Table of contents
1. JavaScript built-in objects
Array：用于在单独的变量名中存储一系列的值
String：用于支持对字符串的处理
Math：用于执行常用的数学任务，包含若干个数字常量和函数
Date：用于操作日期和时间
Second, the Array object
创建数组
new Array(); // 创建一个空数组
new Array(size); // 创建的时候给个大小
new Array(element0, element1, …, elementN); // 创建的时候指定元素，赋值
为数组元素赋值
<script>
// 方法一
var shuzu1 = new Array("apple","orange","peach","banana");
// 方法二
var shuzu2 = ["apple","orange","peach","banana"];
// 方法三
var shuzu3 = new Array(4);
shuzu3[0] = "apple";
shuzu3[1] = "orange";
shuzu3[2] = "peach";
shuzu3[3] = "banana";
</script>
访问数组
数组名[下标]
console.log(shuzu3[2]);
2.1, common attributes and methods
| category | name | describe |
|---|---|---|
| Attributes | length | Sets or returns the number of elements in an array |
| method | join() | Put all the elements of the array into a string, separated by commas or the specified delimiter |
| method | sort() | sort the array |
| method | push() | Adds one or more elements to the end of the array and returns the new length |
| method | forEach() | To traverse the array, the forEach() method will not directly modify the original array, but the callback function may modify |
forEach() method
array.forEach(callback[, thisArg]);
callback参数：为数组中的每个元素执行的函数
callback(currentValue[, index[, array]])
currentValue：数组中正在处理的当前元素
index：可选，数组中正在处理的当前元素的索引
array：可选，forEach()方法正在操作的数组
thisArg参数：可选，callback函数中的this可以引用的对象
示例:
<script>
var arr1 = ["apple","orange","peach"];
console.log(arr1.length);
arr1.push("banana"); // 在末尾追加一个数组元素
console.log(arr1.length); // 重新打印了数组长度，发现长度改变。说明数组是可变的
var arrStr = arr1.join("|"); // 把数组中的元素，通过指定的字符串连接成一个字符串返回
console.log(arrStr);
var arr2 = [4,2,3,1]
console.log(arr2.sort()); // 默认升序排序
// 注意，再循环中，注意数组长度的变化
arr1.forEach(function(val,index){
console.log(val,index);
})
</script>
2.2, basic method
var list = [1, 2, 3]; // 创建数组并赋值。
console.log(list.indexOf(1));// 返回1这个元素在数组中第一次出现的下标，如果没有返回-1
list[1] = 22; // 修改指定下标的值
list.push(4,5);// 向数组末尾追加元素，元素数量不定。返回新的长度
list.unshift(-1,0) // 向数组头部添加元素，元素数量不定。返回新的长度
list.pop(); // 删除最后一位元素，返回被删除的元素
list.shift(); // 删除第一位元素，返回被删除的元素
list.splice(1); // 从指定下标开始删除，删到最后。返回删除的元素数组
list.splice(1,2); // 删除从下标1开始，长度为2的元素。返回删除的元素数组
list.splice(1,1,33,44); // 删除从下标1开始，长度为1的元素，并在删除的位置上添加两个元素，添加的元素个数不定
var str = list.join(''); // 把数组按照指定的间隔符，拼成一个字符串
var list3 = [11,22,33];
var newlist = list.concat(list3); // 合并数组，注意合并数组返回的是新的数组对象，对原对象不做修改
// console.log(list2)
// console.log(str);
console.log(list);
Three, Date object
用于处理日期和时间
使用自UTC（Coordinated Universal Time，国际协调时间）1970年1月1日0时开始经过的毫秒数来保存日期
创建Date对象
语法
new Date() // 不带参数 当前日期
new Date(dateString) // 带参数，指定日期
示例
<script>
var today = new Date();
var sdate = new Date("July 15,2020,10:07:42");
console.log(today); // 返回当前日期和时间
console.log(sdate); // 返回指定日期和时间
</script>
3.1, common methods
| method | illustrate |
|---|---|
| getDate() | Returns each day of the month in the Date object, whose value is between 1 and 31 |
| getDay() | Returns each day of the week in a Date object, with a value between 0 and 6 |
| getHours() | Returns the hour of the Date object, with a value between 0 and 23 |
| getMinutes() | Returns the minute of the Date object, with a value between 0 and 59 |
| getSeconds() | Returns the seconds of the Date object, whose value is between 0 and 59 |
| getMonth() | Returns the month of the Date object, with a value between 0 and 11 |
| getFullYear() | Returns the year of a Date object as a 4-digit value |
| getTime() | Returns the number of milliseconds since a certain moment (January 1, 1970) |
<script>
var mydate = new Date();
console.log("年:",mydate.getFullYear());
console.log("月:",mydate.getMonth()+1); // 需要自己+1
console.log("日:",mydate.getDate());
console.log("时:",mydate.getHours());
console.log("分:",mydate.getMinutes());
console.log("秒:",mydate.getSeconds());
console.log("周:",mydate.getDay()); // 周日是0
console.log("时间戳:",mydate.getTime());
</script>
3.2, small case
<body>
<div id="datediv"></div>
<div>
<input type="button" value="开始" onclick="startLock()">
<input type="button" id="zanting" value="暂停">
</div>
</body>
<script>
var interIndex;
function startLock(){
interIndex = setInterval("showDate()",1000)
}
document.getElementById("zanting").addEventListener('click',function(){
clearInterval(interIndex)
})
function showDate(){
var mydate = new Date();
var year = mydate.getFullYear()
var month = mydate.getMonth()+1
var day = mydate.getDate()
var hour = mydate.getHours()
var minute = mydate.getMinutes()
var second = mydate.getSeconds()
document.getElementById("datediv").innerHTML=year+"年" +month+"月" +day+"日"+hour+":"+minute+":"+second
}
</script>
js example
Four, Math object
提供与数学相关的功能
common method
| method | illustrate | example |
|---|---|---|
| ceil() | round up the argument | Math.ceil(25.5); returns 26 ---- Math.ceil(-25.5); returns -25 |
| floor() | round the argument down | Math.floor(25.5); returns 25 ---- Math.floor(-25.5); returns -26 |
| round() | rounds the argument to the nearest number | Math.round(25.5); returns 26 ---- Math.round(-25.5); returns -26 |
| random() | Returns a random number between 0 and 1 | Math.random(); ---- For example: 0.6273608814137365 |
| method | forEach() | To traverse the array, the forEach() method will not directly modify the original array, but the callback function may modify |