Summary of basic JavaScript knowledge (5) Brief introduction to array or object typeof and type conversion

This time I will bring you a basic JavaScript knowledge summary. There are a total of eleven knowledge points. Basic JavaScript knowledge summary (5) briefly introduces array or object typeof and Type conversionThe following is a practical case , let’s take a look.

Array

Array objects are used to store a series of values ​​in separate variable names. The form is a square bracket, in which many things can be written, separated by commas. Each comma can separate two warehouses. Each warehouse can put things, such as Number, String, undefined, and any type of value can be placed. .

var arr = [1,2,undefined,"abc",["a",1,5],null]//Adding the array document.write(arr.push("Daniel Wu")) ;//Print arr-->[1,2,undefined,"abc",["a",1,5],null,"Daniel Wu"]//Delete array arr.splice(0,1)/ /The printout is [2,undefined,"abc",["a",1,5],null]//The modification of the array arr[0];//The printout is 0arr[0] = 100;//In The printout is 100;//View the array for(var i = 0;i

Other methods about arrays

For...In Statement
Use the for...in statement to loop through the elements in the output array.

Merge two arrays - concat()
How to use the concat() method to merge two arrays.

Use the elements of the array to form a string - join()
How to use the join() method to form a string from all the elements of the array.

Literal Array - sort()
How to use the sort() method to literally sort an array.

Number Array - sort()
How to use the sort() method to numerically sort an array.

Object

JavaScript object is data that has properties and methods. Everything in JavaScript is an object: strings, numbers, arrays, dates, etc. In JavaScript, an object is data that has properties and methods.

var obj = {    key : value,
    建 : 值,
    属性: 属性值; 
}var car = {    type:"Fiat", 
    model:500, 
    color:"white"
    money:undefined,    newCar:false,
}//增加对象属性car.width = "1.6m";//删除对象属性delete.car.width;//修改对象属性car.width = "1.5m";

The difference in programming forms

Procedure-oriented

Object-oriented

The difference is too complicated, I don’t accept it, I’ll look for it on Baidu The difference goes to

typeof

六尊Data type

Number
string
boolean
undefined
object
function

typeof("里面放数据")var num = 123;var str = "123";var a = true;var b = null; 
var c = undefined;console.log(typeof(num));//打印-->numberconsole.log(typeof(str));//打印-->stringconsole.log(typeof(a));//打印-->booleanconsole.log(typeof(b));//打印-->objectconsole.log(typeof(c));//打印-->undefined//第二种方法console.log(typeof c);//打印-->undefined 空格也可以

Type conversion

Display type conversion

Number(mix)
parseInt(string,radix)
parseFloat(string)
toString(radix)
String(mix)
Boolean()
//Number转换成数var num = Number("123");        = true;        = false;        = null;        = undefined;        = "a";        = "123abc";console.log(num)//打印123                //打印1                //打印0                //打印0                //打印NaN                //打印NaN                //打印NaN//parseInt转换成整数//parseInt(String,radix)//radix 是调整进制取值范围是2-36//parseInt 是用数字为一直往后面看,看到截止,一直看到非数字位截止,把之前的数字返回var num = parseInt("123.9");        = true;        = false;        = null;        = undefined;        = "a";        = "123abc";console.log(num)//打印123不是四舍五入                //打印NaN                //打印NaN                //打印NaN                //打印NaN                //打印NaN                //打印123                //parseFloat//把数字转换为浮点数var num = parseFloat("123.9");        = true;        = false;        = null;        = undefined;        = "a";        = "123.2abc";console.log(num)//打印123.9                //打印NaN                //打印NaN                //打印NaN                //打印NaN                //打印NaN                //打印123.2                //String把内容换成字符串var num = String(123.9);        = undefined;console.log(num)//打印"123.9"                //打印"undefined"//Boolean转换成布尔值//除了undefined、null、NaN、""、0、false 打印出来的是false以外, 其他的全是truevar num = Boolean(123.9);        = undefined;console.log(num)//打印true                //打印false                                                //toString(radix) 转换成为字符串//两个不能用一个undefined一个null会报错undefined和unll没有这个toString属性//radix 是以10进制为基底转换为别的进制//用法:要转的数据.toStringvar demo = 123;var str = demo.toString();console.log(str)//打印出"123"

Quote content conversion

isNaN()
++/-- +/-(一元正负)
-，*，/，%
&&，||，!
<，>，<=，>=
== !=

//isNaN()当你把数放在括号里面的时候他能判断这个是是不是NaN,然后给你返回回来console.log(isNaN(NaN))//打印trueconsole.log(isNaN(123))//打印falseconsole.log(isNaN("123"))//打印falseconsole.log(isNaN("adc"))//打印trueconsole.log(isNaN(null)//打印falseconsole.log(isNaN(undefined)//打印true//isNaN在内部 执行了一个Numbar方法//比如isNaN("abc")//首先执行了Numbar("abc")看是不是NaN 如果是就返回NaN//所以这个numbar它没有显示的去调用,是隐式的去调用
//++,先调用numbarvar a = "123";
a++;//打印出a-->124;var a = "abc";
a++;//打印出a-->NaN;//+/- 先调用numbarvar a = + "abc";console.log(typeof(a))//打印出numbar//+ 隐式类型转换调用的是stringvar a = "1" + 1;console.log(typeof(a))//打印出string
//-，*，/，%隐式类型转换调用的是numbar
// <，>，<=，>=如果有数字就调用numbarvar a = "1" < 2;console.log(typeof(a))//打印出boolean
// == ，!=var a = "1" == 1;console.log(typeof(a))//打印出boolean true
//特殊的undefined>0//打印falseundefined<0//打印falseundefined==0//打印falsenull>0//打印falsenull<0//打印falsenull==0//打印falseundefined == null//打印trueNaN == NaN//不等于任何东西

No type conversion occurs

=== !==(绝对的等于 绝对不等于)
1 !== "1" //true1 !== 1 //falseNaN === NaN //false

There is also a special

//a在没有定义的情况下typeof(a);//用console.log打印出undefined//typeof返回的值类型 都是string类型typeof(typeof(a))//用console.log打印出string

I believe you have mastered the method after reading the case in this article. Please pay attention for more exciting things. Other related articles on php Chinese website!

Related reading:

Basic JavaScript knowledge summary (4) Conditional statements, loop statements

Basics Summary of JavaScript knowledge (3) Comparison operators, logical operators

Summary of basic JavaScript knowledge (2) Introduction, variables, value types, operators

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