;
}
default :
{
var res = [arr[i] + ""];
}
}
res[res.length] = typeof arr[i];
res[res.length] = +arr[i];
res[res.length] = f1();
res[res.length] = Number(arr[i]);
res[res.length] = f1();
res[res.length] = parseInt(arr[i]);
res[res.length] = f1();
res[res.length] = parseFloat(arr[i]);
res[res.length] = f1();
res[res.length] = arr[i] + "";
res[res.length] = f1();
if (i == 3 || i == 4) {//null 和undefined没有.toString()方法，导致报错
res[res.length] = "报错";
res[res.length] = "报错";
} else {
res[res.length] = (arr[i]).toString();
res[res.length] = f1();
}
res[res.length] = String(arr[i]);
res[res.length] = f1();
res[res.length] = !!arr[i];
res[res.length] = f1();
res[res.length] = Boolean(arr[i]);
res[res.length] = f1();
var resString = res.join("<br>");
document.write("<div class='details'>" + resString + "</br>" + "</div>");
}
var j = 22;
for (var i = 0; i < 9; i++) {
document.write("<div class='line-short' style='margin-top:" + j + "px'></div>")
document.write("<div class='line-long' style='margin-top:" + j + "px'></div>")
}
document.write("</div>");
</script>
</head>
<body>
</body>
</html>
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